https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94274
--- Comment #3 from z.zhanghaijian at huawei dot com <z.zhanghaijian at huawei
dot com> ---
(In reply to Marc Glisse from comment #1)
> Detecting common beginnings / endings in branches is something gcc does very
> seldom. Even at -Os, for if(cond)f(b);else f(c); we need to wait until
> rtl-optimizations to get a single call to f. (of course the reverse
> transformation of duplicating a statement that was after the branches into
> them, if it simplifies, is nice as well, and they can conflict)
> I don't know if handling one such very specific case (binary operations with
> a common argument) separately is a good idea when we don't even handle unary
> operations.
I tried to test this fold on specint2017 and found some performance gains on
500.perlbench_r. Then compared the assemble and found some improvements.
For example:
S_invlist_max, which is inlined by many functions, such as
S__append_range_to_invlist, S_ssc_anything, Perl__invlist_invert ...
invlist_inline.h:
#define FROM_INTERNAL_SIZE(x) ((x)/ sizeof(UV))
S_invlist_max(inlined by S__append_range_to_invlist, S_ssc_anything,
Perl__invlist_invert, ....)：
return SvLEN(invlist) == 0 /* This happens under _new_invlist_C_array */
? FROM_INTERNAL_SIZE(SvCUR(invlist)) - 1
: FROM_INTERNAL_SIZE(SvLEN(invlist)) - 1;
Dump tree phiopt：
<bb 3> [local count: 536870911]:
_46 = pretmp_112 >> 3;
iftmp.1123_47 = _46 + 18446744073709551615;
goto <bb 5>; [100.00%]
<bb 4> [local count: 536870911]:
_48 = _44 >> 3;
iftmp.1123_49 = _48 + 18446744073709551615;
<bb 5> [local count: 1073741823]:
# iftmp.1123_50 = PHI <iftmp.1123_47(3), iftmp.1123_49(4)>
Which can replaces with：
<bb 3> [local count: 536870912]:
<bb 4> [local count: 1073741823]:
# _48 = PHI <_44(2), pretmp_112(3)>
_49 = _48 >> 3;
iftmp.1123_50 = _49 + 18446744073709551615;
Assemble：
lsr x5, x6, #3
lsr x3, x3, #3
sub x20, x5, #0x1
sub x3, x3, #0x1
csel x20, x3, x20, ne
Replaces with：
csel x3, x3, x4, ne
lsr x3, x3, #3
sub x20, x3, #0x1
This can eliminate two instruction.
