[release-branch.go1.15] cmd/compile: fix left shift constant folding …

…rule

The 32-bit left shift constant folding rule should keep its result
properly sign extended.

Fixes #41720
Fixes #41711

Change-Id: I0fc74444d444274e911952e1725dab0b7737a846
Reviewed-on: https://go-review.googlesource.com/c/go/+/258817
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/cmd/compile/internal/ssa/gen/AMD64.rules

@@ -1454,7 +1454,7 @@
 (XORQ x x) -> (MOVQconst [0])
 (XORL x x) -> (MOVLconst [0])
 
-(SHLLconst [d] (MOVLconst [c])) -> (MOVLconst [int64(int32(c)) << uint64(d)])
+(SHLLconst [d] (MOVLconst [c])) -> (MOVLconst [int64(int32(c) << uint64(d))])
 (SHLQconst [d] (MOVQconst [c])) -> (MOVQconst [c << uint64(d)])
 (SHLQconst [d] (MOVLconst [c])) -> (MOVQconst [int64(int32(c)) << uint64(d)])
 

test/fixedbugs/issue41711.go

@@ -0,0 +1,17 @@
+// compile -d=ssa/check/on
+
+// Copyright 2020 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
+
+func f() uint32 {
+	s := "food"
+	x := uint32(s[0]) + uint32(s[1])<<8 + uint32(s[2])<<16 + uint32(s[3])<<24
+	// x is a constant, but that's not known until lowering.
+	// shifting it by 8 moves the high byte up into the high 32 bits of
+	// a 64-bit word. That word is not properly sign-extended by the faulty
+	// rule, which causes the compiler to fail.
+	return x << 8
+}