Add #cgo noescape directive

cgo/cgo.go

@@ -18,6 +18,7 @@ import (
 	"go/scanner"
 	"go/token"
 	"path/filepath"
+	"sort"
 	"strconv"
 	"strings"
 
@@ -42,6 +43,7 @@ type cgoPackage struct {
 	fset            *token.FileSet
 	tokenFiles      map[string]*token.File
 	definedGlobally map[string]ast.Node
+	noescapingFuncs map[string]*noescapingFunc // #cgo noescape lines
 	anonDecls       map[interface{}]string
 	cflags          []string // CFlags from #cgo lines
 	ldflags         []string // LDFlags from #cgo lines
@@ -80,6 +82,13 @@ type bitfieldInfo struct {
 	endBit   int64 // may be 0 meaning "until the end of the field"
 }
 
+// Information about a #cgo noescape line in the source code.
+type noescapingFunc struct {
+	name string
+	pos  token.Pos
+	used bool // true if used somewhere in the source (for proper error reporting)
+}
+
 // cgoAliases list type aliases between Go and C, for types that are equivalent
 // in both languages. See addTypeAliases.
 var cgoAliases = map[string]string{
@@ -186,6 +195,7 @@ func Process(files []*ast.File, dir, importPath string, fset *token.FileSet, cfl
 		fset:            fset,
 		tokenFiles:      map[string]*token.File{},
 		definedGlobally: map[string]ast.Node{},
+		noescapingFuncs: map[string]*noescapingFunc{},
 		anonDecls:       map[interface{}]string{},
 		visitedFiles:    map[string][]byte{},
 	}
@@ -344,6 +354,22 @@ func Process(files []*ast.File, dir, importPath string, fset *token.FileSet, cfl
 		})
 	}
 
+	// Show an error when a #cgo noescape line isn't used in practice.
+	// This matches upstream Go. I think the goal is to avoid issues with
+	// misspelled function names, which seems very useful.
+	var unusedNoescapeLines []*noescapingFunc
+	for _, value := range p.noescapingFuncs {
+		if !value.used {
+			unusedNoescapeLines = append(unusedNoescapeLines, value)
+		}
+	}
+	sort.SliceStable(unusedNoescapeLines, func(i, j int) bool {
+		return unusedNoescapeLines[i].pos < unusedNoescapeLines[j].pos
+	})
+	for _, value := range unusedNoescapeLines {
+		p.addError(value.pos, fmt.Sprintf("function %#v in #cgo noescape line is not used", value.name))
+	}
+
 	// Print the newly generated in-memory AST, for debugging.
 	//ast.Print(fset, p.generated)
 
@@ -409,6 +435,33 @@ func (p *cgoPackage) parseCGoPreprocessorLines(text string, pos token.Pos) strin
 		}
 		text = text[:lineStart] + string(spaces) + text[lineEnd:]
 
+		allFields := strings.Fields(line[4:])
+		switch allFields[0] {
+		case "noescape":
+			// The code indicates that pointer parameters will not be captured
+			// by the called C function.
+			if len(allFields) < 2 {
+				p.addErrorAfter(pos, text[:lineStart], "missing function name in #cgo noescape line")
+				continue
+			}
+			if len(allFields) > 2 {
+				p.addErrorAfter(pos, text[:lineStart], "multiple function names in #cgo noescape line")
+				continue
+			}
+			name := allFields[1]
+			p.noescapingFuncs[name] = &noescapingFunc{
+				name: name,
+				pos:  pos,
+				used: false,
+			}
+			continue
+		case "nocallback":
+			// We don't do anything special when calling a C function, so there
+			// appears to be no optimization that we can do here.
+			// Accept, but ignore the parameter for compatibility.
+			continue
+		}
+
 		// Get the text before the colon in the #cgo directive.
 		colon := strings.IndexByte(line, ':')
 		if colon < 0 {

cgo/libclang.go

@@ -256,10 +256,18 @@ func (f *cgoFile) createASTNode(name string, c clangCursor) (ast.Node, any) {
 				},
 			},
 		}
+		var doc []string
 		if C.clang_isFunctionTypeVariadic(cursorType) != 0 {
+			doc = append(doc, "//go:variadic")
+		}
+		if _, ok := f.noescapingFuncs[name]; ok {
+			doc = append(doc, "//go:noescape")
+			f.noescapingFuncs[name].used = true
+		}
+		if len(doc) != 0 {
 			decl.Doc.List = append(decl.Doc.List, &ast.Comment{
 				Slash: pos - 1,
-				Text:  "//go:variadic",
+				Text:  strings.Join(doc, "\n"),
 			})
 		}
 		for i := 0; i < numArgs; i++ {

