[release-branch.go1.8] cmd/compile: fix subword store/load elision for MIPS

Apply the fix in CL 44355 to MIPS.

ARM64 has these rules but commented out for performance reason.
Fix the commented rules, in case they are enabled in the future.

Enhance the test so it triggers the failure on ARM and MIPS without
the fix.

Updates #20530.

Change-Id: I82d77448e3939a545fe519d0a29a164f8fa5417c
Reviewed-on: https://go-review.googlesource.com/44430
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-on: https://go-review.googlesource.com/70840
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

4 files changed, 45 insertions, 33 deletions

diff --git a/src/cmd/compile/internal/ssa/gen/ARM64.rules b/src/cmd/compile/internal/ssa/gen/ARM64.rules
index bc58a1f5f5..ba7b00a641 100644
--- a/src/cmd/compile/internal/ssa/gen/ARM64.rules
+++ b/src/cmd/compile/internal/ssa/gen/ARM64.rules
@@ -679,14 +679,14 @@
 (MOVWstore [off] {sym} ptr (MOVDconst [0]) mem) -> (MOVWstorezero [off] {sym} ptr mem)
 (MOVDstore [off] {sym} ptr (MOVDconst [0]) mem) -> (MOVDstorezero [off] {sym} ptr mem)
 
-// replace load from same location as preceding store with copy
+// replace load from same location as preceding store with zero/sign extension (or copy in case of full width)
 // these seem to have bad interaction with other rules, resulting in slower code
-//(MOVBload [off] {sym} ptr (MOVBstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> x
-//(MOVBUload [off] {sym} ptr (MOVBstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> x
-//(MOVHload [off] {sym} ptr (MOVHstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> x
-//(MOVHUload [off] {sym} ptr (MOVHstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> x
-//(MOVWload [off] {sym} ptr (MOVWstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> x
-//(MOVWUload [off] {sym} ptr (MOVWstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> x
+//(MOVBload [off] {sym} ptr (MOVBstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> (MOVBreg x)
+//(MOVBUload [off] {sym} ptr (MOVBstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> (MOVBUreg x)
+//(MOVHload [off] {sym} ptr (MOVHstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> (MOVHreg x)
+//(MOVHUload [off] {sym} ptr (MOVHstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> (MOVHUreg x)
+//(MOVWload [off] {sym} ptr (MOVWstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> (MOVWreg x)
+//(MOVWUload [off] {sym} ptr (MOVWstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> (MOVWUreg x)
 //(MOVDload [off] {sym} ptr (MOVDstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> x
 //(FMOVSload [off] {sym} ptr (FMOVSstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> x
 //(FMOVDload [off] {sym} ptr (FMOVDstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> x
diff --git a/src/cmd/compile/internal/ssa/gen/MIPS.rules b/src/cmd/compile/internal/ssa/gen/MIPS.rules
index ad7a954973..c231cd833f 100644
--- a/src/cmd/compile/internal/ssa/gen/MIPS.rules
+++ b/src/cmd/compile/internal/ssa/gen/MIPS.rules
@@ -522,11 +522,11 @@
 (MOVWstorezero [off1] {sym1} (MOVWaddr [off2] {sym2} ptr) mem) && canMergeSym(sym1,sym2) ->
 	(MOVWstorezero [off1+off2] {mergeSym(sym1,sym2)} ptr mem)
 
