cmp/compile: rewrite CMP $0 with TEST

The CMP* family of instructions are longer than their TEST counterparts by one byte.

After this change, my go tool has 13 cmp.*$0x0 instructions, compared to 5612 before.

Change-Id: Ieb87d65657917e494c0e4b711a7ba2918ae27610
Reviewed-on: https://go-review.googlesource.com/21255Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -1239,6 +1239,12 @@
 (CMPWconst (ANDWconst [c] x) [0]) -> (TESTWconst [c] x)
 (CMPBconst (ANDBconst [c] x) [0]) -> (TESTBconst [c] x)
 
+// TEST %reg,%reg is shorter than CMP
+(CMPQconst x [0]) -> (TESTQ x x)
+(CMPLconst x [0]) -> (TESTL x x)
+(CMPWconst x [0]) -> (TESTW x x)
+(CMPBconst x [0]) -> (TESTB x x)
+
 // Combining byte loads into larger (unaligned) loads.
 // There are many ways these combinations could occur.  This is
 // designed to match the way encoding/binary.LittleEndian does it.

src/cmd/compile/internal/ssa/rewriteAMD64.go

@@ -2608,6 +2608,19 @@ func rewriteValueAMD64_OpAMD64CMPBconst(v *Value, config *Config) bool {
 		v.AddArg(x)
 		return true
 	}
+	// match: (CMPBconst x [0])
+	// cond:
+	// result: (TESTB x x)
+	for {
+		x := v.Args[0]
+		if v.AuxInt != 0 {
+			break
+		}
+		v.reset(OpAMD64TESTB)
+		v.AddArg(x)
+		v.AddArg(x)
+		return true
+	}
 	return false
 }
 func rewriteValueAMD64_OpAMD64CMPL(v *Value, config *Config) bool {
@@ -2782,6 +2795,19 @@ func rewriteValueAMD64_OpAMD64CMPLconst(v *Value, config *Config) bool {
 		v.AddArg(x)
 		return true
 	}
+	// match: (CMPLconst x [0])
+	// cond:
+	// result: (TESTL x x)
+	for {
+		x := v.Args[0]
+		if v.AuxInt != 0 {
+			break
+		}
+		v.reset(OpAMD64TESTL)
+		v.AddArg(x)
+		v.AddArg(x)
+		return true
+	}
 	return false
 }
 func rewriteValueAMD64_OpAMD64CMPQ(v *Value, config *Config) bool {
@@ -2962,6 +2988,19 @@ func rewriteValueAMD64_OpAMD64CMPQconst(v *Value, config *Config) bool {
 		v.AddArg(x)
 		return true
 	}
+	// match: (CMPQconst x [0])
+	// cond:
+	// result: (TESTQ x x)
+	for {
+		x := v.Args[0]
+		if v.AuxInt != 0 {
+			break
+		}
+		v.reset(OpAMD64TESTQ)
+		v.AddArg(x)
+		v.AddArg(x)
+		return true
+	}
 	return false
 }
 func rewriteValueAMD64_OpAMD64CMPW(v *Value, config *Config) bool {
@@ -3136,6 +3175,19 @@ func rewriteValueAMD64_OpAMD64CMPWconst(v *Value, config *Config) bool {
 		v.AddArg(x)
 		return true
 	}
+	// match: (CMPWconst x [0])
+	// cond:
+	// result: (TESTW x x)
+	for {
+		x := v.Args[0]
+		if v.AuxInt != 0 {
+			break
+		}
+		v.reset(OpAMD64TESTW)
+		v.AddArg(x)
+		v.AddArg(x)
+		return true
+	}
 	return false
 }
 func rewriteValueAMD64_OpClosureCall(v *Value, config *Config) bool {