[release-branch.go1.11] cmd/compile: fix MIPS SGTconst-with-shift rules

(SGTconst [c] (SRLconst _ [d])) && 0 <= int32(c) && uint32(d) <= 31 && 1<<(32-uint32(d)) <= int32(c) -> (MOVWconst [1]) This rule is problematic. 1<<(32-uint32(d)) <= int32(c) meant to say that it is true if c is greater than the largest possible value of the right shift. But when d==1, 1<<(32-1) is negative and results in the wrong comparison. Rewrite the rules in a more direct way. Updates #29402. Fixes #29442. Change-Id: I5940fc9538d9bc3a4bcae8aa34672867540dc60e Reviewed-on: https://go-review.googlesource.com/c/155798 Run-TryBot: Cherry Zhang <[email protected]> TryBot-Result: Gobot Gobot <[email protected]> Reviewed-by: Keith Randall <[email protected]> (cherry picked from commit 6a64efc) Reviewed-on: https://go-review.googlesource.com/c/155799 Reviewed-by: David Chase <[email protected]>
golang · Jan 4, 2019 · b2c472f · b2c472f
1 parent 61b8817
commit b2c472f
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Showing 5 changed files with 35 additions and 12 deletions.
diff --git a/src/cmd/compile/internal/ssa/gen/MIPS.rules b/src/cmd/compile/internal/ssa/gen/MIPS.rules
@@ -670,8 +670,8 @@
 (SGTUconst [c] (MOVHUreg _)) && 0xffff < uint32(c) -> (MOVWconst [1])
 (SGTconst [c] (ANDconst [m] _)) && 0 <= int32(m) && int32(m) < int32(c) -> (MOVWconst [1])
 (SGTUconst [c] (ANDconst [m] _)) && uint32(m) < uint32(c) -> (MOVWconst [1])
-(SGTconst [c] (SRLconst _ [d])) && 0 <= int32(c) && uint32(d) <= 31 && 1<<(32-uint32(d)) <= int32(c) -> (MOVWconst [1])
-(SGTUconst [c] (SRLconst _ [d])) && uint32(d) <= 31 && 1<<(32-uint32(d)) <= uint32(c) -> (MOVWconst [1])
+(SGTconst [c] (SRLconst _ [d])) && 0 <= int32(c) && uint32(d) <= 31 && 0xffffffff>>uint32(d) < uint32(c) -> (MOVWconst [1])
+(SGTUconst [c] (SRLconst _ [d])) && uint32(d) <= 31 && 0xffffffff>>uint32(d) < uint32(c) -> (MOVWconst [1])
 
 // absorb constants into branches
 (EQ  (MOVWconst [0]) yes no) -> (First nil yes no)

diff --git a/src/cmd/compile/internal/ssa/gen/MIPS64.rules b/src/cmd/compile/internal/ssa/gen/MIPS64.rules
@@ -667,8 +667,8 @@
 (SGTconst [c] (MOVWUreg _)) && c < 0 -> (MOVVconst [0])
 (SGTconst [c] (ANDconst [m] _)) && 0 <= m && m < c -> (MOVVconst [1])
 (SGTUconst [c] (ANDconst [m] _)) && uint64(m) < uint64(c) -> (MOVVconst [1])
-(SGTconst [c] (SRLVconst _ [d])) && 0 <= c && 0 < d && d <= 63 && 1<<uint64(64-d) <= c -> (MOVVconst [1])
-(SGTUconst [c] (SRLVconst _ [d])) && 0 < d && d <= 63 && 1<<uint64(64-d) <= uint64(c) -> (MOVVconst [1])
+(SGTconst [c] (SRLVconst _ [d])) && 0 <= c && 0 < d && d <= 63 && 0xffffffffffffffff>>uint64(d) < uint64(c) -> (MOVVconst [1])
+(SGTUconst [c] (SRLVconst _ [d])) && 0 < d && d <= 63 && 0xffffffffffffffff>>uint64(d) < uint64(c) -> (MOVVconst [1])
 
 // absorb constants into branches
 (EQ  (MOVVconst [0]) yes no) -> (First nil yes no)

diff --git a/src/cmd/compile/internal/ssa/rewriteMIPS.go b/src/cmd/compile/internal/ssa/rewriteMIPS.go
diff --git a/src/cmd/compile/internal/ssa/rewriteMIPS64.go b/src/cmd/compile/internal/ssa/rewriteMIPS64.go
diff --git a/test/fixedbugs/issue29402.go b/test/fixedbugs/issue29402.go
@@ -0,0 +1,23 @@
+// run
+
+// 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.
+
+// Issue 29402: wrong optimization of comparison of
+// constant and shift on MIPS.
+
+package main
+
+//go:noinline
+func F(s []int) bool {
+	half := len(s) / 2
+	return half >= 0
+}
+
+func main() {
+	b := F([]int{1, 2, 3, 4})
+	if !b {
+		panic("FAIL")
+	}
+}