fanzha02
ba6bd967d2
cmd/compile/internal/ssa: strengthen phiopt pass
...
The current phiopt pass just transforms the following code
x := false
if b { x = true}
into
x = b
But we find code in runtime.atoi like this:
neg := false
if s[0] == '-' {
neg = true
s = s[1:]
}
The current phiopt pass does not covert it into code like:
neg := s[0] == '-'
if neg { s = s[1:] }
Therefore, this patch strengthens the phiopt pass so that the
boolean Phi value "neg" can be replaced with a copy of control
value "s[0] == '-'", thereby using "cmp+cset" instead of a branch.
But in some cases even replacing the boolean Phis cannot eliminate
this branch. In the following case, this patch replaces "d" with a
copy of "a<0", but the regalloc pass will insert the "Load {c}"
value into an empty block to split the live ranges, which causes
the branch to not be eliminated.
For example:
func test(a, b, c int) (bool, int) {
d := false
if (a<0) {
if (b<0) {
c = c+1
}
d = true
}
return d, c
}
The optimized assembly code:
MOVD "".a(FP), R0
TBZ $63, R0, 48
MOVD "".c+16(FP), R1
ADD $1, R1, R2
MOVD "".b+8(FP), R3
CMP ZR, R3
CSEL LT, R2, R1, R1
CMP ZR, R0
CSET LT, R0
MOVB R0, "".~r3+24(FP)
MOVD R1, "".~r4+32(FP)
RET (R30)
MOVD "".c+16(FP), R1
JMP 28
The benchmark:
name old time/op new time/op delta
pkg:cmd/compile/internal/ssa goos:linux goarch:arm64
PhioptPass 117783.250000ns +- 1% 117219.111111ns +- 1% ~ (p=0.074 n=8+9)
Statistical data from compilecmp tool:
compilecmp local/master -> HEAD
local/master (a826f7dc45#37608
Change-Id: Ic6206b22fd1faf570be9fd3c2511aa6c4ce38cdb
Reviewed-on: https://go-review.googlesource.com/c/go/+/252937
Trust: fannie zhang <Fannie.Zhang@arm.com>
Run-TryBot: fannie zhang <Fannie.Zhang@arm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
2021-03-29 05:50:11 +00:00
