Merge pull request #2624 from nspcc-dev/optimize-unused-globals

compiler: do not emit initialisation code for unused global vars

pkg/compiler/analysis.go

@@ -49,11 +49,15 @@ func (c *codegen) getIdentName(pkg string, name string) string {
 func (c *codegen) traverseGlobals() bool {
 	var hasDefer bool
 	var n, nConst int
+	var hasUnusedCall bool
 	var hasDeploy bool
 	c.ForEachFile(func(f *ast.File, pkg *types.Package) {
-		nv, nc := countGlobals(f)
+		nv, nc, huc := countGlobals(f, !hasUnusedCall)
 		n += nv
 		nConst += nc
+		if huc {
+			hasUnusedCall = true
+		}
 		if !hasDeploy || !hasDefer {
 			ast.Inspect(f, func(node ast.Node) bool {
 				switch n := node.(type) {
@@ -85,10 +89,10 @@ func (c *codegen) traverseGlobals() bool {
 
 	lastCnt, maxCnt := -1, -1
 	c.ForEachPackage(func(pkg *packages.Package) {
-		if n+nConst > 0 {
+		if n+nConst > 0 || hasUnusedCall {
 			for _, f := range pkg.Syntax {
 				c.fillImportMap(f, pkg)
-				c.convertGlobals(f, pkg.Types)
+				c.convertGlobals(f)
 			}
 		}
 		for _, f := range pkg.Syntax {
@@ -143,26 +147,37 @@ func (c *codegen) traverseGlobals() bool {
 // countGlobals counts the global variables in the program to add
 // them with the stack size of the function.
 // Second returned argument contains the amount of global constants.
-func countGlobals(f ast.Node) (int, int) {
+// If checkUnusedCalls set to true then unnamed global variables containing call
+// will be searched for and their presence is returned as the last argument.
+func countGlobals(f ast.Node, checkUnusedCalls bool) (int, int, bool) {
 	var numVar, numConst int
+	var hasUnusedCall bool
 	ast.Inspect(f, func(node ast.Node) bool {
 		switch n := node.(type) {
 		// Skip all function declarations if we have already encountered `defer`.
 		case *ast.FuncDecl:
 			return false
 		// After skipping all funcDecls, we are sure that each value spec
-		// is a global declared variable or constant.
+		// is a globally declared variable or constant.
 		case *ast.GenDecl:
 			isVar := n.Tok == token.VAR
 			if isVar || n.Tok == token.CONST {
 				for _, s := range n.Specs {
-					for _, id := range s.(*ast.ValueSpec).Names {
-						if id.Name != "_" {
+					valueSpec := s.(*ast.ValueSpec)
+					multiRet := len(valueSpec.Values) != 0 && len(valueSpec.Names) != len(valueSpec.Values) // e.g. var A, B = f() where func f() (int, int)
+					for j, id := range valueSpec.Names {
+						if id.Name != "_" { // If variable has name, then it's treated as used - that's countGlobals' caller responsibility to guarantee that.
 							if isVar {
 								numVar++
 							} else {
 								numConst++
 							}
+						} else if isVar && len(valueSpec.Values) != 0 && checkUnusedCalls && !hasUnusedCall {
+							indexToCheck := j
+							if multiRet {
+								indexToCheck = 0
+							}
+							hasUnusedCall = containsCall(valueSpec.Values[indexToCheck])
 						}
 					}
 				}
@@ -171,7 +186,23 @@ func countGlobals(f ast.Node) (int, int) {
 		}
 		return true
 	})
-	return numVar, numConst
+	return numVar, numConst, hasUnusedCall
+}
+
+// containsCall traverses node and looks if it contains a function or method call.
+func containsCall(n ast.Node) bool {
+	var hasCall bool
+	ast.Inspect(n, func(node ast.Node) bool {
+		switch node.(type) {
+		case *ast.CallExpr:
+			hasCall = true
+		case *ast.Ident:
+			// Can safely skip idents immediately, we're interested at function calls only.
+			return false
+		}
+		return !hasCall
+	})
+	return hasCall
 }
 
 // isExprNil looks if the given expression is a `nil`.
@@ -250,21 +281,45 @@ func (c *codegen) fillDocumentInfo() {
 	})
 }
 
-// analyzeFuncUsage traverses all code and returns a map with functions
+// analyzeFuncAndGlobalVarUsage traverses all code and returns a map with functions
 // which should be present in the emitted code.
 // This is done using BFS starting from exported functions or
 // the function used in variable declarations (graph edge corresponds to
-// the function being called in declaration).
-func (c *codegen) analyzeFuncUsage() funcUsage {
+// the function being called in declaration). It also analyzes global variables
+// usage preserving the same traversal strategy and rules. Unused global variables
+// are renamed to "_" in the end. Global variable is treated as "used" iff:
+// 1. It belongs either to main or to exported package AND is used directly from the exported (or _init\_deploy) method of the main package.
+// 2. It belongs either to main or to exported package AND is used non-directly from the exported (or _init\_deploy) method of the main package
+// (e.g. via series of function calls or in some expression that is "used").
+// 3. It belongs either to main or to exported package AND contains function call inside its value definition.
+func (c *codegen) analyzeFuncAndGlobalVarUsage() funcUsage {
 	type declPair struct {
 		decl      *ast.FuncDecl
 		importMap map[string]string
 		path      string
 	}
-
-	// nodeCache contains top-level function declarations .
+	// globalVar represents a global variable declaration node with the corresponding package context.
+	type globalVar struct {
+		decl      *ast.GenDecl // decl contains global variables declaration node (there can be multiple declarations in a single node).
+		specIdx   int          // specIdx is the index of variable specification in the list of GenDecl specifications.
+		varIdx    int          // varIdx is the index of variable name in the specification names.
+		ident     *ast.Ident   // ident is a named global variable identifier got from the specified node.
+		importMap map[string]string
+		path      string
+	}
+	// nodeCache contains top-level function declarations.
 	nodeCache := make(map[string]declPair)
+	// globalVarsCache contains both used and unused declared named global vars.
+	globalVarsCache := make(map[string]globalVar)
+	// diff contains used functions that are not yet marked as "used" and those definition
+	// requires traversal in the subsequent stages.
 	diff := funcUsage{}
+	// globalVarsDiff contains used named global variables that are not yet marked as "used"
+	// and those declaration requires traversal in the subsequent stages.
+	globalVarsDiff := funcUsage{}
+	// usedExpressions contains a set of ast.Nodes that are used in the program and need to be evaluated
+	// (either they are used from the used functions OR belong to global variable declaration and surrounded by a function call)
+	var usedExpressions []nodeContext
 	c.ForEachFile(func(f *ast.File, pkg *types.Package) {
 		var pkgPath string
 		isMain := pkg == c.mainPkg.Types
@@ -316,6 +371,44 @@ func (c *codegen) analyzeFuncUsage() funcUsage {
 				}
 				nodeCache[name] = declPair{n, c.importMap, pkgPath}
 				return false // will be processed in the next stage
+			case *ast.GenDecl:
+				// After skipping all funcDecls, we are sure that each value spec
+				// is a globally declared variable or constant. We need to gather global
+				// vars from both main and imported packages.
+				if n.Tok == token.VAR {
+					for i, s := range n.Specs {
+						valSpec := s.(*ast.ValueSpec)
+						for j, id := range valSpec.Names {
+							if id.Name != "_" {
+								name := c.getIdentName(pkgPath, id.Name)
+								globalVarsCache[name] = globalVar{
+									decl:      n,
+									specIdx:   i,
+									varIdx:    j,
+									ident:     id,
+									importMap: c.importMap,
+									path:      pkgPath,
+								}
+							}
+							// Traverse both named/unnamed global variables, check whether function/method call
+							// is present inside variable value and if so, mark all its children as "used" for
+							// further traversal and evaluation.
+							if len(valSpec.Values) == 0 {
+								continue
+							}
+							multiRet := len(valSpec.Values) != len(valSpec.Names)
+							if (j == 0 || !multiRet) && containsCall(valSpec.Values[j]) {
+								usedExpressions = append(usedExpressions, nodeContext{
+									node:      valSpec.Values[j],
+									path:      pkgPath,
+									importMap: c.importMap,
+									typeInfo:  c.typeInfo,
+									currPkg:   c.currPkg,
+								})
+							}
+						}
+					}
+				}
 			}
 			return true
 		})
@@ -324,9 +417,24 @@ func (c *codegen) analyzeFuncUsage() funcUsage {
 		return nil
 	}
 
