cmd/cgo: use C exact-width integer types to represent Go types

The exact-width integer types are required to use two’s complement
representation and may not have padding bits, cf. §7.20.1.1/1 in the C11
standard or https://en.cppreference.com/w/c/types/integer.  This ensures that
they have the same domain and representation as the corresponding Go types.

Fixes #29878

misc/cgo/test/cgo_test.go

@@ -56,6 +56,7 @@ func Test25143(t *testing.T)                 { test25143(t) }
 func Test26066(t *testing.T)                 { test26066(t) }
 func Test27660(t *testing.T)                 { test27660(t) }
 func Test28896(t *testing.T)                 { test28896(t) }
+func Test29878(t *testing.T)                 { test29878(t) }
 func Test30065(t *testing.T)                 { test30065(t) }
 func TestAlign(t *testing.T)                 { testAlign(t) }
 func TestAtol(t *testing.T)                  { testAtol(t) }

misc/cgo/test/issue29878.go

@@ -0,0 +1,20 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cgotest
+
+// #include <stdint.h>
+// uint64_t issue29878exported(int8_t);  // prototype must match
+// int16_t issue29878function(uint32_t arg) { return issue29878exported(arg); }
+import "C"
+
+import "testing"
+
+func test29878(t *testing.T) {
+	const arg uint32 = 123                    // fits into all integer types
+	var ret int16 = C.issue29878function(arg) // no conversions needed
+	if int64(ret) != int64(arg) {
+		t.Errorf("return value unexpected: got %d, want %d", ret, arg)
+	}
+}

misc/cgo/test/issue29878export.go

@@ -0,0 +1,12 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cgotest
+
+import "C"
+
+//export issue29878exported
+func issue29878exported(arg int8) uint64 {
+	return uint64(arg)
+}

src/cmd/cgo/doc.go

@@ -148,6 +148,8 @@ C.long, C.ulong (unsigned long), C.longlong (long long),
 C.ulonglong (unsigned long long), C.float, C.double,
 C.complexfloat (complex float), and C.complexdouble (complex double).
 The C type void* is represented by Go's unsafe.Pointer.
+The C sized integer types (int8_t, uint8_t, …) are represented by their Go
+counterparts (int8, uint8, …).
 The C types __int128_t and __uint128_t are represented by [16]byte.
 
 A few special C types which would normally be represented by a pointer
@@ -296,7 +298,7 @@ Go functions can be exported for use by C code in the following way:
 
 They will be available in the C code as:
 
-	extern int64 MyFunction(int arg1, int arg2, GoString arg3);
+	extern int64_t MyFunction(int arg1, int arg2, GoString arg3);
 	extern struct MyFunction2_return MyFunction2(int arg1, int arg2, GoString arg3);
 
 found in the _cgo_export.h generated header, after any preambles

src/cmd/cgo/gcc.go

@@ -23,6 +23,7 @@ import (
 	"internal/xcoff"
 	"math"
 	"os"
+	"regexp"
 	"strconv"
 	"strings"
 	"unicode"
@@ -2046,6 +2047,8 @@ type typeConv struct {
 
 	ptrSize int64
 	intSize int64
+
+	exactWidthIntegerTypes map[string]*Type
 }
 
 var tagGen int
@@ -2088,6 +2091,21 @@ func (c *typeConv) Init(ptrSize, intSize int64) {
 	} else {
 		c.goVoidPtr = c.Ident("unsafe.Pointer")
 	}
+
+	c.exactWidthIntegerTypes = make(map[string]*Type)
+	for _, t := range []ast.Expr{
+		c.int8, c.int16, c.int32, c.int64,
+		c.uint8, c.uint16, c.uint32, c.uint64,
+	} {
+		name := t.(*ast.Ident).Name
+		u := new(Type)
+		*u = *goTypes[name]
+		if u.Align > ptrSize {
+			u.Align = ptrSize
+		}
+		u.Go = t
+		c.exactWidthIntegerTypes[name] = u
+	}
 }
 
