[release-branch.go1.9] cmd/compile: fix constant folding of right shifts

The sub-word shifts need to sign-extend before shifting, to avoid
bringing in data from higher in the argument.

Fixes #23812

Change-Id: I0a95a0b49c48f3b40b85765bb4a9bb492be0cd73
Reviewed-on: https://go-review.googlesource.com/93716
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-on: https://go-review.googlesource.com/102775
Run-TryBot: Andrew Bonventre <andybons@golang.org>

4 files changed, 55 insertions, 21 deletions

diff --git a/src/cmd/compile/internal/ssa/gen/AMD64.rules b/src/cmd/compile/internal/ssa/gen/AMD64.rules
index ff38be550e..89d082ac7f 100644
--- a/src/cmd/compile/internal/ssa/gen/AMD64.rules
+++ b/src/cmd/compile/internal/ssa/gen/AMD64.rules
@@ -1499,9 +1499,9 @@
 (SUBQconst (MOVQconst [d]) [c]) -> (MOVQconst [d-c])
 (SUBQconst (SUBQconst x [d]) [c]) && is32Bit(-c-d) -> (ADDQconst [-c-d] x)
 (SARQconst [c] (MOVQconst [d])) -> (MOVQconst [d>>uint64(c)])
-(SARLconst [c] (MOVQconst [d])) -> (MOVQconst [d>>uint64(c)])
-(SARWconst [c] (MOVQconst [d])) -> (MOVQconst [d>>uint64(c)])
-(SARBconst [c] (MOVQconst [d])) -> (MOVQconst [d>>uint64(c)])
+(SARLconst [c] (MOVQconst [d])) -> (MOVQconst [int64(int32(d))>>uint64(c)])
+(SARWconst [c] (MOVQconst [d])) -> (MOVQconst [int64(int16(d))>>uint64(c)])
+(SARBconst [c] (MOVQconst [d])) -> (MOVQconst [int64(int8(d))>>uint64(c)])
 (NEGQ (MOVQconst [c])) -> (MOVQconst [-c])
 (NEGL (MOVLconst [c])) -> (MOVLconst [int64(int32(-c))])
 (MULQconst [c] (MOVQconst [d])) -> (MOVQconst [c*d])
diff --git a/src/cmd/compile/internal/ssa/gen/AMD64Ops.go b/src/cmd/compile/internal/ssa/gen/AMD64Ops.go
index c984cbfb12..12757a7d5a 100644
--- a/src/cmd/compile/internal/ssa/gen/AMD64Ops.go
+++ b/src/cmd/compile/internal/ssa/gen/AMD64Ops.go
@@ -268,22 +268,22 @@ func init() {
 		// Note: x86 is weird, the 16 and 8 byte shifts still use all 5 bits of shift amount!
 