+	// Handle nodes that contain (or surrounded by) function calls and are a part
+	// of global variable declaration.
+	c.pickVarsFromNodes(usedExpressions, func(name string) {
+		if _, gOK := globalVarsCache[name]; gOK {
+			globalVarsDiff[name] = true
+		}
+	})
+
+	// Traverse the set of upper-layered used functions and construct the functions' usage map.
+	// At the same time, go through the whole set of used functions and mark global vars used
+	// from these functions as "used". Also mark the global variables from the previous step
+	// and their children as "used".
 	usage := funcUsage{}
-	for len(diff) != 0 {
+	globalVarsUsage := funcUsage{}
+	for len(diff) != 0 || len(globalVarsDiff) != 0 {
 		nextDiff := funcUsage{}
+		nextGlobalVarsDiff := funcUsage{}
+		usedExpressions = usedExpressions[:0]
 		for name := range diff {
 			fd, ok := nodeCache[name]
 			if !ok || usage[name] {
@@ -354,12 +462,157 @@ func (c *codegen) analyzeFuncUsage() funcUsage {
 				}
 				return true
 			})
+			usedExpressions = append(usedExpressions, nodeContext{
+				node:      fd.decl.Body,
+				path:      fd.path,
+				importMap: c.importMap,
+				typeInfo:  c.typeInfo,
+				currPkg:   c.currPkg,
+			})
 		}
+
+		// Traverse used global vars in a separate cycle so that we're sure there's no other unrelated vars.
+		// Mark their children as "used".
+		for name := range globalVarsDiff {
+			fd, ok := globalVarsCache[name]
+			if !ok || globalVarsUsage[name] {
+				continue
+			}
+			globalVarsUsage[name] = true
+			pkg := c.mainPkg
+			if fd.path != "" {
+				pkg = c.packageCache[fd.path]
+			}
+			valSpec := fd.decl.Specs[fd.specIdx].(*ast.ValueSpec)
+			if len(valSpec.Values) == 0 {
+				continue
+			}
+			multiRet := len(valSpec.Values) != len(valSpec.Names)
+			if fd.varIdx == 0 || !multiRet {
+				usedExpressions = append(usedExpressions, nodeContext{
+					node:      valSpec.Values[fd.varIdx],
+					path:      fd.path,
+					importMap: fd.importMap,
+					typeInfo:  pkg.TypesInfo,
+					currPkg:   pkg,
+				})
+			}
+		}
+		c.pickVarsFromNodes(usedExpressions, func(name string) {
+			if _, gOK := globalVarsCache[name]; gOK {
+				nextGlobalVarsDiff[name] = true
+			}
+		})
 		diff = nextDiff
+		globalVarsDiff = nextGlobalVarsDiff
+	}
+
+	// Tiny hack: rename all remaining unused global vars. After that these unused
+	// vars will be handled as any other unnamed unused variables, i.e.
+	// c.traverseGlobals() won't take them into account during static slot creation
+	// and the code won't be emitted for them.
+	for name, node := range globalVarsCache {
+		if _, ok := globalVarsUsage[name]; !ok {
+			node.ident.Name = "_"
+		}
 	}
 	return usage
 }
 
+// nodeContext contains ast node with the corresponding import map, type info and package information
+// required to retrieve fully qualified node name (if so).
+type nodeContext struct {
+	node      ast.Node
+	path      string
+	importMap map[string]string
+	typeInfo  *types.Info
+	currPkg   *packages.Package
+}
+
+// derive returns provided node with the parent's context.
+func (c nodeContext) derive(n ast.Node) nodeContext {
+	return nodeContext{
+		node:      n,
+		path:      c.path,
+		importMap: c.importMap,
+		typeInfo:  c.typeInfo,
+		currPkg:   c.currPkg,
+	}
+}
+
+// pickVarsFromNodes searches for variables used in the given set of nodes
+// calling markAsUsed for each variable. Be careful while using codegen after
+// pickVarsFromNodes, it changes importMap, currPkg and typeInfo.
+func (c *codegen) pickVarsFromNodes(nodes []nodeContext, markAsUsed func(name string)) {
+	for len(nodes) != 0 {
+		var nextExprToCheck []nodeContext
+		for _, val := range nodes {
+			// Set variable context for proper name extraction.
+			c.importMap = val.importMap
+			c.currPkg = val.currPkg
+			c.typeInfo = val.typeInfo
+			ast.Inspect(val.node, func(node ast.Node) bool {
+				switch n := node.(type) {
+				case *ast.KeyValueExpr: // var _ = f() + CustomInt{Int: Unused}.Int + 3 => mark Unused as "used".
+					nextExprToCheck = append(nextExprToCheck, val.derive(n.Value))
+					return false
+				case *ast.CallExpr:
+					switch t := n.Fun.(type) {
+					case *ast.Ident:
+						// Do nothing, used functions are handled in a separate cycle.
+					case *ast.SelectorExpr:
+						nextExprToCheck = append(nextExprToCheck, val.derive(t))
+					}
+					for _, arg := range n.Args {
+						switch arg.(type) {
+						case *ast.BasicLit:
+						default:
+							nextExprToCheck = append(nextExprToCheck, val.derive(arg))
+						}
+					}
+					return false
+				case *ast.SelectorExpr:
+					if c.typeInfo.Selections[n] != nil {
+						switch t := n.X.(type) {
+						case *ast.Ident:
+							nextExprToCheck = append(nextExprToCheck, val.derive(t))
+						case *ast.CompositeLit:
+							nextExprToCheck = append(nextExprToCheck, val.derive(t))
+						case *ast.SelectorExpr: // imp_pkg.Anna.GetAge() => mark Anna (exported global struct) as used.
+							nextExprToCheck = append(nextExprToCheck, val.derive(t))
+						}
+					} else {
+						ident := n.X.(*ast.Ident)
+						name := c.getIdentName(ident.Name, n.Sel.Name)
+						markAsUsed(name)
+					}
+					return false
+				case *ast.CompositeLit: // var _ = f(1) + []int{1, Unused, 3}[1] => mark Unused as "used".
+					for _, e := range n.Elts {
+						switch e.(type) {
+						case *ast.BasicLit:
+						default:
+							nextExprToCheck = append(nextExprToCheck, val.derive(e))
+						}
+					}
+					return false
+				case *ast.Ident:
+					name := c.getIdentName(val.path, n.Name)
+					markAsUsed(name)
+					return false
+				case *ast.DeferStmt:
+					nextExprToCheck = append(nextExprToCheck, val.derive(n.Call.Fun))
+					return false
+				case *ast.BasicLit:
+					return false
+				}
+				return true
+			})
+		}
+		nodes = nextExprToCheck
+	}
+}
+
 func isGoBuiltin(name string) bool {
 	for i := range goBuiltins {
 		if name == goBuiltins[i] {

