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cmd/internal/obj/riscv: improve code generation for loading of constants

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Loading of constants that are 12 bits or smaller is currently performed using a single
ADDIW instruction, while constants between 13 bits and 32 bits are loaded using a
LUI+ADDIW pair.

Instead, use a single ADDI instruction for the 12 bits or smaller case - this
translates to the LI pseudo-instruction, making objdump more readable and giving:

   11c7c:       fff00293                li      t0,-1
   11c80:       00000313                li      t1,0

Rather than:

   11c7c:       fff0029b                addiw   t0,zero,-1
   11c80:       0000031b                sext.w  t1,zero

In the case where a constant exceeds 12 bits, an LUI instruction is required,
however if the lower 12 bits are zero, the ADDIW instruction can be omitted.
The same applies to the case where immediate splitting is performed for other
immediate instructions.

This removes around 900 instructions from the Go binary.

Change-Id: Id6c77774b3b429fa525da018a6926b85df838a2f
Reviewed-on: https://go-review.googlesource.com/c/go/+/344457
Trust: Joel Sing <joel@sing.id.au>
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
This commit is contained in:
Joel Sing 2021-08-09 08:14:04 +00:00
parent 37e9c1d6fe
commit 28dae3defb
2 changed files with 15 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
src/cmd/asm/internal/asm/testdata/riscv64.s vendored
View File

@ -295,10 +295,14 @@ start:
// MOV pseudo-instructions
MOV X5, X6 // 13830200
MOV $2047, X5 // 9b02f07f
MOV $-2048, X5 // 9b020080
MOV $2047, X5 // 9302f07f
MOV $-2048, X5 // 93020080
MOV $2048, X5 // b71200009b820280
MOV $-2049, X5 // b7f2ffff9b82f27f
MOV $4096, X5 // b7120000
MOV $2147479552, X5 // b7f2ff7f
MOV $2147483647, X5 // b70200809b82f2ff
MOV $-2147483647, X5 // b70200809b821200
// Converted to load of symbol (AUIPC + LD)
MOV $4294967296, X5 // 9702000083b20200

12
src/cmd/internal/obj/riscv/obj.go
View File

@ -1669,6 +1669,9 @@ func instructionsForOpImmediate(p *obj.Prog, as obj.As, rs int16) []*instruction
return nil
}
ins.rs2 = REG_TMP
if low == 0 {
return []*instruction{insLUI, ins}
}
return []*instruction{insLUI, insADDIW, ins}
}
@ -1768,7 +1771,7 @@ func instructionsForMOV(p *obj.Prog) []*instruction {
}
// MOV $c, R -> ADD $c, ZERO, R
ins.as, ins.rs1, ins.rs2, ins.imm = AADDIW, REG_ZERO, obj.REG_NONE, low
ins.as, ins.rs1, ins.rs2, ins.imm = AADDI, REG_ZERO, obj.REG_NONE, low
// LUI is only necessary if the constant does not fit in 12 bits.
if high == 0 {
@ -1778,8 +1781,11 @@ func instructionsForMOV(p *obj.Prog) []*instruction {
// LUI top20bits(c), R
// ADD bottom12bits(c), R, R
insLUI := &instruction{as: ALUI, rd: ins.rd, imm: high}
ins.rs1 = ins.rd
inss = []*instruction{insLUI, ins}
inss = []*instruction{insLUI}
if low != 0 {
ins.as, ins.rs1 = AADDIW, ins.rd
inss = append(inss, ins)
}
case p.From.Type == obj.TYPE_REG && p.To.Type == obj.TYPE_REG:
// Handle register to register moves.