cmd/compile: fold shift through AND for slice operations

Fold a shift through AND when the AND gets a zero-or-one operand (e.g.
from arithmetic shift by 63 of a 64-bit value) for a common case with
slice operations:

    ASR     $63, R2, R2
    AND     R3<<3, R2, R2
    ADD     R2, R0, R2

As the operands are 64-bit, we can transform it to:

    AND     R2->63, R3, R2
    ADD     R2<<3, R0, R2

Code size improvement:
compile: .text:     9088004 ->  9086292 (-0.02%)
etcd:    .text:    10500276 -> 10498964 (-0.01%)

Change-Id: Ibcd5e67173da39b77ceff77ca67812fb8be5a7b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/679895
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Mark Freeman <mark@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/test/codegen/slices.go b/test/codegen/slices.go
index 30a131a5a5..1d918a3a0a 100644
--- a/test/codegen/slices.go
+++ b/test/codegen/slices.go
@@ -418,6 +418,15 @@ func SliceWithSubtractBound(a []int, b int) []int {
 }
 
 // --------------------------------------- //
+//   ARM64 folding for slice masks         //
+// --------------------------------------- //
+
+func SliceAndIndex(a []int, b int) int {
+	// arm64:"AND\tR[0-9]+->63","ADD\tR[0-9]+<<3"
+	return a[b:][b]
+}
+
+// --------------------------------------- //
 //   Code generation for unsafe.Slice      //
 // --------------------------------------- //