pkg/compiler/codegen.go

@@ -350,7 +350,7 @@ func (c *codegen) emitDefault(t types.Type) {
 // convertGlobals traverses the AST and only converts global declarations.
 // If we call this in convertFuncDecl, it will load all global variables
 // into the scope of the function.
-func (c *codegen) convertGlobals(f *ast.File, _ *types.Package) {
+func (c *codegen) convertGlobals(f *ast.File) {
 	ast.Inspect(f, func(node ast.Node) bool {
 		switch n := node.(type) {
 		case *ast.FuncDecl:
@@ -596,15 +596,32 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 						}
 					}
 				}
-				for i := range t.Names {
-					if len(t.Values) != 0 {
-						if i == 0 || !multiRet {
-							ast.Walk(c, t.Values[i])
+				for i, id := range t.Names {
+					if id.Name != "_" {
+						if len(t.Values) != 0 {
+							if i == 0 || !multiRet {
+								ast.Walk(c, t.Values[i])
+							}
+						} else {
+							c.emitDefault(c.typeOf(t.Type))
 						}
-					} else {
-						c.emitDefault(c.typeOf(t.Type))
+						c.emitStoreVar("", t.Names[i].Name)
+						continue
+					}
+					// If var decl contains call then the code should be emitted for it, otherwise - do not evaluate.
+					if len(t.Values) == 0 {
+						continue
+					}
+					var hasCall bool
+					if i == 0 || !multiRet {
+						hasCall = containsCall(t.Values[i])
+					}
+					if hasCall {
+						ast.Walk(c, t.Values[i])
+					}
+					if hasCall || i != 0 && multiRet {
+						c.emitStoreVar("", "_") // drop unused after walk
 					}
-					c.emitStoreVar("", t.Names[i].Name)
 				}
 			}
 		}
@@ -2119,7 +2136,7 @@ func (c *codegen) compile(info *buildInfo, pkg *packages.Package) error {
 	c.mainPkg = pkg
 	c.analyzePkgOrder()
 	c.fillDocumentInfo()
-	funUsage := c.analyzeFuncUsage()
+	funUsage := c.analyzeFuncAndGlobalVarUsage()
 	if c.prog.Err != nil {
 		return c.prog.Err
 	}

pkg/compiler/global_test.go

@@ -9,34 +9,652 @@ import (
 
 	"github.com/nspcc-dev/neo-go/pkg/compiler"
 	"github.com/nspcc-dev/neo-go/pkg/vm"
+	"github.com/nspcc-dev/neo-go/pkg/vm/opcode"
 	"github.com/stretchr/testify/require"
 )
 