 // base strips away qualifiers and typedefs to get the underlying type
@@ -2459,6 +2477,24 @@ func (c *typeConv) Type(dtype dwarf.Type, pos token.Pos) *Type {
 			t.Align = c.ptrSize
 			break
 		}
+		// Exact-width integer types.  These are always compatible with
+		// the corresponding Go types since the C standard requires
+		// them to have no padding bit and use the two’s complement
+		// representation.
+		if exactWidthIntegerType.MatchString(dt.Name) {
+			sub := c.Type(dt.Type, pos)
+			u := c.exactWidthIntegerTypes[strings.TrimSuffix(dt.Name, "_t")]
+			if sub.Size != u.Size {
+				fatalf("%s: unexpected size: %d vs. %d – %s", lineno(pos), sub.Size, u.Size, dtype)
+			}
+			if sub.Align != u.Align {
+				fatalf("%s: unexpected alignment: %d vs. %d – %s", lineno(pos), sub.Align, u.Align, dtype)
+			}
+			t.Size = u.Size
+			t.Align = u.Align
+			t.Go = u.Go
+			break
+		}
 		name := c.Ident("_Ctype_" + dt.Name)
 		goIdent[name.Name] = name
 		sub := c.Type(dt.Type, pos)
@@ -2594,6 +2630,8 @@ func (c *typeConv) Type(dtype dwarf.Type, pos token.Pos) *Type {
 	return t
 }
 
+var exactWidthIntegerType = regexp.MustCompile(`^u?int(8|16|32|64)_t$`)
+
 // isStructUnionClass reports whether the type described by the Go syntax x
 // is a struct, union, or class with a tag.
 func isStructUnionClass(x ast.Expr) bool {

src/cmd/cgo/out.go

@@ -1361,19 +1361,19 @@ func c(repr string, args ...interface{}) *TypeRepr {
 
 // Map predeclared Go types to Type.
 var goTypes = map[string]*Type{
-	"bool":       {Size: 1, Align: 1, C: c("GoUint8")},
-	"byte":       {Size: 1, Align: 1, C: c("GoUint8")},
+	"bool":       {Size: 1, Align: 1, C: c("uint8_t")},
+	"byte":       {Size: 1, Align: 1, C: c("uint8_t")},
 	"int":        {Size: 0, Align: 0, C: c("GoInt")},
 	"uint":       {Size: 0, Align: 0, C: c("GoUint")},
-	"rune":       {Size: 4, Align: 4, C: c("GoInt32")},
-	"int8":       {Size: 1, Align: 1, C: c("GoInt8")},
-	"uint8":      {Size: 1, Align: 1, C: c("GoUint8")},
-	"int16":      {Size: 2, Align: 2, C: c("GoInt16")},
-	"uint16":     {Size: 2, Align: 2, C: c("GoUint16")},
-	"int32":      {Size: 4, Align: 4, C: c("GoInt32")},
-	"uint32":     {Size: 4, Align: 4, C: c("GoUint32")},
-	"int64":      {Size: 8, Align: 8, C: c("GoInt64")},
-	"uint64":     {Size: 8, Align: 8, C: c("GoUint64")},
+	"rune":       {Size: 4, Align: 4, C: c("int32_t")},
+	"int8":       {Size: 1, Align: 1, C: c("int8_t")},
+	"uint8":      {Size: 1, Align: 1, C: c("uint8_t")},
+	"int16":      {Size: 2, Align: 2, C: c("int16_t")},
+	"uint16":     {Size: 2, Align: 2, C: c("uint16_t")},
+	"int32":      {Size: 4, Align: 4, C: c("int32_t")},
+	"uint32":     {Size: 4, Align: 4, C: c("uint32_t")},
+	"int64":      {Size: 8, Align: 8, C: c("int64_t")},
+	"uint64":     {Size: 8, Align: 8, C: c("uint64_t")},
 	"float32":    {Size: 4, Align: 4, C: c("GoFloat32")},
 	"float64":    {Size: 8, Align: 8, C: c("GoFloat64")},
 	"complex64":  {Size: 8, Align: 4, C: c("GoComplex64")},
@@ -1865,16 +1865,10 @@ const gccExportHeaderProlog = `
 #ifndef GO_CGO_PROLOGUE_H
 #define GO_CGO_PROLOGUE_H
 
-typedef signed char GoInt8;
-typedef unsigned char GoUint8;
-typedef short GoInt16;
-typedef unsigned short GoUint16;
-typedef int GoInt32;
-typedef unsigned int GoUint32;
-typedef long long GoInt64;
-typedef unsigned long long GoUint64;
-typedef GoIntGOINTBITS GoInt;
-typedef GoUintGOINTBITS GoUint;
+#include <stdint.h>
+
+typedef intGOINTBITS_t GoInt;
+typedef uintGOINTBITS_t GoUint;
 typedef __SIZE_TYPE__ GoUintptr;
 typedef float GoFloat32;
 typedef double GoFloat64;