cmd/compile/internal/ssa: remove useless zero extension

We generate MOVBLZX for byte-sized LoadReg, so
(MOVBQZX (LoadReg (Arg))) is the same as
(LoadReg (Arg)). Remove those zero extension where possible.
Triggers several times during all.bash.

Fixes #25378
Updates #15300

Change-Id: If50656e66f217832a13ee8f49c47997f4fcc093a
Reviewed-on: https://go-review.googlesource.com/115617
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

@@ -988,6 +988,8 @@
 (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
+(MOVWQZX x) && zeroUpper48Bits(x,3) -> x
+(MOVBQZX x) && zeroUpper56Bits(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)

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

@@ -894,6 +894,54 @@ func zeroUpper32Bits(x *Value, depth int) bool {
 	return false
 }
 
+// zeroUpper48Bits is similar to zeroUpper32Bits, but for upper 48 bits
+func zeroUpper48Bits(x *Value, depth int) bool {
+	switch x.Op {
+	case OpAMD64MOVWQZX, OpAMD64MOVWload, OpAMD64MOVWloadidx1, OpAMD64MOVWloadidx2:
+		return true
+	case OpArg:
+		return x.Type.Width == 2
+	case OpPhi, OpSelect0, OpSelect1:
+		// 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 !zeroUpper48Bits(x.Args[i], depth-1) {
+				return false
+			}
+		}
+		return true
+
+	}
+	return false
+}
+
+// zeroUpper56Bits is similar to zeroUpper32Bits, but for upper 56 bits
+func zeroUpper56Bits(x *Value, depth int) bool {
+	switch x.Op {
+	case OpAMD64MOVBQZX, OpAMD64MOVBload, OpAMD64MOVBloadidx1:
+		return true
+	case OpArg:
+		return x.Type.Width == 1
+	case OpPhi, OpSelect0, OpSelect1:
+		// 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 !zeroUpper56Bits(x.Args[i], depth-1) {
+				return false
+			}
+		}
+		return true
+
+	}
+	return false
+}
+
 // isInlinableMemmove reports whether the given arch performs a Move of the given size
 // faster than memmove. It will only return true if replacing the memmove with a Move is
 // safe, either because Move is small or because the arguments are disjoint.

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

@@ -10270,6 +10270,19 @@ func rewriteValueAMD64_OpAMD64MOVBQZX_0(v *Value) bool {
 		v0.AddArg(mem)
 		return true
 	}
+	// match: (MOVBQZX x)
+	// cond: zeroUpper56Bits(x,3)
+	// result: x
+	for {
+		x := v.Args[0]
+		if !(zeroUpper56Bits(x, 3)) {
+			break
+		}
+		v.reset(OpCopy)
+		v.Type = x.Type
+		v.AddArg(x)
+		return true
+	}
 	// match: (MOVBQZX x:(MOVBloadidx1 [off] {sym} ptr idx mem))
 	// cond: x.Uses == 1 && clobber(x)
 	// result: @x.Block (MOVBloadidx1 <v.Type> [off] {sym} ptr idx mem)
@@ -18893,6 +18906,19 @@ func rewriteValueAMD64_OpAMD64MOVWQZX_0(v *Value) bool {
 		v0.AddArg(mem)
 		return true
 	}
+	// match: (MOVWQZX x)
+	// cond: zeroUpper48Bits(x,3)
+	// result: x
+	for {
+		x := v.Args[0]
+		if !(zeroUpper48Bits(x, 3)) {
+			break
+		}
+		v.reset(OpCopy)
+		v.Type = x.Type
+		v.AddArg(x)
+		return true
+	}
 	// match: (MOVWQZX x:(MOVWloadidx1 [off] {sym} ptr idx mem))
 	// cond: x.Uses == 1 && clobber(x)
 	// result: @x.Block (MOVWloadidx1 <v.Type> [off] {sym} ptr idx mem)

test/codegen/issue25378.go

@@ -0,0 +1,22 @@
+// asmcheck
+
+// 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 codegen
+
+var wsp = [256]bool{
+	' ':  true,
+	'\t': true,
+	'\n': true,
+	'\r': true,
+}
+
+func zeroExtArgByte(ch byte) bool {
+	return wsp[ch] // amd64:-"MOVBLZX\t..,.."
+}
+
+func zeroExtArgUint16(ch uint16) bool {
+	return wsp[ch] // amd64:-"MOVWLZX\t..,.."
+}