 func TestUnusedGlobal(t *testing.T) {
-	src := `package foo
-	const (
-		_ int = iota
-		a
-	)
-	func Main() int {
-		return 1
-	}`
-	eval(t, src, big.NewInt(1))
+	t.Run("simple unused", func(t *testing.T) {
+		src := `package foo
+				const (
+					_ int = iota
+					a
+				)
+				func Main() int {
+					return 1
+				}`
+		prog := eval(t, src, big.NewInt(1))
+		require.Equal(t, 2, len(prog)) // PUSH1 + RET
+	})
+	t.Run("unused with function call inside", func(t *testing.T) {
+		t.Run("specification names count matches values count", func(t *testing.T) {
+			src := `package foo
+				var control int
+				var _ = f()
+				func Main() int {
+					return control
+				}
+				func f() int {
+					control = 1
+					return 5
+				}`
+			eval(t, src, big.NewInt(1))
+		})
+		t.Run("specification names count differs from values count", func(t *testing.T) {
+			src := `package foo
+				var control int
+				var _, _ = f()
+				func Main() int {
+					return control
+				}
+				func f() (int, int) {
+					control = 1
+					return 5, 6
+				}`
+			eval(t, src, big.NewInt(1))
+		})
+		t.Run("used", func(t *testing.T) {
+			src := `package foo
+				var _, A = f()
+				func Main() int {
+					return A
+				}
+				func f() (int, int) {
+					return 5, 6
+				}`
+			eval(t, src, big.NewInt(6))
+			checkInstrCount(t, src, 1, 1, 0, 0) // sslot for A, single call to f
+		})
+	})
+	t.Run("unused without function call", func(t *testing.T) {
+		src := `package foo
+				var _ = 1
+				var (
+					_ = 2 + 3
+					_, _ = 3 + 4, 5
+				)
+				func Main() int {
+					return 1
+				}`
+		prog := eval(t, src, big.NewInt(1))
+		require.Equal(t, 2, len(prog)) // PUSH1 + RET
+	})
+}
+
+func TestUnusedOptimizedGlobalVar(t *testing.T) {
+	t.Run("unused, no initialization", func(t *testing.T) {
+		src := `package foo
+				var A int
+				var (
+					B int
+					C, D, E int
+				)
+				func Main() int {
+					return 1
+				}`
+		prog := eval(t, src, big.NewInt(1))
+		require.Equal(t, 2, len(prog)) // Main
+	})
+	t.Run("used, no initialization", func(t *testing.T) {
+		src := `package foo
+				var A int
+				func Main() int {
+					return A
+				}`
+		eval(t, src, big.NewInt(0))
+		checkInstrCount(t, src, 1, 0, 0, 0) // sslot for A
+	})
+	t.Run("used by unused var, no initialization", func(t *testing.T) {
+		src := `package foo
+				var Unused int
+				var Unused2 = Unused + 1
+				func Main() int {
+					return 1
+				}`
+		prog := eval(t, src, big.NewInt(1))
+		require.Equal(t, 2, len(prog)) // Main
+	})
+	t.Run("unused, with initialization", func(t *testing.T) {
+		src := `package foo
+				var Unused = 1
+				func Main() int {
+					return 2
+				}`
+		prog := eval(t, src, big.NewInt(2))
+		require.Equal(t, 2, len(prog)) // Main
+	})
+	t.Run("unused, with initialization by used var", func(t *testing.T) {
+		src := `package foo
+				var (
+					A = 1
+					B, Unused, C = f(), A + 2, 3 // the code for Unused initialization won't be emitted as it's a pure expression without function calls
+					Unused2 = 4
+				)
+				var Unused3 = 5
+				func Main() int {
+					return A + C
+				}
+				func f() int {
+					return 4
+				}`
+		eval(t, src, big.NewInt(4), []interface{}{opcode.INITSSLOT, []byte{2}}, // sslot for A and C
+			opcode.PUSH1, opcode.STSFLD0, // store A
+			[]interface{}{opcode.CALL, []byte{10}}, opcode.DROP, // evaluate B and drop
+			opcode.PUSH3, opcode.STSFLD1, opcode.RET, // store C
+			opcode.LDSFLD0, opcode.LDSFLD1, opcode.ADD, opcode.RET, // Main
+			opcode.PUSH4, opcode.RET) // f
+	})
+	t.Run("used by unused var, with initialization", func(t *testing.T) {
+		src := `package foo
+				var (
+					Unused1 = 1
+					Unused2 = Unused1 + 1
+				)
+				func Main() int {
+					return 1
+				}`
+		prog := eval(t, src, big.NewInt(1))
+		require.Equal(t, 2, len(prog)) // Main
+	})
+	t.Run("used with combination of nested unused", func(t *testing.T) {
+		src := `package foo
+				var (
+					A = 1
+					Unused1 = 2
+					Unused2 = Unused1 + 1
+				)
+				func Main() int {
+					return A
+				}`
+		eval(t, src, big.NewInt(1), []interface{}{opcode.INITSSLOT, []byte{1}}, // sslot for A
+			opcode.PUSH1, opcode.STSFLD0, opcode.RET, // store A
+			opcode.LDSFLD0, opcode.RET) // Main
+	})
+	t.Run("single var stmt with both used and unused vars", func(t *testing.T) {
+		src := `package foo
+				var A, Unused1, B, Unused2 = 1, 2, 3, 4
+				func Main() int {
+					return A + B
+				}`
+		eval(t, src, big.NewInt(4), []interface{}{opcode.INITSSLOT, []byte{2}}, // sslot for A and B
+			opcode.PUSH1, opcode.STSFLD0, // store A
+			opcode.PUSH3, opcode.STSFLD1, opcode.RET, // store B
+			opcode.LDSFLD0, opcode.LDSFLD1, opcode.ADD, opcode.RET) // Main
+	})
+	t.Run("single var decl token with multiple var specifications", func(t *testing.T) {
+		src := `package foo
+				var (
+					A, Unused1, B, Unused2 = 1, 2, 3, 4
+					C, Unused3 int
+				)
+				func Main() int {
+					return A + B + C
+				}`
+		eval(t, src, big.NewInt(4), []interface{}{opcode.INITSSLOT, []byte{3}}, // sslot for A, B, C
+			opcode.PUSH1, opcode.STSFLD0, // store A
+			opcode.PUSH3, opcode.STSFLD1, // store B
+			opcode.PUSH0, opcode.STSFLD2, opcode.RET, // store C
+			opcode.LDSFLD0, opcode.LDSFLD1, opcode.ADD, opcode.LDSFLD2, opcode.ADD, opcode.RET) // Main
+	})
+	t.Run("function as unused var value", func(t *testing.T) {
+		src := `package foo
+				var A, Unused1 = 1, f()
+				func Main() int {
+					return A
+				}
+				func f() int {
+					return 2
+				}`
+		eval(t, src, big.NewInt(1), []interface{}{opcode.INITSSLOT, []byte{1}}, // sslot for A
+			opcode.PUSH1, opcode.STSFLD0, // store A
+			[]interface{}{opcode.CALL, []byte{6}}, opcode.DROP, opcode.RET, // evaluate Unused1 (call to f) and drop its value
+			opcode.LDSFLD0, opcode.RET, // Main
+			opcode.PUSH2, opcode.RET) // f
+	})
+	t.Run("function as unused struct field", func(t *testing.T) {
+		src := `package foo
+				type Str struct { Int int }
+				var _ = Str{Int: f()}
+				func Main() int {
+					return 1
+				}
+				func f() int {
+					return 2
+				}`
+		eval(t, src, big.NewInt(1), []interface{}{opcode.CALL, []byte{8}}, opcode.PUSH1, opcode.PACKSTRUCT, opcode.DROP, opcode.RET, // evaluate struct val
+			opcode.PUSH1, opcode.RET, // Main
+			opcode.PUSH2, opcode.RET) // f
+	})
+	t.Run("used in unused function", func(t *testing.T) {
+		src := `package foo
+				var Unused1, Unused2, Unused3 = 1, 2, 3
+				func Main() int {
+					return 1
+				}
+				func unused1() int {
+					return Unused1
+				}
+				func unused2() int {
+					return Unused1 + unused1()
+				}
+				func unused3() int {
+					return Unused2 + unused2()
+				}`
+		prog := eval(t, src, big.NewInt(1))
+		require.Equal(t, 2, len(prog)) // Main
+	})
+	t.Run("used in used function", func(t *testing.T) {
+		src := `package foo
+				var A = 1
+				func Main() int {
+					return f()
+				}
+				func f() int {
+					return A
+				}`
+		eval(t, src, big.NewInt(1))
+		checkInstrCount(t, src, 1, 1, 0, 0)
+	})
+	t.Run("unused, initialized via init", func(t *testing.T) {
+		src := `package foo
+				var A int
+				func Main() int {
+					return 2
+				}
+				func init() {
+					A = 1		// Although A is unused from exported functions, it's used from init(), so it should be mark as "used" and stored.
+				}`
+		eval(t, src, big.NewInt(2))
+		checkInstrCount(t, src, 1, 0, 0, 0)
+	})
+	t.Run("used, initialized via init", func(t *testing.T) {
+		src := `package foo
+				var A int
+				func Main() int {
+					return A
+				}
+				func init() {
+					A = 1
+				}`
+		eval(t, src, big.NewInt(1))
+		checkInstrCount(t, src, 1, 0, 0, 0)
+	})
+	t.Run("unused, initialized by function call", func(t *testing.T) {
+		t.Run("unnamed", func(t *testing.T) {
+			src := `package foo
+					var _ = f()
+					func Main() int {
+						return 1
+					}
+					func f() int {
+						return 2
+					}`
+			eval(t, src, big.NewInt(1))
+			checkInstrCount(t, src, 0, 1, 0, 0)
+		})
+		t.Run("named", func(t *testing.T) {
+			src := `package foo
+					var A = f()
+					func Main() int {
+						return 1
+					}
+					func f() int {
+						return 2
+					}`
+			eval(t, src, big.NewInt(1))
+			checkInstrCount(t, src, 0, 1, 0, 0)
+		})
+		t.Run("named, with dependency on unused var", func(t *testing.T) {
+			src := `package foo
+					var (
+						A = 1
+						B = A + 1 // To check nested ident values.
+						C = 3
+						D = B + f() + C // To check that both idents (before and after the call to f) will be marked as "used".
+						E = C + 1 		// Unused, no code expected.
+					)
+					func Main() int {
+						return 1
+					}
+					func f() int {
+						return 2
+					}`
+			eval(t, src, big.NewInt(1), []interface{}{opcode.INITSSLOT, []byte{3}}, // sslot for A
+				opcode.PUSH1, opcode.STSFLD0, // store A
+				opcode.LDSFLD0, opcode.PUSH1, opcode.ADD, opcode.STSFLD1, // store B
+				opcode.PUSH3, opcode.STSFLD2, // store C
+				opcode.LDSFLD1, []interface{}{opcode.CALL, []byte{9}}, opcode.ADD, opcode.LDSFLD2, opcode.ADD, opcode.DROP, opcode.RET, // evaluate D and drop
+				opcode.PUSH1, opcode.RET, // Main
+				opcode.PUSH2, opcode.RET) // f
+		})
+		t.Run("named, with dependency on unused var ident inside function call", func(t *testing.T) {
+			src := `package foo
+					var A = 1
+					var B = f(A)
+					func Main() int {
+						return 1
+					}
+					func f(a int) int {
+						return a
+					}`
+			eval(t, src, big.NewInt(1), []interface{}{opcode.INITSSLOT, []byte{1}}, // sslot for A
+				opcode.PUSH1, opcode.STSFLD0, // store A
