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cmd/compile: optimise subtraction with const on riscv64

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Convert subtraction from const to a negated ADDI with negative const
value, where possible. At worst this avoids a register load and uses
the same number of instructions. At best, this allows for further
optimisation to occur, particularly where equality is involved.

For example, this sequence:

   li      t0,-1
   sub     t1,t0,a0
   snez    t1,t1

Becomes:

   addi    t0,a0,1
   snez    t0,t0

Removes more than 2000 instructions from the Go binary on linux/riscv64.

Change-Id: I68f3be897bc645d4a8fa3ab3cef165a00a74df19
Reviewed-on: https://go-review.googlesource.com/c/go/+/426263
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Joel Sing <joel@sing.id.au>
This commit is contained in:
Joel Sing 2022-08-28 05:23:28 +10:00
parent 646c3eee06
commit 3e11e61f3c
2 changed files with 23 additions and 1 deletions
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3
src/cmd/compile/internal/ssa/gen/RISCV64.rules
View File

@ -748,8 +748,9 @@
(SRL x (MOVDconst [val])) => (SRLI [int64(val&63)] x)
(SRA x (MOVDconst [val])) => (SRAI [int64(val&63)] x)
// Convert subtraction of a const into ADDI with negative immediate, where possible.
// Convert const subtraction into ADDI with negative immediate, where possible.
(SUB x (MOVDconst [val])) && is32Bit(-val) => (ADDI [-val] x)
(SUB <t> (MOVDconst [val]) y) && is32Bit(-val) => (NEG (ADDI <t> [-val] y))
// Subtraction of zero.
(SUB x (MOVDconst [0])) => x

21
src/cmd/compile/internal/ssa/rewriteRISCV64.go
View File

@ -5495,6 +5495,7 @@ func rewriteValueRISCV64_OpRISCV64SRLI(v *Value) bool {
func rewriteValueRISCV64_OpRISCV64SUB(v *Value) bool {
v_1 := v.Args[1]
v_0 := v.Args[0]
b := v.Block
// match: (SUB x (MOVDconst [val]))
// cond: is32Bit(-val)
// result: (ADDI [-val] x)
@ -5512,6 +5513,26 @@ func rewriteValueRISCV64_OpRISCV64SUB(v *Value) bool {
v.AddArg(x)
return true
}
// match: (SUB <t> (MOVDconst [val]) y)
// cond: is32Bit(-val)
// result: (NEG (ADDI <t> [-val] y))
for {
t := v.Type
if v_0.Op != OpRISCV64MOVDconst {
break
}
val := auxIntToInt64(v_0.AuxInt)
y := v_1
if !(is32Bit(-val)) {
break
}
v.reset(OpRISCV64NEG)
v0 := b.NewValue0(v.Pos, OpRISCV64ADDI, t)
v0.AuxInt = int64ToAuxInt(-val)
v0.AddArg(y)
v.AddArg(v0)
return true
}
// match: (SUB x (MOVDconst [0]))
// result: x
for {