cmd/compile: fix bit length intrinsic for 16/8 bits on GOAMD64=v3

Upper bits of registers for uint8/uint16 are junk. Make sure we
mask those off before using LZCNT (leading zeros count).

Fixes #52681

Change-Id: I0ca9e62f23bcb1f6ad2a787fa9895322afaa2533
Reviewed-on: https://go-review.googlesource.com/c/go/+/403815
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@google.com>

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

@@ -105,7 +105,9 @@
 (BitLen8  x) && buildcfg.GOAMD64 <  3 => (BSRL (LEAL1 <typ.UInt32> [1] (MOVBQZX <typ.UInt32> x) (MOVBQZX <typ.UInt32> x)))
 (BitLen64 <t> x)        && buildcfg.GOAMD64 >= 3 => (NEGQ (ADDQconst <t> [-64] (LZCNTQ x)))
 // Use 64-bit version to allow const-fold remove unnecessary arithmetic.
-(BitLen(32|16|8) <t> x) && buildcfg.GOAMD64 >= 3 => (NEGQ (ADDQconst <t> [-32] (LZCNTL x)))
+(BitLen32 <t> x) && buildcfg.GOAMD64 >= 3 => (NEGQ (ADDQconst <t> [-32] (LZCNTL x)))
+(BitLen16 <t> x) && buildcfg.GOAMD64 >= 3 => (NEGQ (ADDQconst <t> [-32] (LZCNTL (MOVWQZX <x.Type> x))))
+(BitLen8 <t> x) && buildcfg.GOAMD64 >= 3 => (NEGQ (ADDQconst <t> [-32] (LZCNTL (MOVBQZX <x.Type> x))))
 
 (Bswap(64|32) ...) => (BSWAP(Q|L) ...)
 

src/cmd/compile/internal/test/intrinsics_test.go

@@ -0,0 +1,62 @@
+// Copyright 2022 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 test
+
+import (
+	"math/bits"
+	"testing"
+)
+
+func TestBitLen64(t *testing.T) {
+	for i := 0; i <= 64; i++ {
+		got := bits.Len64(1 << i)
+		want := i + 1
+		if want == 65 {
+			want = 0
+		}
+		if got != want {
+			t.Errorf("Len64(1<<%d) = %d, want %d", i, got, want)
+		}
+	}
+}
+
+func TestBitLen32(t *testing.T) {
+	for i := 0; i <= 32; i++ {
+		got := bits.Len32(1 << i)
+		want := i + 1
+		if want == 33 {
+			want = 0
+		}
+		if got != want {
+			t.Errorf("Len32(1<<%d) = %d, want %d", i, got, want)
+		}
+	}
+}
+
+func TestBitLen16(t *testing.T) {
+	for i := 0; i <= 16; i++ {
+		got := bits.Len16(1 << i)
+		want := i + 1
+		if want == 17 {
+			want = 0
+		}
+		if got != want {
+			t.Errorf("Len16(1<<%d) = %d, want %d", i, got, want)
+		}
+	}
+}
+
+func TestBitLen8(t *testing.T) {
+	for i := 0; i <= 8; i++ {
+		got := bits.Len8(1 << i)
+		want := i + 1
+		if want == 9 {
+			want = 0
+		}
+		if got != want {
+			t.Errorf("Len8(1<<%d) = %d, want %d", i, got, want)
+		}
+	}
+}