https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71336
Daniel Henrique Barboza <daniel.barboza at oss dot qualcomm.com> changed:
What |Removed |Added
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CC| |daniel.barboza at oss dot
qualcomm
| |.com
--- Comment #13 from Daniel Henrique Barboza <daniel.barboza at oss dot
qualcomm.com> ---
A fix proposal was sent to the ML (currently on v2):
https://gcc.gnu.org/pipermail/gcc-patches/2026-March/710154.html
The idea is to generalize the idea Jeff gave in comment #12: for a pattern
like:
int test(int a) {
return a & 1 ? 7 : 3;
}
We can simplify it to
3 + (a & 1) * (7 - 3);
This will remove the conditional and targets can optimize the PLUS pattern at
their leisure. In an armv8-a target, using current trunk, this function will
generate:
test:
.LFB0:
.cfi_startproc
tst x0, 1 // 38 [c=8 l=4] *anddi3nr_compare0_zextract
mov w1, 3 // 41 [c=4 l=4] *movsi_aarch64/3
mov w0, 7 // 42 [c=4 l=4] *movsi_aarch64/3
csel w0, w1, w0, eq // 17 [c=4 l=4] *cmovsi_insn/0
ret // 47 [c=0 l=4] *do_return
.cfi_endproc
With the proposed fix:
.LFB0:
.cfi_startproc
ubfiz w0, w0, 2, 1 // 7 [c=4 l=4] *andim_ashiftsi_bfiz
add w0, w0, 3 // 13 [c=4 l=4] *addsi3_aarch64/0
ret // 21 [c=0 l=4] *do_return
.cfi_endproc
For RISC-V the generated .s is:
test:
.LFB0:
.cfi_startproc
andi a0,a0,1 # 6 [c=4 l=4] *anddi3/1
slliw a0,a0,2 # 7 [c=8 l=4] ashlsi3_extend
addi a0,a0,3 # 16 [c=4 l=4] *adddi3/1
ret # 29 [c=0 l=4] simple_return
.cfi_endproc
This idea of using a mult to re-create an immediate to eliminate a conditional
is also used in 56110 and 123967. I'll send patches for those shortly.
design is going to be used in 56110 and 123967 as well.
