cmd/internal/obj/arm64: avoid unnecessary literal pool usage for moves

In a number of load and store cases, the use of the literal pool can be
entirely avoided by simply adding or subtracting the offset from the
register. This uses the same number of instructions, while avoiding a
load from memory, along with the need for the value to be in the literal
pool. Overall this reduces the size of binaries slightly and should have
lower overhead.

Updates #59615

Change-Id: I9cb6a403dc71e34a46af913f5db87dbf52f8688c
Reviewed-on: https://go-review.googlesource.com/c/go/+/512539
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>

1 files changed, 42 insertions, 4 deletions

diff --git a/src/cmd/internal/obj/arm64/asm7.go b/src/cmd/internal/obj/arm64/asm7.go
index 72e6aadce4..05cf62773e 100644
--- a/src/cmd/internal/obj/arm64/asm7.go
+++ b/src/cmd/internal/obj/arm64/asm7.go
@@ -1959,6 +1959,10 @@ func (c *ctxt7) loadStoreClass(p *obj.Prog, lsc int, v int64) int {
 	}
 
 	needsPool := true
+	if v >= -4095 && v <= 4095 {
+		needsPool = false
+	}
+
 	switch p.As {
 	case AMOVB, AMOVBU:
 		if cmp(C_UAUTO4K, lsc) || cmp(C_UOREG4K, lsc) {
@@ -4015,10 +4019,13 @@ func (c *ctxt7) asmout(p *obj.Prog, o *Optab, out []uint32) {
 		o1 |= uint32(p.From.Reg&31)<<5 | uint32(p.To.Reg&31)
 
 	case 30: /* movT R,L(R) -> strT */
-		// if offset L can be split into hi+lo, and both fit into instructions, do
+		// If offset L fits in a 12 bit unsigned immediate:
+		//	add $L, R, Rtmp  or  sub $L, R, Rtmp
+		//	str R, (Rtmp)
+		// Otherwise, if offset L can be split into hi+lo, and both fit into instructions:
 		//	add $hi, R, Rtmp
 		//	str R, lo(Rtmp)
-		// otherwise, use constant pool
+		// Otherwise, use constant pool:
 		//	mov $L, Rtmp (from constant pool)
 		//	str R, (R+Rtmp)
 		s := movesize(o.as)
@@ -4032,6 +4039,20 @@ func (c *ctxt7) asmout(p *obj.Prog, o *Optab, out []uint32) {
 		}
 
 		v := c.regoff(&p.To)
+		if v >= -256 && v <= 256 {
+			c.ctxt.Diag("%v: bad type for offset %d (should be 9 bit signed immediate store)", p, v)
+		}
+		if v >= 0 && v <= 4095 && v&((1<<int32(s))-1) == 0 {
+			c.ctxt.Diag("%v: bad type for offset %d (should be 12 bit unsigned immediate store)", p, v)
+		}
+
+		// Handle smaller unaligned and negative offsets via addition or subtraction.
+		if v >= -4095 && v <= 4095 {
+			o1 = c.oaddi12(p, v, REGTMP, int16(r))
+			o2 = c.olsr12u(p, c.opstr(p, p.As), 0, REGTMP, p.From.Reg)
+			break
+		}
+
 		hi, lo, err := splitImm24uScaled(v, s)
 		if err != nil {
 			goto storeusepool
@@ -4054,10 +4075,13 @@ func (c *ctxt7) asmout(p *obj.Prog, o *Optab, out []uint32) {
 		o2 = c.olsxrr(p, int32(c.opstrr(p, p.As, false)), int(p.From.Reg), int(r), REGTMP)
 
 	case 31: /* movT L(R), R -> ldrT */
-		// if offset L can be split into hi+lo, and both fit into instructions, do
+		// If offset L fits in a 12 bit unsigned immediate:
+		//	add $L, R, Rtmp  or  sub $L, R, Rtmp
+		//	ldr R, (Rtmp)
+		// Otherwise, if offset L can be split into hi+lo, and both fit into instructions:
 		//	add $hi, R, Rtmp
 		//	ldr lo(Rtmp), R
-		// otherwise, use constant pool
+		// Otherwise, use constant pool:
 		//	mov $L, Rtmp (from constant pool)
 		//	ldr (R+Rtmp), R
 		s := movesize(o.as)
@@ -4071,6 +4095,20 @@ func (c *ctxt7) asmout(p *obj.Prog, o *Optab, out []uint32) {
 		}
 
 		v := c.regoff(&p.From)
+		if v >= -256 && v <= 256 {
+			c.ctxt.Diag("%v: bad type for offset %d (should be 9 bit signed immediate load)", p, v)
+		}
+		if v >= 0 && v <= 4095 && v&((1<<int32(s))-1) == 0 {
+			c.ctxt.Diag("%v: bad type for offset %d (should be 12 bit unsigned immediate load)", p, v)
+		}
+
+		// Handle smaller unaligned and negative offsets via addition or subtraction.
+		if v >= -4095 && v <= 4095 {
+			o1 = c.oaddi12(p, v, REGTMP, int16(r))
+			o2 = c.olsr12u(p, c.opldr(p, p.As), 0, REGTMP, p.To.Reg)
+			break
+		}
+
 		hi, lo, err := splitImm24uScaled(v, s)
 		if err != nil {
 			goto loadusepool