cmd/compile: don't ICE on unaligned offsets for pointer writes

User code is unlikely to be correct, but don't crash the compiler
when the offset of a pointer in an object is not a multiple of the
pointer size.

Fixes #61187

Change-Id: Ie56bfcb38556c5dd6f702ae4ec1d4534c6acd420
Reviewed-on: https://go-review.googlesource.com/c/go/+/508555
Reviewed-by: Cherry Mui <[email protected]>
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: randall77

src/cmd/compile/internal/ssa/writebarrier.go

@@ -53,7 +53,10 @@ func mightContainHeapPointer(ptr *Value, size int64, mem *Value, zeroes map[ID]Z
 	}
 
 	ptrSize := ptr.Block.Func.Config.PtrSize
-	if off%ptrSize != 0 || size%ptrSize != 0 {
+	if off%ptrSize != 0 {
+		return true // see issue 61187
+	}
+	if size%ptrSize != 0 {
 		ptr.Fatalf("unaligned pointer write")
 	}
 	if off < 0 || off+size > 64*ptrSize {
@@ -130,7 +133,7 @@ func needWBdst(ptr, mem *Value, zeroes map[ID]ZeroRegion) bool {
 	}
 	ptrSize := ptr.Block.Func.Config.PtrSize
 	if off%ptrSize != 0 {
-		ptr.Fatalf("unaligned pointer write")
+		return true // see issue 61187
 	}
 	if off < 0 || off >= 64*ptrSize {
 		// write goes off end of tracked offsets

test/fixedbugs/issue61187.go

@@ -0,0 +1,22 @@
+// compile
+
+// Copyright 2023 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 (
+	"fmt"
+	"reflect"
+	"unsafe"
+)
+
+var slice = []byte{'H', 'e', 'l', 'l', 'o', ','}
+
+func main() {
+	ptr := uintptr(unsafe.Pointer(&slice)) + 100
+	header := (*reflect.SliceHeader)(unsafe.Pointer(ptr))
+	header.Data += 1
+	fmt.Printf("%d %d\n", cap(slice), header.Cap)
+}