cgo/testdata/errors.go

@@ -10,6 +10,11 @@ typedef struct {
 
 typedef someType noType; // undefined type
 
+// Some invalid noescape lines
+#cgo noescape
+#cgo noescape foo bar
+#cgo noescape unusedFunction
+
 #define SOME_CONST_1 5) // invalid const syntax
 #define SOME_CONST_2 6) // const not used (so no error)
 #define SOME_CONST_3 1234 // const too large for byte

cgo/testdata/errors.out.go

@@ -1,14 +1,17 @@
 // CGo errors:
+//     testdata/errors.go:14:1: missing function name in #cgo noescape line
+//     testdata/errors.go:15:1: multiple function names in #cgo noescape line
 //     testdata/errors.go:4:2: warning: some warning
 //     testdata/errors.go:11:9: error: unknown type name 'someType'
-//     testdata/errors.go:26:5: warning: another warning
-//     testdata/errors.go:13:23: unexpected token ), expected end of expression
-//     testdata/errors.go:21:26: unexpected token ), expected end of expression
-//     testdata/errors.go:16:33: unexpected token ), expected end of expression
-//     testdata/errors.go:17:34: unexpected token ), expected end of expression
+//     testdata/errors.go:31:5: warning: another warning
+//     testdata/errors.go:18:23: unexpected token ), expected end of expression
+//     testdata/errors.go:26:26: unexpected token ), expected end of expression
+//     testdata/errors.go:21:33: unexpected token ), expected end of expression
+//     testdata/errors.go:22:34: unexpected token ), expected end of expression
 //     -: unexpected token INT, expected end of expression
-//     testdata/errors.go:30:35: unexpected number of parameters: expected 2, got 3
-//     testdata/errors.go:31:31: unexpected number of parameters: expected 2, got 1
+//     testdata/errors.go:35:35: unexpected number of parameters: expected 2, got 3
+//     testdata/errors.go:36:31: unexpected number of parameters: expected 2, got 1
+//     testdata/errors.go:3:1: function "unusedFunction" in #cgo noescape line is not used
 
 // Type checking errors after CGo processing:
 //     testdata/errors.go:102: cannot use 2 << 10 (untyped int constant 2048) as C.char value in variable declaration (overflows)

cgo/testdata/symbols.go

@@ -9,6 +9,10 @@ static void staticfunc(int x);
 
 // Global variable signatures.
 extern int someValue;
+
+void notEscapingFunction(int *a);
+
+#cgo noescape notEscapingFunction
 */
 import "C"
 
@@ -18,6 +22,7 @@ func accessFunctions() {
 	C.variadic0()
 	C.variadic2(3, 5)
 	C.staticfunc(3)
+	C.notEscapingFunction(nil)
 }
 
 func accessGlobals() {

cgo/testdata/symbols.out.go

@@ -67,5 +67,10 @@ func C.staticfunc!symbols.go(x C.int)
 
 var C.staticfunc!symbols.go$funcaddr unsafe.Pointer
 
+//export notEscapingFunction
+//go:noescape
+func C.notEscapingFunction(a *C.int)
+
+var C.notEscapingFunction$funcaddr unsafe.Pointer
 //go:extern someValue
 var C.someValue C.int

compiler/calls.go

@@ -19,18 +19,19 @@ const maxFieldsPerParam = 3
 // useful while declaring or defining a function.
 type paramInfo struct {
 	llvmType llvm.Type
-	name     string // name, possibly with suffixes for e.g. struct fields
-	elemSize uint64 // size of pointer element type, or 0 if this isn't a pointer
+	name     string     // name, possibly with suffixes for e.g. struct fields
+	elemSize uint64     // size of pointer element type, or 0 if this isn't a pointer
+	flags    paramFlags // extra flags for this parameter
 }
 
 // paramFlags identifies parameter attributes for flags. Most importantly, it
 // determines which parameters are dereferenceable_or_null and which aren't.
 type paramFlags uint8
 
 const (
-	// Parameter may have the deferenceable_or_null attribute. This attribute
-	// cannot be applied to unsafe.Pointer and to the data pointer of slices.
-	paramIsDeferenceableOrNull = 1 << iota
+	// Whether this is a full or partial Go parameter (int, slice, etc).
+	// The extra context parameter is not a Go parameter.
+	paramIsGoParam = 1 << iota
 )
 