 		{name: "SHRQ", argLength: 2, reg: gp21shift, asm: "SHRQ", resultInArg0: true, clobberFlags: true},              // unsigned arg0 >> arg1, shift amount is mod 64
-		{name: "SHRL", argLength: 2, reg: gp21shift, asm: "SHRL", resultInArg0: true, clobberFlags: true},              // unsigned arg0 >> arg1, shift amount is mod 32
-		{name: "SHRW", argLength: 2, reg: gp21shift, asm: "SHRW", resultInArg0: true, clobberFlags: true},              // unsigned arg0 >> arg1, shift amount is mod 32
-		{name: "SHRB", argLength: 2, reg: gp21shift, asm: "SHRB", resultInArg0: true, clobberFlags: true},              // unsigned arg0 >> arg1, shift amount is mod 32
+		{name: "SHRL", argLength: 2, reg: gp21shift, asm: "SHRL", resultInArg0: true, clobberFlags: true},              // unsigned uint32(arg0) >> arg1, shift amount is mod 32
+		{name: "SHRW", argLength: 2, reg: gp21shift, asm: "SHRW", resultInArg0: true, clobberFlags: true},              // unsigned uint16(arg0) >> arg1, shift amount is mod 32
+		{name: "SHRB", argLength: 2, reg: gp21shift, asm: "SHRB", resultInArg0: true, clobberFlags: true},              // unsigned uint8(arg0) >> arg1, shift amount is mod 32
 		{name: "SHRQconst", argLength: 1, reg: gp11, asm: "SHRQ", aux: "Int8", resultInArg0: true, clobberFlags: true}, // unsigned arg0 >> auxint, shift amount 0-63
-		{name: "SHRLconst", argLength: 1, reg: gp11, asm: "SHRL", aux: "Int8", resultInArg0: true, clobberFlags: true}, // unsigned arg0 >> auxint, shift amount 0-31
-		{name: "SHRWconst", argLength: 1, reg: gp11, asm: "SHRW", aux: "Int8", resultInArg0: true, clobberFlags: true}, // unsigned arg0 >> auxint, shift amount 0-15
-		{name: "SHRBconst", argLength: 1, reg: gp11, asm: "SHRB", aux: "Int8", resultInArg0: true, clobberFlags: true}, // unsigned arg0 >> auxint, shift amount 0-7
+		{name: "SHRLconst", argLength: 1, reg: gp11, asm: "SHRL", aux: "Int8", resultInArg0: true, clobberFlags: true}, // unsigned uint32(arg0) >> auxint, shift amount 0-31
+		{name: "SHRWconst", argLength: 1, reg: gp11, asm: "SHRW", aux: "Int8", resultInArg0: true, clobberFlags: true}, // unsigned uint16(arg0) >> auxint, shift amount 0-15
+		{name: "SHRBconst", argLength: 1, reg: gp11, asm: "SHRB", aux: "Int8", resultInArg0: true, clobberFlags: true}, // unsigned uint8(arg0) >> auxint, shift amount 0-7
 
 		{name: "SARQ", argLength: 2, reg: gp21shift, asm: "SARQ", resultInArg0: true, clobberFlags: true},              // signed arg0 >> arg1, shift amount is mod 64
-		{name: "SARL", argLength: 2, reg: gp21shift, asm: "SARL", resultInArg0: true, clobberFlags: true},              // signed arg0 >> arg1, shift amount is mod 32
-		{name: "SARW", argLength: 2, reg: gp21shift, asm: "SARW", resultInArg0: true, clobberFlags: true},              // signed arg0 >> arg1, shift amount is mod 32
-		{name: "SARB", argLength: 2, reg: gp21shift, asm: "SARB", resultInArg0: true, clobberFlags: true},              // signed arg0 >> arg1, shift amount is mod 32
+		{name: "SARL", argLength: 2, reg: gp21shift, asm: "SARL", resultInArg0: true, clobberFlags: true},              // signed int32(arg0) >> arg1, shift amount is mod 32
+		{name: "SARW", argLength: 2, reg: gp21shift, asm: "SARW", resultInArg0: true, clobberFlags: true},              // signed int16(arg0) >> arg1, shift amount is mod 32
+		{name: "SARB", argLength: 2, reg: gp21shift, asm: "SARB", resultInArg0: true, clobberFlags: true},              // signed int8(arg0) >> arg1, shift amount is mod 32
 		{name: "SARQconst", argLength: 1, reg: gp11, asm: "SARQ", aux: "Int8", resultInArg0: true, clobberFlags: true}, // signed arg0 >> auxint, shift amount 0-63
-		{name: "SARLconst", argLength: 1, reg: gp11, asm: "SARL", aux: "Int8", resultInArg0: true, clobberFlags: true}, // signed arg0 >> auxint, shift amount 0-31
-		{name: "SARWconst", argLength: 1, reg: gp11, asm: "SARW", aux: "Int8", resultInArg0: true, clobberFlags: true}, // signed arg0 >> auxint, shift amount 0-15
-		{name: "SARBconst", argLength: 1, reg: gp11, asm: "SARB", aux: "Int8", resultInArg0: true, clobberFlags: true}, // signed arg0 >> auxint, shift amount 0-7
+		{name: "SARLconst", argLength: 1, reg: gp11, asm: "SARL", aux: "Int8", resultInArg0: true, clobberFlags: true}, // signed int32(arg0) >> auxint, shift amount 0-31
+		{name: "SARWconst", argLength: 1, reg: gp11, asm: "SARW", aux: "Int8", resultInArg0: true, clobberFlags: true}, // signed int16(arg0) >> auxint, shift amount 0-15
+		{name: "SARBconst", argLength: 1, reg: gp11, asm: "SARB", aux: "Int8", resultInArg0: true, clobberFlags: true}, // signed int8(arg0) >> auxint, shift amount 0-7
 
