[dev.ssa] cmd/compile: simplify repeated OCOM

Rewrite ^{n}x to be ^{n % 2}x.  This will eventually resolve a fuzz
issue that breaks v1.5.

Updates #11352

Change-Id: I1b3f93872d06222f9ff5f6fd5580178ebaf4c003
Reviewed-on: https://go-review.googlesource.com/13110Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/testdata/arith_ssa.go

@@ -156,6 +156,21 @@ func testSubqToNegq_ssa(a, b, c, d, e, f, g, h, i, j, k int64) int64 {
 	return a + 8207351403619448057 - b - 1779494519303207690 + c*8810076340510052032*d - 4465874067674546219 - e*4361839741470334295 - f + 8688847565426072650*g*8065564729145417479
 }
 
+func testOcom() {
+	want1, want2 := int32(0x55555555), int32(-0x55555556)
+	if got1, got2 := testOcom_ssa(0x55555555, 0x55555555); want1 != got1 || want2 != got2 {
+		println("testSubqToNegq failed, wanted", want1, "and", want2,
+			"got", got1, "and", got2)
+		failed = true
+	}
+}
+
+func testOcom_ssa(a, b int32) (int32, int32) {
+	switch { // prevent inlining
+	}
+	return ^^^^a, ^^^^^b
+}
+
 var failed = false
 
 func main() {
@@ -165,6 +180,7 @@ func main() {
 	testRegallocCVSpill()
 	testSubqToNegq()
 	testBitwiseLogic()
+	testOcom()
 
 	if failed {
 		panic("failed")

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

@@ -34,6 +34,11 @@
 (Neq16 x x) -> (ConstBool {false})
 (Neq8 x x) -> (ConstBool {false})
 
+(Com8 (Com8 x)) -> (Copy x)
+(Com16 (Com16 x)) -> (Copy x)
+(Com32 (Com32 x)) -> (Copy x)
+(Com64 (Com64 x)) -> (Copy x)
+
 // tear apart slices
 // TODO: anything that generates a slice needs to go in here.
 (SlicePtr (Load ptr mem)) -> (Load ptr mem)

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

@@ -76,6 +76,82 @@ func rewriteValuegeneric(v *Value, config *Config) bool {
 		goto end4894dd7b58383fee5f8a92be08437c33
 	end4894dd7b58383fee5f8a92be08437c33:
 		;
+	case OpCom16:
+		// match: (Com16 (Com16 x))
+		// cond:
+		// result: (Copy x)
+		{
+			if v.Args[0].Op != OpCom16 {
+				goto end388d572e5a72fd87a07da5cab243ebdc
+			}
+			x := v.Args[0].Args[0]
+			v.Op = OpCopy
+			v.AuxInt = 0
+			v.Aux = nil
+			v.resetArgs()
+			v.AddArg(x)
+			return true
+		}
+		goto end388d572e5a72fd87a07da5cab243ebdc
+	end388d572e5a72fd87a07da5cab243ebdc:
+		;
+	case OpCom32:
+		// match: (Com32 (Com32 x))
+		// cond:
+		// result: (Copy x)
+		{
+			if v.Args[0].Op != OpCom32 {
+				goto end5b2b3834acc7313649923604f685e7c5
+			}
+			x := v.Args[0].Args[0]
+			v.Op = OpCopy
+			v.AuxInt = 0
+			v.Aux = nil
+			v.resetArgs()
+			v.AddArg(x)
+			return true
+		}
+		goto end5b2b3834acc7313649923604f685e7c5
+	end5b2b3834acc7313649923604f685e7c5:
+		;
+	case OpCom64:
+		// match: (Com64 (Com64 x))
+		// cond:
+		// result: (Copy x)
+		{
+			if v.Args[0].Op != OpCom64 {
+				goto end6d6312f25d06a327d92f028b1ce50566
+			}
+			x := v.Args[0].Args[0]
+			v.Op = OpCopy
+			v.AuxInt = 0
+			v.Aux = nil
+			v.resetArgs()
+			v.AddArg(x)
+			return true
+		}
+		goto end6d6312f25d06a327d92f028b1ce50566
+	end6d6312f25d06a327d92f028b1ce50566:
+		;
+	case OpCom8:
+		// match: (Com8 (Com8 x))
+		// cond:
+		// result: (Copy x)
+		{
+			if v.Args[0].Op != OpCom8 {
+				goto end70cbd85c4b8e82c170dba7c23f8bc0f3
+			}
+			x := v.Args[0].Args[0]
+			v.Op = OpCopy
+			v.AuxInt = 0
+			v.Aux = nil
+			v.resetArgs()
+			v.AddArg(x)
+			return true
+		}
+		goto end70cbd85c4b8e82c170dba7c23f8bc0f3
+	end70cbd85c4b8e82c170dba7c23f8bc0f3:
+		;
 	case OpConstString:
 		// match: (ConstString {s})
 		// cond: