cmd/compile/internal/ssa: remove redundant zeroextensions on amd64

Some instructions operating on <= 32 bits also zero out upper 32bits.
Remove zeroextensions of such values. Triggers a few times during
all.bash. Also removes ugly code like:
MOVL CX,CX

Change-Id: I66a46c190dd6929b7e3c52f3fe6b967768d00638
Reviewed-on: https://go-review.googlesource.com/58090
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -995,6 +995,8 @@
 (MOVLQZX x:(MOVLload [off] {sym} ptr mem)) && x.Uses == 1 && clobber(x) -> @x.Block (MOVLload <v.Type> [off] {sym} ptr mem)
 (MOVLQZX x:(MOVQload [off] {sym} ptr mem)) && x.Uses == 1 && clobber(x) -> @x.Block (MOVLload <v.Type> [off] {sym} ptr mem)
 
+(MOVLQZX x) && zeroUpper32Bits(x,3) -> x
+
 (MOVBQZX x:(MOVBloadidx1 [off] {sym} ptr idx mem)) && x.Uses == 1 && clobber(x) -> @x.Block (MOVBloadidx1 <v.Type> [off] {sym} ptr idx mem)
 (MOVWQZX x:(MOVWloadidx1 [off] {sym} ptr idx mem)) && x.Uses == 1 && clobber(x) -> @x.Block (MOVWloadidx1 <v.Type> [off] {sym} ptr idx mem)
 (MOVWQZX x:(MOVWloadidx2 [off] {sym} ptr idx mem)) && x.Uses == 1 && clobber(x) -> @x.Block (MOVWloadidx2 <v.Type> [off] {sym} ptr idx mem)

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

@@ -648,3 +648,35 @@ func overlap(offset1, size1, offset2, size2 int64) bool {
 	}
 	return false
 }
+
+// check if value zeroes out upper 32-bit of 64-bit register.
+// depth limits recursion depth. In AMD64.rules 3 is used as limit,
+// because it catches same amount of cases as 4.
+func zeroUpper32Bits(x *Value, depth int) bool {
+	switch x.Op {
+	case OpAMD64MOVLconst, OpAMD64MOVLload, OpAMD64MOVLQZX, OpAMD64MOVLloadidx1,
+		OpAMD64MOVWload, OpAMD64MOVWloadidx1, OpAMD64MOVBload, OpAMD64MOVBloadidx1,
+		OpAMD64MOVLloadidx4, OpAMD64ADDLmem, OpAMD64SUBLmem, OpAMD64ANDLmem,
+		OpAMD64ORLmem, OpAMD64XORLmem, OpAMD64CVTTSD2SL,
+		OpAMD64ADDL, OpAMD64ADDLconst, OpAMD64SUBL, OpAMD64SUBLconst,
+		OpAMD64ANDL, OpAMD64ANDLconst, OpAMD64ORL, OpAMD64ORLconst,
+		OpAMD64XORL, OpAMD64XORLconst, OpAMD64NEGL, OpAMD64NOTL:
+		return true
+	case OpArg, OpSelect0, OpSelect1:
+		return x.Type.Width == 4
+	case OpPhi:
+		// Phis can use each-other as an arguments, instead of tracking visited values,
+		// just limit recursion depth.
+		if depth <= 0 {
+			return false
+		}
+		for i := range x.Args {
+			if !zeroUpper32Bits(x.Args[i], depth-1) {
+				return false
+			}
+		}
+		return true
+
+	}
+	return false
+}

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

@@ -6957,6 +6957,19 @@ func rewriteValueAMD64_OpAMD64MOVLQZX_0(v *Value) bool {
 		v0.AddArg(mem)
 		return true
 	}
+	// match: (MOVLQZX x)
+	// cond: zeroUpper32Bits(x,3)
+	// result: x
+	for {
+		x := v.Args[0]
+		if !(zeroUpper32Bits(x, 3)) {
+			break
+		}
+		v.reset(OpCopy)
+		v.Type = x.Type
+		v.AddArg(x)
+		return true
+	}
 	// match: (MOVLQZX x:(MOVLloadidx1 [off] {sym} ptr idx mem))
 	// cond: x.Uses == 1 && clobber(x)
 	// result: @x.Block (MOVLloadidx1 <v.Type> [off] {sym} ptr idx mem)