 		{name: "ROLQ", argLength: 2, reg: gp21shift, asm: "ROLQ", resultInArg0: true, clobberFlags: true},              // arg0 rotate left arg1 bits.
 		{name: "ROLL", argLength: 2, reg: gp21shift, asm: "ROLL", resultInArg0: true, clobberFlags: true},              // arg0 rotate left arg1 bits.
diff --git a/src/cmd/compile/internal/ssa/rewriteAMD64.go b/src/cmd/compile/internal/ssa/rewriteAMD64.go
index 9213616f83..379ea55b88 100644
--- a/src/cmd/compile/internal/ssa/rewriteAMD64.go
+++ b/src/cmd/compile/internal/ssa/rewriteAMD64.go
@@ -33234,7 +33234,7 @@ func rewriteValueAMD64_OpAMD64SARBconst_0(v *Value) bool {
 	}
 	// match: (SARBconst [c] (MOVQconst [d]))
 	// cond:
-	// result: (MOVQconst [d>>uint64(c)])
+	// result: (MOVQconst [int64(int8(d))>>uint64(c)])
 	for {
 		c := v.AuxInt
 		v_0 := v.Args[0]
@@ -33243,7 +33243,7 @@ func rewriteValueAMD64_OpAMD64SARBconst_0(v *Value) bool {
 		}
 		d := v_0.AuxInt
 		v.reset(OpAMD64MOVQconst)
-		v.AuxInt = d >> uint64(c)
+		v.AuxInt = int64(int8(d)) >> uint64(c)
 		return true
 	}
 	return false
@@ -33489,7 +33489,7 @@ func rewriteValueAMD64_OpAMD64SARLconst_0(v *Value) bool {
 	}
 	// match: (SARLconst [c] (MOVQconst [d]))
 	// cond:
-	// result: (MOVQconst [d>>uint64(c)])
+	// result: (MOVQconst [int64(int32(d))>>uint64(c)])
 	for {
 		c := v.AuxInt
 		v_0 := v.Args[0]
@@ -33498,7 +33498,7 @@ func rewriteValueAMD64_OpAMD64SARLconst_0(v *Value) bool {
 		}
 		d := v_0.AuxInt
 		v.reset(OpAMD64MOVQconst)
-		v.AuxInt = d >> uint64(c)
+		v.AuxInt = int64(int32(d)) >> uint64(c)
 		return true
 	}
 	return false
@@ -33809,7 +33809,7 @@ func rewriteValueAMD64_OpAMD64SARWconst_0(v *Value) bool {
 	}
 	// match: (SARWconst [c] (MOVQconst [d]))
 	// cond:
-	// result: (MOVQconst [d>>uint64(c)])
+	// result: (MOVQconst [int64(int16(d))>>uint64(c)])
 	for {
 		c := v.AuxInt
 		v_0 := v.Args[0]
@@ -33818,7 +33818,7 @@ func rewriteValueAMD64_OpAMD64SARWconst_0(v *Value) bool {
 		}
 		d := v_0.AuxInt
 		v.reset(OpAMD64MOVQconst)
-		v.AuxInt = d >> uint64(c)
+		v.AuxInt = int64(int16(d)) >> uint64(c)
 		return true
 	}
 	return false
diff --git a/test/fixedbugs/issue23812.go b/test/fixedbugs/issue23812.go
new file mode 100644
index 0000000000..0a40deb212
--- /dev/null
+++ b/test/fixedbugs/issue23812.go
@@ -0,0 +1,34 @@
+// 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.
+
+package main
+
+import "fmt"
+
+func main() {
+	want := int32(0x3edae8)
+	got := foo(1)
+	if want != got {
+		panic(fmt.Sprintf("want %x, got %x", want, got))
+	}
+}
+
+func foo(a int32) int32 {
+	return shr1(int32(shr2(int64(0x14ff6e2207db5d1f), int(a))), 4)
+}
+
+func shr1(n int32, m int) int32 { return n >> uint(m) }
+
+func shr2(n int64, m int) int64 {
+	if m < 0 {
+		m = -m
+	}
+	if m >= 64 {
+		return n
+	}
+
+	return n >> uint(m)
+}