[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>

diff --git a/src/cmd/compile/internal/ssa/gen/AMD64.rules b/src/cmd/compile/internal/ssa/gen/AMD64.rules
index 9967c7b0305ea76a2a98b4f9ee54f9ddac6503fa..ee9ccfb41c64cbcc03cddb89af6bcbee34b40b29 100644 (file)
--- a/src/cmd/compile/internal/ssa/gen/AMD64.rules
+++ b/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)])
 

diff --git a/src/cmd/compile/internal/ssa/rewriteAMD64.go b/src/cmd/compile/internal/ssa/rewriteAMD64.go
index 20eab05e9ca2ab02df8494eaa1c2c6b98e7bc347..72ed1eb62c8692514e44862c9184ab352feff262 100644 (file)
--- a/src/cmd/compile/internal/ssa/rewriteAMD64.go
+++ b/src/cmd/compile/internal/ssa/rewriteAMD64.go
@@ -25454,7 +25454,7 @@ func rewriteValueAMD64_OpAMD64SHLLconst(v *Value) bool {
                return true
        }
        // match: (SHLLconst [d] (MOVLconst [c]))
-       // result: (MOVLconst [int64(int32(c)) << uint64(d)])
+       // result: (MOVLconst [int64(int32(c) << uint64(d))])
        for {
                d := v.AuxInt
                if v_0.Op != OpAMD64MOVLconst {
@@ -25462,7 +25462,7 @@ func rewriteValueAMD64_OpAMD64SHLLconst(v *Value) bool {
                }
                c := v_0.AuxInt
                v.reset(OpAMD64MOVLconst)
-               v.AuxInt = int64(int32(c)) << uint64(d)
+               v.AuxInt = int64(int32(c) << uint64(d))
                return true
        }
        return false

diff --git a/test/fixedbugs/issue41711.go b/test/fixedbugs/issue41711.go
new file mode 100644 (file)
index 0000000..4e09440
--- /dev/null
+++ b/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
+}