cmd/internal/obj/arm64: shorten constant integer loads

Large integer constants can take up to 4 instructions to encode.

We can encode some large constants with a single instruction, namely
those which are bit patterns (repetitions of certain runs of 0s and 1s).

Often the constants we want to encode are *close* to those bit patterns,
but don't exactly match. For those, we can use 2 instructions, one to
load the close-by bit pattern and one to fix up any mismatches.

The constants we use to strength reduce divides often fit this pattern.
For unsigned divides by 1 through 15, this CL applies to the constant
for N=3,5,6,10,12,15.

Triggers 17 times in hello world.

Change-Id: I623abf32961fb3e74d0a163f6822f0647cd94499
Reviewed-on: https://go-review.googlesource.com/c/go/+/717900
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

3 files changed, 59 insertions, 1 deletions

diff --git a/src/cmd/internal/obj/arm64/asm7.go b/src/cmd/internal/obj/arm64/asm7.go
index 7e7f028bfb..ccf8eda495 100644
--- a/src/cmd/internal/obj/arm64/asm7.go
+++ b/src/cmd/internal/obj/arm64/asm7.go
@@ -38,6 +38,7 @@ import (
 	"fmt"
 	"log"
 	"math"
+	"math/bits"
 	"slices"
 	"strings"
 )
@@ -1976,7 +1977,18 @@ func (c *ctxt7) con64class(a *obj.Addr) int {
 		return C_MOVCON
 	} else if zeroCount == 2 || negCount == 2 {
 		return C_MOVCON2
-	} else if zeroCount == 1 || negCount == 1 {
+	}
+	// See omovlconst for description of this loop.
+	for i := 0; i < 4; i++ {
+		mask := uint64(0xffff) << (i * 16)
+		for period := 2; period <= 32; period *= 2 {
+			x := uint64(a.Offset)&^mask | bits.RotateLeft64(uint64(a.Offset), max(period, 16))&mask
+			if isbitcon(x) {
+				return C_MOVCON2
+			}
+		}
+	}
+	if zeroCount == 1 || negCount == 1 {
 		return C_MOVCON3
 	} else {
 		return C_VCON
@@ -7555,6 +7567,31 @@ func (c *ctxt7) omovlconst(as obj.As, p *obj.Prog, a *obj.Addr, rt int, os []uin
 				}
 			}
 			return 2
+		}
+
+		// Look for a two instruction pair, a bit pattern encodeable
+		// as a bitcon immediate plus a fixup MOVK instruction.
+		// Constants like this often occur from strength reduction of divides.
+		for i = 0; i < 4; i++ {
+			mask := uint64(0xffff) << (i * 16)
+			for period := 2; period <= 32; period *= 2 { // TODO: handle period==64 somehow?
+				// Copy in bits from outside of the masked region
+				x := uint64(d)&^mask | bits.RotateLeft64(uint64(d), max(period, 16))&mask
+				if isbitcon(x) {
+					// ORR $c1, ZR, rt
+					os[0] = c.opirr(p, AORR)
+					os[0] |= bitconEncode(x, 64) | uint32(REGZERO&31)<<5 | uint32(rt&31)
+					// MOVK $c2<<(i*16), rt
+					os[1] = c.opirr(p, AMOVK)
+					os[1] |= MOVCONST(d, i, rt)
+					return 2
+				}
+			}
+		}
+		// TODO: other fixups, like ADD or SUB?
+		// TODO: 3-instruction variant, instead of the full MOVD+3*MOVK version below?
+
+		switch {
 
 		case zeroCount == 1:
 			// one MOVZ and two MOVKs
diff --git a/src/cmd/internal/obj/arm64/asm_arm64_test.go b/src/cmd/internal/obj/arm64/asm_arm64_test.go
index 83d137a084..b83db60b40 100644
--- a/src/cmd/internal/obj/arm64/asm_arm64_test.go
+++ b/src/cmd/internal/obj/arm64/asm_arm64_test.go
@@ -38,3 +38,16 @@ func TestMOVK(t *testing.T) {
 		t.Errorf("Got %x want %x\n", x, want)
 	}
 }
+
+func testCombined() (a uint64, b uint64)
+func TestCombined(t *testing.T) {
+	got1, got2 := testCombined()
+	want1 := uint64(0xaaaaaaaaaaaaaaab)
+	want2 := uint64(0x0ff019940ff00ff0)
+	if got1 != want1 {
+		t.Errorf("First result, got %x want %x", got1, want1)
+	}
+	if got2 != want2 {
+		t.Errorf("First result, got %x want %x", got2, want2)
+	}
+}
diff --git a/src/cmd/internal/obj/arm64/asm_arm64_test.s b/src/cmd/internal/obj/arm64/asm_arm64_test.s
index e3fda57775..65d80d1380 100644
--- a/src/cmd/internal/obj/arm64/asm_arm64_test.s
+++ b/src/cmd/internal/obj/arm64/asm_arm64_test.s
@@ -37,3 +37,11 @@ TEXT ·testmovk(SB), NOSPLIT, $0-8
 	MOVK	$(40000<<48), R0
 	MOVD	R0, ret+0(FP)
 	RET
+
+// testCombined() (uint64, uint64)
+TEXT ·testCombined(SB), NOSPLIT, $0-16
+	MOVD	$0xaaaaaaaaaaaaaaab, R0
+	MOVD	$0x0ff019940ff00ff0, R1
+	MOVD	R0, a+0(FP)
+	MOVD	R1, b+8(FP)
+	RET