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 <[email protected]>
Reviewed-by: Mark Freeman <[email protected]>
Auto-Submit: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: amusman

src/cmd/compile/internal/ssa/_gen/ARM64.rules

@@ -1666,6 +1666,10 @@
 (SRLconst [rc] (MOVHUreg x)) && rc >= 16 => (MOVDconst [0])
 (SRLconst [rc] (MOVBUreg x)) && rc >= 8 => (MOVDconst [0])
 
+// Special cases for slice operations
+(ADD x0 x1:(ANDshiftRA x2:(SLLconst [sl] y) z [63])) && x1.Uses == 1 && x2.Uses == 1 => (ADDshiftLL x0 (ANDshiftRA <y.Type> y z [63]) [sl])
+(ADD x0 x1:(ANDshiftLL x2:(SRAconst [63] z) y [sl])) && x1.Uses == 1 && x2.Uses == 1 => (ADDshiftLL x0 (ANDshiftRA <y.Type> y z [63]) [sl])
+
 // bitfield ops
 
 // sbfiz

src/cmd/compile/internal/ssa/rewriteARM64.go

@@ -417,6 +417,15 @@ func SliceWithSubtractBound(a []int, b int) []int {
 	return a[(3 - b):]
 }
 
+// --------------------------------------- //
+//   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      //
 // --------------------------------------- //