 // createRuntimeCallCommon creates a runtime call. Use createRuntimeCall or
@@ -195,6 +196,7 @@ func (c *compilerContext) getParamInfo(t llvm.Type, name string, goType types.Ty
 	info := paramInfo{
 		llvmType: t,
 		name:     name,
+		flags:    paramIsGoParam,
 	}
 	if goType != nil {
 		switch underlying := goType.Underlying().(type) {

compiler/symbol.go

@@ -33,6 +33,7 @@ type functionInfo struct {
 	exported      bool       // go:export, CGo
 	interrupt     bool       // go:interrupt
 	nobounds      bool       // go:nobounds
+	noescape      bool       // go:noescape
 	variadic      bool       // go:variadic (CGo only)
 	inline        inlineType // go:inline
 }
@@ -127,11 +128,20 @@ func (c *compilerContext) getFunction(fn *ssa.Function) (llvm.Type, llvm.Value)
 	c.addStandardDeclaredAttributes(llvmFn)
 
 	dereferenceableOrNullKind := llvm.AttributeKindID("dereferenceable_or_null")
-	for i, info := range paramInfos {
-		if info.elemSize != 0 {
-			dereferenceableOrNull := c.ctx.CreateEnumAttribute(dereferenceableOrNullKind, info.elemSize)
+	for i, paramInfo := range paramInfos {
+		if paramInfo.elemSize != 0 {
+			dereferenceableOrNull := c.ctx.CreateEnumAttribute(dereferenceableOrNullKind, paramInfo.elemSize)
 			llvmFn.AddAttributeAtIndex(i+1, dereferenceableOrNull)
 		}
+		if info.noescape && paramInfo.flags&paramIsGoParam != 0 && paramInfo.llvmType.TypeKind() == llvm.PointerTypeKind {
+			// Parameters to functions with a //go:noescape parameter should get
+			// the nocapture attribute. However, the context parameter should
+			// not.
+			// (It may be safe to add the nocapture parameter to the context
+			// parameter, but I'd like to stay on the safe side here).
+			nocapture := c.ctx.CreateEnumAttribute(llvm.AttributeKindID("nocapture"), 0)
+			llvmFn.AddAttributeAtIndex(i+1, nocapture)
+		}
 	}
 
 	// Set a number of function or parameter attributes, depending on the
@@ -394,6 +404,13 @@ func (c *compilerContext) parsePragmas(info *functionInfo, f *ssa.Function) {
 			if hasUnsafeImport(f.Pkg.Pkg) {
 				info.nobounds = true
 			}
+		case "//go:noescape":
+			// Don't let pointer parameters escape.
+			// Following the upstream Go implementation, we only do this for
+			// declarations, not definitions.
+			if len(f.Blocks) == 0 {
+				info.noescape = true
+			}
 		case "//go:variadic":
 			// The //go:variadic pragma is emitted by the CGo preprocessing
 			// pass for C variadic functions. This includes both explicit

compiler/testdata/pragma.go

@@ -106,3 +106,12 @@ var undefinedGlobalNotInSection uint32
 //go:align 1024
 //go:section .global_section
 var multipleGlobalPragmas uint32
+
+//go:noescape
+func doesNotEscapeParam(a *int, b []int, c chan int, d *[0]byte)
+
+// The //go:noescape pragma only works on declarations, not definitions.
+//
+//go:noescape
+func stillEscapes(a *int, b []int, c chan int, d *[0]byte) {
+}

compiler/testdata/pragma.ll

@@ -85,6 +85,14 @@ entry:
 
 declare void @main.undefinedFunctionNotInSection(ptr) #1
 
+declare void @main.doesNotEscapeParam(ptr nocapture dereferenceable_or_null(4), ptr nocapture, i32, i32, ptr nocapture dereferenceable_or_null(32), ptr nocapture, ptr) #1
+
+; Function Attrs: nounwind
+define hidden void @main.stillEscapes(ptr dereferenceable_or_null(4) %a, ptr %b.data, i32 %b.len, i32 %b.cap, ptr dereferenceable_or_null(32) %c, ptr %d, ptr %context) unnamed_addr #2 {
+entry:
+  ret void
+}
+
 attributes #0 = { allockind("alloc,zeroed") allocsize(0) "alloc-family"="runtime.alloc" "target-features"="+bulk-memory,+mutable-globals,+nontrapping-fptoint,+sign-ext" }
 attributes #1 = { "target-features"="+bulk-memory,+mutable-globals,+nontrapping-fptoint,+sign-ext" }
 attributes #2 = { nounwind "target-features"="+bulk-memory,+mutable-globals,+nontrapping-fptoint,+sign-ext" }