+				opcode.LDSFLD0, []interface{}{opcode.CALL, []byte{6}}, opcode.DROP, opcode.RET, // evaluate B and drop
+				opcode.PUSH1, opcode.RET, // Main
+				[]interface{}{opcode.INITSLOT, []byte{0, 1}}, opcode.LDARG0, opcode.RET) // f
+		})
+		t.Run("named, inside multi-specs and multi-vals var declaration", func(t *testing.T) {
+			src := `package foo
+					var (
+						Unused = 1
+						Unused1, A, Unused2 = 2, 3 + f(), 4
+					)
+					func Main() int {
+						return 1
+					}
+					func f() int {
+						return 5
+					}`
+			eval(t, src, big.NewInt(1), opcode.PUSH3, []interface{}{opcode.CALL, []byte{7}}, opcode.ADD, opcode.DROP, opcode.RET, // evaluate and drop A
+				opcode.PUSH1, opcode.RET, // Main
+				opcode.PUSH5, opcode.RET) // f
+		})
+		t.Run("unnamed + unused", func(t *testing.T) {
+			src := `package foo
+					var A = 1 // At least one global variable is used, thus, the whole set of package variables will be walked.
+					var B = 2
+					var _ = B + 1 // This variable is unnamed and doesn't contain call, thus its children won't be marked as "used".
+					func Main() int {
+						return A
+					}`
+			eval(t, src, big.NewInt(1), []interface{}{opcode.INITSSLOT, []byte{1}}, //  sslot for A
+				opcode.PUSH1, opcode.STSFLD0, opcode.RET, // store A
+				opcode.LDSFLD0, opcode.RET) // Main
+		})
+		t.Run("mixed value", func(t *testing.T) {
+			src := `package foo
+					var control int // At least one global variable is used, thus the whole set of package variables will be walked.
+					var B = 2
+					var _ = 1 + f() + B // This variable is unnamed but contains call, thus its children will be marked as "used".
+					func Main() int {
+						return control
+					}
+					func f() int {
+						control = 1
+						return 3
+					}`
+			eval(t, src, big.NewInt(1))
+			checkInstrCount(t, src, 2 /* control + B */, 1, 0, 0)
+		})
+		t.Run("multiple function return values", func(t *testing.T) {
+			src := `package foo
+					var A, B = f()
+					func Main() int {
+						return A
+					}
+					func f() (int, int) {
+						return 3, 4
+					}`
+			eval(t, src, big.NewInt(3), []interface{}{opcode.INITSSLOT, []byte{1}}, // sslot for A
+				[]interface{}{opcode.CALL, []byte{7}}, opcode.STSFLD0, opcode.DROP, opcode.RET, // evaluate and store A, drop B
+				opcode.LDSFLD0, opcode.RET, // Main
+				opcode.PUSH4, opcode.PUSH3, opcode.RET) // f
+		})
+		t.Run("constant in declaration", func(t *testing.T) {
+			src := `package foo
+					const A = 5
+					var Unused = 1 + A
+					func Main() int {
+						return 1
+					}`
+			prog := eval(t, src, big.NewInt(1))
+			require.Equal(t, 2, len(prog)) // Main
+		})
+		t.Run("mixed expression", func(t *testing.T) {
+			src := `package foo
+					type CustomInt struct {
+						Int int
+					}
+					var A = CustomInt{Int: 2}
+					var B = f(3) + A.f(1)
+					func Main() int {
+						return 1
+					}
+					func f(a int) int {
+						return a
+					}
+					func (i CustomInt) f(a int) int {	// has the same name as f
+						return i.Int + a
+					}`
+			eval(t, src, big.NewInt(1))
+			checkInstrCount(t, src, 1 /* A */, 2, 2, 0)
+		})
+	})
+	t.Run("mixed nested expressions", func(t *testing.T) {
+		src := `package foo
+				type CustomInt struct {	Int int}	// has the same field name as Int variable, important for test
+				var A = CustomInt{Int: 2}
+				var B = f(A.Int)
+				var Unused = 4
+				var Int = 5		// unused and MUST NOT be treated as "used"
+				var C = CustomInt{Int: Unused}.Int + f(1) 	// uses Unused => Unused should be marked as "used"
+				func Main() int {
+					return 1
+				}
+				func f(a int) int {
+					return a
+				}
+				func (i CustomInt) f(a int) int {	// has the same name as f
+					return i.Int + a
+				}`
+		eval(t, src, big.NewInt(1))
+	})
+	t.Run("composite literal", func(t *testing.T) {
+		src := `package foo
+				var A = 2
+				var B = []int{1, A, 3}[1]
+				var C = f(1) + B
+				func Main() int {
+					return 1
+				}
+				func f(a int) int {
+					return a
+				}`
+		eval(t, src, big.NewInt(1), []interface{}{opcode.INITSSLOT, []byte{2}}, // sslot for A, B
+			opcode.PUSH2, opcode.STSFLD0, // store A
+			opcode.PUSH3, opcode.LDSFLD0, opcode.PUSH1, opcode.PUSH3, opcode.PACK, opcode.PUSH1, opcode.PICKITEM, opcode.STSFLD1, // evaluate B
+			opcode.PUSH1, []interface{}{opcode.CALL, []byte{8}}, opcode.LDSFLD1, opcode.ADD, opcode.DROP, opcode.RET, // evalute C and drop
+			opcode.PUSH1, opcode.RET, // Main
+			[]interface{}{opcode.INITSLOT, []byte{0, 1}}, opcode.LDARG0, opcode.RET) // f
+	})
+	t.Run("index expression", func(t *testing.T) {
+		src := `package foo
+				var Unused = 2
+				var A = f(1) + []int{1, 2, 3}[Unused] // index expression
+				func Main() int {
+					return 1
+				}
+				func f(a int) int {
+					return a
+				}`
+		eval(t, src, big.NewInt(1), []interface{}{opcode.INITSSLOT, []byte{1}}, // sslot for Unused
+			opcode.PUSH2, opcode.STSFLD0, // store Unused
+			opcode.PUSH1, []interface{}{opcode.CALL, []byte{14}}, // call f(1)
+			opcode.PUSH3, opcode.PUSH2, opcode.PUSH1, opcode.PUSH3, opcode.PACK, opcode.LDSFLD0, opcode.PICKITEM, // eval index expression
+			opcode.ADD, opcode.DROP, opcode.RET, // eval and drop A
+			opcode.PUSH1, opcode.RET, // Main
+			[]interface{}{opcode.INITSLOT, []byte{0, 1}}, opcode.LDARG0, opcode.RET) // f(a)
+	})
+	t.Run("used via nested function calls", func(t *testing.T) {
+		src := `package foo
+				var A = 1
+				func Main() int {
+					return f()
+				}
+				func f() int {
+					return g()
+				}
+				func g() int {
+					return A
+				}`
+		eval(t, src, big.NewInt(1), []interface{}{opcode.INITSSLOT, []byte{1}}, // sslot for A
+			opcode.PUSH1, opcode.STSFLD0, opcode.RET, // store A
+			[]interface{}{opcode.CALL, []byte{3}}, opcode.RET, // Main
+			[]interface{}{opcode.CALL, []byte{3}}, opcode.RET, // f
+			opcode.LDSFLD0, opcode.RET) // g
+	})
+	t.Run("struct field name matches global var name", func(t *testing.T) {
+		src := `package foo
+				type CustomStr struct { Int int	}
+				var str = CustomStr{Int: 2}
+				var Int = 5 // Unused and the code must not be emitted.
+				func Main() int {
+					return str.Int
+				}`
+		eval(t, src, big.NewInt(2), []interface{}{opcode.INITSSLOT, []byte{1}}, // sslot for str
+			opcode.PUSH2, opcode.PUSH1, opcode.PACKSTRUCT, opcode.STSFLD0, opcode.RET, // store str
+			opcode.LDSFLD0, opcode.PUSH0, opcode.PICKITEM, opcode.RET) // Main
+	})
+	t.Run("var as a struct field initializer", func(t *testing.T) {
+		src := `package foo
+				type CustomStr struct { Int int	}
+				var A = 5
+				var Int = 6 // Unused
+				func Main() int {
+					return CustomStr{Int: A}.Int
+				}`
+		eval(t, src, big.NewInt(5))
+	})
+	t.Run("argument of globally called function", func(t *testing.T) {
+		src := `package foo
+				var Unused = 5
+				var control int
+				var _, A = f(Unused)
+				func Main() int {
+					return control
+				}
+				func f(int) (int, int) {
+					control = 5
+					return 1, 2
+				}`
+		eval(t, src, big.NewInt(5))
+	})
+	t.Run("argument of locally called function", func(t *testing.T) {
+		src := `package foo
+				var Unused = 5
+				func Main() int {
+					var _, a = f(Unused)
+					return a
+				}
+				func f(i int) (int, int) {
+					return i, i
+				}`
+		eval(t, src, big.NewInt(5))
+	})
+	t.Run("used in globally called defer", func(t *testing.T) {
+		src := `package foo
+				var control1, control2 int
+				var Unused = 5
+				var _ = f()
+				func Main() int {
+					return control1 + control2
+				}
+				func f() int {
+					control1 = 1
+					defer func(){
+						control2 = Unused
+					}()
+					return 2
+				}`
+		eval(t, src, big.NewInt(6))
+	})
+	t.Run("used in locally called defer", func(t *testing.T) {
+		src := `package foo
+				var control1, control2 int
+				var Unused = 5
+				func Main() int {
+					_ = f()
+					return control1 + control2
+				}
+				func f() int {
+					control1 = 1
+					defer func(){
+						control2 = Unused
+					}()
+					return 2
+				}`
+		eval(t, src, big.NewInt(6))
+	})
+	t.Run("imported", func(t *testing.T) {
+		t.Run("init by func call", func(t *testing.T) {
+			src := `package foo
+					import "github.com/nspcc-dev/neo-go/pkg/compiler/testdata/globalvar"
+					func Main() int {
+						return globalvar.Default
+					}`
+			eval(t, src, big.NewInt(0))
+			checkInstrCount(t, src, 1 /* Default */, 1 /* f */, 0, 0)
+		})
+		t.Run("nested var call", func(t *testing.T) {
+			src := `package foo
+					import "github.com/nspcc-dev/neo-go/pkg/compiler/testdata/globalvar/nested1"
+					func Main() int {
+						return nested1.C
+					}`
+			eval(t, src, big.NewInt(81))
+			checkInstrCount(t, src, 6 /* dependant vars of nested1.C */, 3, 1, 1)
+		})
+		t.Run("nested func call", func(t *testing.T) {
+			src := `package foo
+					import "github.com/nspcc-dev/neo-go/pkg/compiler/testdata/globalvar/funccall"
+					func Main() int {
+						return funccall.F()
+					}`
+			eval(t, src, big.NewInt(56))
+			checkInstrCount(t, src, 2 /* nested2.Argument + nested1.Argument */, -1, -1, -1)
+		})
+		t.Run("nested method call", func(t *testing.T) {
+			src := `package foo
+					import "github.com/nspcc-dev/neo-go/pkg/compiler/testdata/globalvar/funccall"
+					func Main() int {
+						return funccall.GetAge()
+					}`
+			eval(t, src, big.NewInt(24))
+			checkInstrCount(t, src, 3, /* nested3.Anna + nested2.Argument + nested3.Argument */
+				5, /* funccall.GetAge() + Anna.GetAge() + nested1.f + nested1.f + nested2.f */
+				2 /* nested1.f + nested2.f */, 0)
+		})
+	})
 }
 
