cmd/compile/internal/gc: OMUL should be evaluated when using soft-float

When using soft-float, OMUL might be rewritten to function call
so we should ensure it was evaluated first.

Fixes #28688

Change-Id: I30b87501782fff62d35151f394a1c22b0d490c6c
Reviewed-on: https://go-review.googlesource.com/c/148837
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -1006,7 +1006,7 @@ func calcHasCall(n *Node) bool {
 
 	// When using soft-float, these ops might be rewritten to function calls
 	// so we ensure they are evaluated first.
-	case OADD, OSUB, OMINUS:
+	case OADD, OSUB, OMINUS, OMUL:
 		if thearch.SoftFloat && (isFloat[n.Type.Etype] || isComplex[n.Type.Etype]) {
 			return true
 		}

test/fixedbugs/issue28688.go

@@ -0,0 +1,31 @@
+// run -gcflags=-d=softfloat
+
+// Copyright 2018 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"
+)
+
+// When using soft-float, OMUL might be rewritten to function
+// call so we should ensure it was evaluated first. Stack frame
+// setup for "test" function call should happen after call to runtime.fmul32
+
+var x int32 = 1
+
+func main() {
+	var y float32 = 1.0
+	test(x, y*y)
+}
+
+//go:noinline
+func test(id int32, a float32) {
+
+	if id != x {
+		fmt.Printf("got: %d, want: %d\n", id, x)
+		panic("FAIL")
+	}
+}