cmd/internal/obj/riscv: optimize loading large immediate with trailing zeros

This CL can avoid a memory load for some immediate.
Reduce binary size slightly.

compilecmp HEAD~1 -> HEAD
HEAD~1 (9860faa512): math/big: remove underscores from Binomial docs
HEAD (ac85312f10): cmd/internal/obj/riscv: optimize loading large immediate with many trailing zeros
platform: linux/riscv64

file      before    after     Δ       %
addr2line 3892904   3889844   -3060   -0.079%
api       5558527   5555073   -3454   -0.062%
asm       5191725   5187853   -3872   -0.075%
buildid   2631958   2629356   -2602   -0.099%
cgo       4722642   4718516   -4126   -0.087%
compile   25579930  25564496  -15434  -0.060%
cover     4989560   4985580   -3980   -0.080%
dist      3538991   3536299   -2692   -0.076%
doc       3990845   3987605   -3240   -0.081%
fix       3410945   3407097   -3848   -0.113%
link      6701702   6697926   -3776   -0.056%
nm        3802030   3798664   -3366   -0.089%
objdump   4211373   4207651   -3722   -0.088%
pack      2423248   2421048   -2200   -0.091%
pprof     14302052  14296714  -5338   -0.037%
test2json 2724359   2721881   -2478   -0.091%
trace     13598607  13590879  -7728   -0.057%
vet       7612108   7607722   -4386   -0.058%
go        15092773  15087323  -5450   -0.036%
total     133976279 133891527 -84752  -0.063%

Change-Id: I8615c7830ebfee0386f95d0c0fc4d29dc0b4c7fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/448635
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Wayne Zuo <wdvxdr@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>

src/cmd/asm/internal/asm/testdata/riscv64.s

@@ -318,7 +318,9 @@ start:
 	MOV	$-2147483647, X5			// b70200809b821200
 
 	// Converted to load of symbol (AUIPC + LD)
-	MOV	$4294967296, X5				// 9702000083b20200
+	MOV	$4294967295, X5				// 9702000083b20200
+	// Converted to MOV $1, X5 + SLLI $32, X5
+	MOV	$4294967296, X5				// 9302100093920202
 
 	MOV	(X5), X6				// 03b30200
 	MOV	4(X5), X6				// 03b34200

src/cmd/internal/obj/riscv/obj.go

@@ -26,6 +26,7 @@ import (
 	"cmd/internal/sys"
 	"fmt"
 	"log"
+	"math/bits"
 )
 
 func buildop(ctxt *obj.Link) {}
@@ -140,8 +141,14 @@ func progedit(ctxt *obj.Link, p *obj.Prog, newprog obj.ProgAlloc) {
 		p.As = AEBREAK
 
 	case AMOV:
-		// Put >32-bit constants in memory and load them.
 		if p.From.Type == obj.TYPE_CONST && p.From.Name == obj.NAME_NONE && p.From.Reg == obj.REG_NONE && int64(int32(p.From.Offset)) != p.From.Offset {
+			ctz := bits.TrailingZeros64(uint64(p.From.Offset))
+			val := p.From.Offset >> ctz
+			if int64(int32(val)) == val {
+				// It's ok. We can handle constants with many trailing zeros.
+				break
+			}
+			// Put >32-bit constants in memory and load them.
 			p.From.Type = obj.TYPE_MEM
 			p.From.Sym = ctxt.Int64Sym(p.From.Offset)
 			p.From.Name = obj.NAME_EXTERN
@@ -1838,6 +1845,23 @@ func instructionsForMOV(p *obj.Prog) []*instruction {
 			return nil
 		}
 
+		// For constants larger than 32 bits in size that have trailing zeros,
+		// use the value with the trailing zeros removed and then use a SLLI
+		// instruction to restore the original constant.
+		// For example:
+		// 	MOV $0x8000000000000000, X10
+		// becomes
+		// 	MOV $1, X10
+		// 	SLLI $63, X10, X10
+		var insSLLI *instruction
+		if !immIFits(ins.imm, 32) {
+			ctz := bits.TrailingZeros64(uint64(ins.imm))
+			if immIFits(ins.imm>>ctz, 32) {
+				ins.imm = ins.imm >> ctz
+				insSLLI = &instruction{as: ASLLI, rd: ins.rd, rs1: ins.rd, imm: int64(ctz)}
+			}
+		}
+
 		low, high, err := Split32BitImmediate(ins.imm)
 		if err != nil {
 			p.Ctxt.Diag("%v: constant %d too large: %v", p, ins.imm, err)
@@ -1849,6 +1873,9 @@ func instructionsForMOV(p *obj.Prog) []*instruction {
 
 		// LUI is only necessary if the constant does not fit in 12 bits.
 		if high == 0 {
+			if insSLLI != nil {
+				inss = append(inss, insSLLI)
+			}
 			break
 		}
 
@@ -1860,6 +1887,9 @@ func instructionsForMOV(p *obj.Prog) []*instruction {
 			ins.as, ins.rs1 = AADDIW, ins.rd
 			inss = append(inss, ins)
 		}
+		if insSLLI != nil {
+			inss = append(inss, insSLLI)
+		}
 
 	case p.From.Type == obj.TYPE_CONST && p.To.Type != obj.TYPE_REG:
 		p.Ctxt.Diag("%v: constant load must target register", p)