 func TestChangeGlobal(t *testing.T) {
-	src := `package foo
-	var a int
-	func Main() int {
-		setLocal()
-		set42()
-		setLocal()
-		return a
-	}
-	func set42() { a = 42 }
-	func setLocal() { a := 10; _ = a }`
-
-	eval(t, src, big.NewInt(42))
+	t.Run("from Main", func(t *testing.T) {
+		src := `package foo
+				var a int
+				func Main() int {
+					setLocal()
+					set42()
+					setLocal()
+					return a
+				}
+				func set42() { a = 42 }
+				func setLocal() { a := 10; _ = a }`
+		eval(t, src, big.NewInt(42))
+	})
+	t.Run("from other global", func(t *testing.T) {
+		t.Skip("see https://github.com/nspcc-dev/neo-go/issues/2661")
+		src := `package foo
+				var A = f()
+				var B int
+				func Main() int {
+					return B
+				}
+				func f() int {
+					B = 3
+					return B
+				}`
+		eval(t, src, big.NewInt(3))
+	})
 }
 
 func TestMultiDeclaration(t *testing.T) {
@@ -293,3 +911,29 @@ func TestUnderscoreVarsDontUseSlots(t *testing.T) {
 	src := buf.String()
 	eval(t, src, big.NewInt(count))
 }
+
+func TestUnderscoreGlobalVarDontEmitCode(t *testing.T) {
+	src := `package foo
+		var _ int
+		var _ = 1
+		var (
+			A = 2
+			_ = A + 3
+			_, B, _ = 4, 5, 6
+			_, C, _ = f(A, B)
+		)
+		var D = 7 // named unused, after global codegen optimisation no code expected
+		func Main() int {
+			return 1
+		}
+		func f(a, b int) (int, int, int) {
+			return 8, 9, 10
+		}`
+	eval(t, src, big.NewInt(1), []interface{}{opcode.INITSSLOT, []byte{2}}, // sslot for A, B
+		opcode.PUSH2, opcode.STSFLD0, // store A
+		opcode.PUSH5, opcode.STSFLD1, // store B
+		opcode.LDSFLD0, opcode.LDSFLD1, opcode.SWAP, []interface{}{opcode.CALL, []byte{8}}, // evaluate f(A,B)
+		opcode.DROP, opcode.DROP, opcode.DROP, opcode.RET, // drop result of f(A,B)
+		opcode.PUSH1, opcode.RET, // Main
+		[]interface{}{opcode.INITSLOT, []byte{0, 2}}, opcode.PUSH10, opcode.PUSH9, opcode.PUSH8, opcode.RET) // f
+}

pkg/compiler/init_test.go

@@ -100,7 +100,7 @@ func TestInitWithNoGlobals(t *testing.T) {
 	func Main() int {
 		return 42
 	}`
-	v, s := vmAndCompileInterop(t, src)
+	v, s, _ := vmAndCompileInterop(t, src)
 	require.NoError(t, v.Run())
 	assertResult(t, v, big.NewInt(42))
 	require.True(t, len(s.events) == 1)

pkg/compiler/inline_test.go

@@ -3,6 +3,7 @@ package compiler_test
 import (
 	"fmt"
 	"math/big"
+	"os"
 	"strings"
 	"testing"
 