-// replace load from same location as preceding store with copy
-(MOVBload [off] {sym} ptr (MOVBstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) && isSigned(x.Type) -> x
-(MOVBUload [off] {sym} ptr (MOVBstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) && !isSigned(x.Type) -> x
-(MOVHload [off] {sym} ptr (MOVHstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) && isSigned(x.Type) -> x
-(MOVHUload [off] {sym} ptr (MOVHstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) && !isSigned(x.Type) -> x
+// replace load from same location as preceding store with zero/sign extension (or copy in case of full width)
+(MOVBload [off] {sym} ptr (MOVBstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> (MOVBreg x)
+(MOVBUload [off] {sym} ptr (MOVBstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> (MOVBUreg x)
+(MOVHload [off] {sym} ptr (MOVHstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> (MOVHreg x)
+(MOVHUload [off] {sym} ptr (MOVHstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> (MOVHUreg x)
 (MOVWload [off] {sym} ptr (MOVWstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> x
 (MOVFload [off] {sym} ptr (MOVFstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> x
 (MOVDload [off] {sym} ptr (MOVDstore [off2] {sym2} ptr2 x _)) && sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) -> x
diff --git a/src/cmd/compile/internal/ssa/rewriteMIPS.go b/src/cmd/compile/internal/ssa/rewriteMIPS.go
index 76eac5b1cf..2590611ab2 100644
--- a/src/cmd/compile/internal/ssa/rewriteMIPS.go
+++ b/src/cmd/compile/internal/ssa/rewriteMIPS.go
@@ -3332,8 +3332,8 @@ func rewriteValueMIPS_OpMIPSMOVBUload(v *Value, config *Config) bool {
 		return true
 	}
 	// match: (MOVBUload [off] {sym} ptr (MOVBstore [off2] {sym2} ptr2 x _))
-	// cond: sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) && !isSigned(x.Type)
-	// result: x
+	// cond: sym == sym2 && off == off2 && isSamePtr(ptr, ptr2)
+	// result: (MOVBUreg x)
 	for {
 		off := v.AuxInt
 		sym := v.Aux
@@ -3346,11 +3346,10 @@ func rewriteValueMIPS_OpMIPSMOVBUload(v *Value, config *Config) bool {
 		sym2 := v_1.Aux
 		ptr2 := v_1.Args[0]
 		x := v_1.Args[1]
-		if !(sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) && !isSigned(x.Type)) {
+		if !(sym == sym2 && off == off2 && isSamePtr(ptr, ptr2)) {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		v.reset(OpMIPSMOVBUreg)
 		v.AddArg(x)
 		return true
 	}
@@ -3490,8 +3489,8 @@ func rewriteValueMIPS_OpMIPSMOVBload(v *Value, config *Config) bool {
 		return true
 	}
 	// match: (MOVBload [off] {sym} ptr (MOVBstore [off2] {sym2} ptr2 x _))
-	// cond: sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) && isSigned(x.Type)
-	// result: x
+	// cond: sym == sym2 && off == off2 && isSamePtr(ptr, ptr2)
+	// result: (MOVBreg x)
 	for {
 		off := v.AuxInt
 		sym := v.Aux
@@ -3504,11 +3503,10 @@ func rewriteValueMIPS_OpMIPSMOVBload(v *Value, config *Config) bool {
 		sym2 := v_1.Aux
 		ptr2 := v_1.Args[0]
 		x := v_1.Args[1]
-		if !(sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) && isSigned(x.Type)) {
+		if !(sym == sym2 && off == off2 && isSamePtr(ptr, ptr2)) {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		v.reset(OpMIPSMOVBreg)
 		v.AddArg(x)
 		return true
 	}
@@ -4148,8 +4146,8 @@ func rewriteValueMIPS_OpMIPSMOVHUload(v *Value, config *Config) bool {
 		return true
 	}
 	// match: (MOVHUload [off] {sym} ptr (MOVHstore [off2] {sym2} ptr2 x _))
-	// cond: sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) && !isSigned(x.Type)
-	// result: x
+	// cond: sym == sym2 && off == off2 && isSamePtr(ptr, ptr2)
+	// result: (MOVHUreg x)
 	for {
 		off := v.AuxInt
 		sym := v.Aux
@@ -4162,11 +4160,10 @@ func rewriteValueMIPS_OpMIPSMOVHUload(v *Value, config *Config) bool {
 		sym2 := v_1.Aux
 		ptr2 := v_1.Args[0]
 		x := v_1.Args[1]
-		if !(sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) && !isSigned(x.Type)) {
+		if !(sym == sym2 && off == off2 && isSamePtr(ptr, ptr2)) {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		v.reset(OpMIPSMOVHUreg)
 		v.AddArg(x)
 		return true
 	}
@@ -4330,8 +4327,8 @@ func rewriteValueMIPS_OpMIPSMOVHload(v *Value, config *Config) bool {
 		return true
 	}
 	// match: (MOVHload [off] {sym} ptr (MOVHstore [off2] {sym2} ptr2 x _))
-	// cond: sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) && isSigned(x.Type)
-	// result: x
+	// cond: sym == sym2 && off == off2 && isSamePtr(ptr, ptr2)
+	// result: (MOVHreg x)
 	for {
 		off := v.AuxInt
 		sym := v.Aux
@@ -4344,11 +4341,10 @@ func rewriteValueMIPS_OpMIPSMOVHload(v *Value, config *Config) bool {
 		sym2 := v_1.Aux
 		ptr2 := v_1.Args[0]
 		x := v_1.Args[1]
-		if !(sym == sym2 && off == off2 && isSamePtr(ptr, ptr2) && isSigned(x.Type)) {
+		if !(sym == sym2 && off == off2 && isSamePtr(ptr, ptr2)) {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		v.reset(OpMIPSMOVHreg)
 		v.AddArg(x)
 		return true
 	}
diff --git a/test/fixedbugs/issue20530.go b/test/fixedbugs/issue20530.go
index 7027b10745..51f0bd8e39 100644
--- a/test/fixedbugs/issue20530.go
+++ b/test/fixedbugs/issue20530.go
@@ -8,7 +8,8 @@ package main
 
 var a uint8
 
-func main() {
+//go:noinline
+func f() {
 	b := int8(func() int32 { return -1 }())
 	a = uint8(b)
 	if int32(a) != 255 {
@@ -16,3 +17,18 @@ func main() {
 		println("got", a, "expected 255")
 	}
 }
+
+//go:noinline
+func g() {
+	b := int8(func() uint32 { return 0xffffffff }())
+	a = uint8(b)
+	if int32(a) != 255 {
+		// Failing case prints 'got 255 expected 255'
+		println("got", a, "expected 255")
+	}
+}
+
+func main() {
+	f()
+	g()
+}