cmd/compile: emit sensible go_asm.h consts for big ints

Currently, the compiler will emit any const that doesn't fit in an
int64 to go_asm.h like

    #define const_stackPreempt constant.intVal{val:(*big.Int)(0xc000c06c40)}

This happens because dumpasmhdr formats the constant.Value using the
verb "%#v". Since constant.Value doesn't implement the GoString()
method, this just prints the Go-syntax representation of the value.
This happens to work for small integer constants, which go/constant
represents directly as an int64, but not for integer constants that
don't fit in an int64, which go/constant represents as a big.Int.

Make these constants usable by changing the formatting verb to "%v",
which will call the String() method, giving a reasonable result in all
cases.

Change-Id: I365eeb88c8acfc43ff377cc873432269bde3f541
Reviewed-on: https://go-review.googlesource.com/c/go/+/359954
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/gc/export.go

@@ -31,7 +31,7 @@ func dumpasmhdr() {
 			if t == constant.Float || t == constant.Complex {
 				break
 			}
-			fmt.Fprintf(b, "#define const_%s %#v\n", n.Sym().Name, n.Val())
+			fmt.Fprintf(b, "#define const_%s %v\n", n.Sym().Name, n.Val())
 
 		case ir.OTYPE:
 			t := n.Type()

test/asmhdr.dir/main.go

@@ -0,0 +1,66 @@
+// Copyright 2021 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 main
+
+import "unsafe"
+
+const (
+	smallInt = 42
+
+	// For bigInt, we use a value that's too big for an int64, but still
+	// fits in uint64. go/constant uses a different representation for
+	// values larger than int64, but the cmd/asm parser can't parse
+	// anything bigger than a uint64.
+	bigInt = 0xffffffffffffffff
+
+	stringVal = "test"
+)
+
+var (
+	smallIntAsm int64
+	bigIntAsm   uint64
+	stringAsm   [len(stringVal)]byte
+)
+
+type typ struct {
+	a uint64
+	b [100]uint8
+	c uint8
+}
+
+var (
+	typSize uint64
+
+	typA, typB, typC uint64
+)
+
+func main() {
+	if smallInt != smallIntAsm {
+		println("smallInt", smallInt, "!=", smallIntAsm)
+	}
+	if bigInt != bigIntAsm {
+		println("bigInt", uint64(bigInt), "!=", bigIntAsm)
+	}
+	if stringVal != string(stringAsm[:]) {
+		println("stringVal", stringVal, "!=", string(stringAsm[:]))
+	}
+
+	// We also include boolean consts in go_asm.h, but they're
+	// defined to be "true" or "false", and it's not clear how to
+	// use that in assembly.
+
+	if want := unsafe.Sizeof(typ{}); want != uintptr(typSize) {
+		println("typSize", want, "!=", typSize)
+	}
+	if want := unsafe.Offsetof(typ{}.a); want != uintptr(typA) {
+		println("typA", want, "!=", typA)
+	}
+	if want := unsafe.Offsetof(typ{}.b); want != uintptr(typB) {
+		println("typB", want, "!=", typB)
+	}
+	if want := unsafe.Offsetof(typ{}.c); want != uintptr(typC) {
+		println("typC", want, "!=", typC)
+	}
+}

test/asmhdr.dir/main.s

@@ -0,0 +1,27 @@
+// Copyright 2021 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.
+
+#include "go_asm.h"
+#define RODATA	8
+
+DATA ·smallIntAsm(SB)/8, $const_smallInt
+GLOBL ·smallIntAsm(SB),RODATA,$8
+
+DATA ·bigIntAsm(SB)/8, $const_bigInt
+GLOBL ·bigIntAsm(SB),RODATA,$8
+
+DATA ·stringAsm(SB)/4, $const_stringVal
+GLOBL ·stringAsm(SB),RODATA,$4
+
+DATA ·typSize(SB)/8, $typ__size
+GLOBL ·typSize(SB),RODATA,$8
+
+DATA ·typA(SB)/8, $typ_a
+GLOBL ·typA(SB),RODATA,$8
+
+DATA ·typB(SB)/8, $typ_b
+GLOBL ·typB(SB),RODATA,$8
+
+DATA ·typC(SB)/8, $typ_c
+GLOBL ·typC(SB),RODATA,$8

test/asmhdr.go

@@ -0,0 +1,9 @@
+// buildrundir
+
+// Copyright 2021 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.
+
+// Test the -asmhdr output of the compiler.
+
+package ignored