@@ -12,8 +13,12 @@ import (
 )
 
 func checkCallCount(t *testing.T, src string, expectedCall, expectedInitSlot, expectedLocalsMain int) {
-	v, sp := vmAndCompileInterop(t, src)
+	checkInstrCount(t, src, -1, expectedCall, expectedInitSlot, expectedLocalsMain)
+}
 
+func checkInstrCount(t *testing.T, src string, expectedSSlotCount, expectedCall, expectedInitSlot, expectedLocalsMain int) {
+	v, sp, _ := vmAndCompileInterop(t, src)
+	v.PrintOps(os.Stdout)
 	mainStart := -1
 	for _, m := range sp.info.Methods {
 		if m.Name.Name == "main" {
@@ -29,6 +34,14 @@ func checkCallCount(t *testing.T, src string, expectedCall, expectedInitSlot, ex
 	for op, param, err := ctx.Next(); ; op, param, err = ctx.Next() {
 		require.NoError(t, err)
 		switch op {
+		case opcode.INITSSLOT:
+			if expectedSSlotCount == -1 {
+				continue
+			}
+			if expectedSSlotCount == 0 {
+				t.Fatalf("no INITSSLOT expected, found at %d with %d cells", ctx.IP(), param[0])
+			}
+			require.Equal(t, expectedSSlotCount, int(param[0]))
 		case opcode.CALL, opcode.CALLL:
 			actualCall++
 		case opcode.INITSLOT:
@@ -41,8 +54,12 @@ func checkCallCount(t *testing.T, src string, expectedCall, expectedInitSlot, ex
 			break
 		}
 	}
-	require.Equal(t, expectedCall, actualCall)
-	require.True(t, expectedInitSlot == actualInitSlot)
+	if expectedCall != -1 {
+		require.Equal(t, expectedCall, actualCall)
+	}
+	if expectedInitSlot != -1 {
+		require.Equal(t, expectedInitSlot, actualInitSlot)
+	}
 }
 
 func TestInline(t *testing.T) {
@@ -55,13 +72,13 @@ func TestInline(t *testing.T) {
 		a int
 		b pair
 	}
-	// local alias
-	func sum(a, b int) int {
-		return 42
-	}
 	var Num = 1
 	func Main() int {
 		%s
+	}
+	// local alias
+	func sum(a, b int) int {
+		return 42
 	}`
 	t.Run("no return", func(t *testing.T) {
 		src := fmt.Sprintf(srcTmpl, `inline.NoArgsNoReturn()

pkg/compiler/syscall_test.go

@@ -193,7 +193,7 @@ func TestNotify(t *testing.T) {
 		runtime.Notify("single")
 	}`
 
-	v, s := vmAndCompileInterop(t, src)
+	v, s, _ := vmAndCompileInterop(t, src)
 	v.Estack().PushVal(11)
 
 	require.NoError(t, v.Run())
@@ -224,7 +224,7 @@ func TestSyscallInGlobalInit(t *testing.T) {
 		func Main() bool {
 			return a
 		}`
-	v, s := vmAndCompileInterop(t, src)
+	v, s, _ := vmAndCompileInterop(t, src)
 	s.interops[interopnames.ToID([]byte(interopnames.SystemRuntimeCheckWitness))] = func(v *vm.VM) error {
 		s := v.Estack().Pop().Value().([]byte)
 		require.Equal(t, "5T", string(s))
@@ -479,7 +479,7 @@ func TestInteropTypesComparison(t *testing.T) {
 			b := struct{}{}
 			return a.Equals(b)
 		}`
-				vm, _ := vmAndCompileInterop(t, src)
+				vm, _, _ := vmAndCompileInterop(t, src)
 				err := vm.Run()
 				require.Error(t, err)
 				require.True(t, strings.Contains(err.Error(), "invalid conversion: Struct/ByteString"), err)

pkg/compiler/testdata/foo/foo.go

@@ -5,7 +5,7 @@ func NewBar() int {
 	return 10
 }
 
-// Dummy is dummy constant.
+// Dummy is dummy variable.
 var Dummy = 1
 
 // Foo is a type.

pkg/compiler/testdata/globalvar/funccall/main.go

@@ -0,0 +1,18 @@
+package funccall
+
+import (
+	"github.com/nspcc-dev/neo-go/pkg/compiler/testdata/globalvar/nested1"
+	"github.com/nspcc-dev/neo-go/pkg/compiler/testdata/globalvar/nested2"
+	alias "github.com/nspcc-dev/neo-go/pkg/compiler/testdata/globalvar/nested3"
+)
+
+// F should be called from the main package to check usage analyzer against
+// nested constructions handling.
+func F() int {
+	return nested1.F(nested2.Argument + alias.Argument)
+}
+
+// GetAge calls method on the global struct.
+func GetAge() int {
+	return alias.Anna.GetAge()
+}

pkg/compiler/testdata/globalvar/main.go

@@ -0,0 +1,14 @@
+package globalvar
+
+// Unused shouldn't produce any initialization code if it's not used anywhere.
+var Unused = 3
+
+// Default is initialized by default value.
+var Default int
+
+// A initialized by function call, thus the initialization code should always be emitted.
+var A = f()
+
+func f() int {
+	return 5
+}

pkg/compiler/testdata/globalvar/nested1/main.go

@@ -0,0 +1,29 @@
+package nested1
+
+import (
+	"github.com/nspcc-dev/neo-go/pkg/compiler/testdata/globalvar/nested2"
+	alias "github.com/nspcc-dev/neo-go/pkg/compiler/testdata/globalvar/nested3"
+)
+
+// Unused shouldn't produce any code if unused.
+var Unused = 11
+
+// A should produce call to f and should not be DROPped if C is used. It uses
+// aliased package var as an argument to check analizator.
+var A = f(alias.Argument)
+
+// B should produce call to f and be DROPped if unused. It uses foreign package var as an argument
+// to check analizator.
+var B = f(nested2.Argument)
+
+// C shouldn't produce any code if unused. It uses
+var C = A + nested2.A + nested2.Unique
+
+func f(i int) int {
+	return i
+}
+
+// F is used for nested calls check.
+func F(i int) int {
+	return i
+}

pkg/compiler/testdata/globalvar/nested2/main.go

@@ -0,0 +1,20 @@
+package nested2
+
+// Unused shouldn't produce any code if unused.
+var Unused = 21
+
+// Argument is an argument used from external package to call nested1.f.
+var Argument = 22
+
+// A has the same name as nested1.A.
+var A = 23
+
+// B should produce call to f and be DROPped if unused.
+var B = f()
+
+// Unique has unique name.
+var Unique = 24
+
+func f() int {
+	return 25
+}

pkg/compiler/testdata/globalvar/nested3/main.go

@@ -0,0 +1,18 @@
+package nested3
+
+// Argument is used as a function argument.
+var Argument = 34
+
+// Anna is used to check struct-related usage analyzer logic (calls to methods
+// and fields).
+var Anna = Person{Age: 24}
+
+// Person is an auxiliary structure containing simple field.
+type Person struct {
+	Age int
+}
+
+// GetAge is used to check method calls inside usage analyzer.
+func (p Person) GetAge() int {
+	return p.Age
+}

pkg/compiler/vardecl_test.go

@@ -3,6 +3,8 @@ package compiler_test
 import (
 	"math/big"
 	"testing"
+
+	"github.com/nspcc-dev/neo-go/pkg/vm/opcode"
 )
 
 func TestGenDeclWithMultiRet(t *testing.T) {
@@ -29,3 +31,46 @@ func TestGenDeclWithMultiRet(t *testing.T) {
 		eval(t, src, big.NewInt(3))
 	})
 }
+
+func TestUnderscoreLocalVarDontEmitCode(t *testing.T) {
+	src := `package foo
+		type Foo struct { Int int }
+		func Main() int {
+			var _ int
+			var _ = 1
+			var (
+				A = 2
+				_ = A + 3
+				_, B, _ = 4, 5, 6
+				_, _, _ = f(A, B) // unused, but has function call, so the code is expected
+				_, C, _ = f(A, B)
+			)
+			var D = 7 // unused but named, so the code is expected
+			_ = D
+			var _ = Foo{ Int: 5 }
+			var fo = Foo{ Int: 3 }
+			var _ = 1 + A + fo.Int
+			var _ = fo.GetInt()	// unused, but has method call, so the code is expected
+			return C
+		}
+		func f(a, b int) (int, int, int) {
+			return 8, 9, 10
+		}
+		func (fo Foo) GetInt() int {
+			return fo.Int
+		}`
+	eval(t, src, big.NewInt(9), []interface{}{opcode.INITSLOT, []byte{5, 0}}, // local slot for A, B, C, D, fo
+		opcode.PUSH2, opcode.STLOC0, // store A
+		opcode.PUSH5, opcode.STLOC1, // store B
+		opcode.LDLOC0, opcode.LDLOC1, opcode.SWAP, []interface{}{opcode.CALL, []byte{27}}, // evaluate f() first time
+		opcode.DROP, opcode.DROP, opcode.DROP, // drop all values from f
+		opcode.LDLOC0, opcode.LDLOC1, opcode.SWAP, []interface{}{opcode.CALL, []byte{19}}, // evaluate f() second time
+		opcode.DROP, opcode.STLOC2, opcode.DROP, // store C
+		opcode.PUSH7, opcode.STLOC3, // store D
+		opcode.LDLOC3, opcode.DROP, // empty assignment
+		opcode.PUSH3, opcode.PUSH1, opcode.PACKSTRUCT, opcode.STLOC4, // fo decl
+		opcode.LDLOC4, []interface{}{opcode.CALL, []byte{12}}, opcode.DROP, // fo.GetInt()
+		opcode.LDLOC2, opcode.RET, // return C
+		[]interface{}{opcode.INITSLOT, []byte{0, 2}}, opcode.PUSH10, opcode.PUSH9, opcode.PUSH8, opcode.RET, // f
+		[]interface{}{opcode.INITSLOT, []byte{0, 1}}, opcode.LDARG0, opcode.PUSH0, opcode.PICKITEM, opcode.RET) // (fo Foo) GetInt() int
+}

pkg/compiler/verify_test.go

@@ -18,7 +18,7 @@ func TestVerifyGood(t *testing.T) {
 	pub, sig := signMessage(t, msg)
 	src := getVerifyProg(pub, sig)
 
-	v, p := vmAndCompileInterop(t, src)
+	v, p, _ := vmAndCompileInterop(t, src)
 	p.interops[interopnames.ToID([]byte(interopnames.SystemCryptoCheckSig))] = func(v *vm.VM) error {
 		assert.Equal(t, pub, v.Estack().Pop().Bytes())
 		assert.Equal(t, sig, v.Estack().Pop().Bytes())

pkg/compiler/vm_test.go

@@ -9,9 +9,12 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/compiler"
 	"github.com/nspcc-dev/neo-go/pkg/core/interop/interopnames"
 	"github.com/nspcc-dev/neo-go/pkg/core/state"
+	"github.com/nspcc-dev/neo-go/pkg/io"
 	"github.com/nspcc-dev/neo-go/pkg/smartcontract/callflag"
 	"github.com/nspcc-dev/neo-go/pkg/smartcontract/manifest"
 	"github.com/nspcc-dev/neo-go/pkg/vm"
+	"github.com/nspcc-dev/neo-go/pkg/vm/emit"
+	"github.com/nspcc-dev/neo-go/pkg/vm/opcode"
 	"github.com/nspcc-dev/neo-go/pkg/vm/stackitem"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -32,9 +35,25 @@ func runTestCases(t *testing.T, tcases []testCase) {
 	}
 }
 
-func eval(t *testing.T, src string, result interface{}) {
-	vm, _ := vmAndCompileInterop(t, src)
+func eval(t *testing.T, src string, result interface{}, expectedOps ...interface{}) []byte {
+	vm, _, script := vmAndCompileInterop(t, src)
+	if len(expectedOps) != 0 {
+		expected := io.NewBufBinWriter()
+		for _, op := range expectedOps {
+			switch typ := op.(type) {
+			case opcode.Opcode:
+				emit.Opcodes(expected.BinWriter, typ)
+			case []interface{}:
+				emit.Instruction(expected.BinWriter, typ[0].(opcode.Opcode), typ[1].([]byte))
+			default:
+				t.Fatalf("unexpected evaluation operation: %v", typ)
+			}
+		}
+
+		require.Equal(t, expected.Bytes(), script)
+	}
 	runAndCheck(t, vm, result)
+	return script
 }
 
 func runAndCheck(t *testing.T, v *vm.VM, result interface{}) {
@@ -61,11 +80,11 @@ func assertResult(t *testing.T, vm *vm.VM, result interface{}) {
 }
 
 func vmAndCompile(t *testing.T, src string) *vm.VM {
-	v, _ := vmAndCompileInterop(t, src)
+	v, _, _ := vmAndCompileInterop(t, src)
 	return v
 }
 
-func vmAndCompileInterop(t *testing.T, src string) (*vm.VM, *storagePlugin) {
+func vmAndCompileInterop(t *testing.T, src string) (*vm.VM, *storagePlugin, []byte) {
 	vm := vm.New()
 
 	storePlugin := newStoragePlugin()
@@ -77,7 +96,7 @@ func vmAndCompileInterop(t *testing.T, src string) (*vm.VM, *storagePlugin) {
 
 	storePlugin.info = di
 	invokeMethod(t, testMainIdent, b.Script, vm, di)
-	return vm, storePlugin
+	return vm, storePlugin, b.Script
 }
 
 func invokeMethod(t *testing.T, method string, script []byte, v *vm.VM, di *compiler.DebugInfo) {