public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
* [Bug tree-optimization/103565] New: GCC emits more assembly than clang for carry flag
@ 2021-12-05 15:41 unlvsur at live dot com
2021-12-05 15:48 ` [Bug tree-optimization/103565] " unlvsur at live dot com
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: unlvsur at live dot com @ 2021-12-05 15:41 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103565
Bug ID: 103565
Summary: GCC emits more assembly than clang for carry flag
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: unlvsur at live dot com
Target Milestone: ---
I tried both examples. One emulation with the pattern. Another is with the x86
intrinsic. GCC emits more instructions than clang.
https://godbolt.org/z/d15WEY85T
https://godbolt.org/z/cob36P8nz
Also can GCC be able to understand the pattern of add_carry_pattern just like
it understands std::rotl?
https://github.com/gcc-mirror/gcc/blob/8d4ef2299cbf9517877dab60d48f34835758a6ee/libstdc%2B%2B-v3/include/std/bit#L135
template<typename T>
inline constexpr bool add_carry_pattern(bool carry,T a,T b,T& out) noexcept
{
T temp{carry+a};
out=temp+b;
return (out < b) | (temp < a);
}
So we do not need that intrinsic anymore and the whole thing can be optimized
at SSA level, rather than RTL level??
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/103565] GCC emits more assembly than clang for carry flag
2021-12-05 15:41 [Bug tree-optimization/103565] New: GCC emits more assembly than clang for carry flag unlvsur at live dot com
@ 2021-12-05 15:48 ` unlvsur at live dot com
2021-12-06 21:47 ` pinskia at gcc dot gnu.org
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: unlvsur at live dot com @ 2021-12-05 15:48 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103565
--- Comment #1 from cqwrteur <unlvsur at live dot com> ---
https://godbolt.org/z/bGfY6zh9x
#include<concepts>
template<std::unsigned_integral T>
inline constexpr bool add_carry_pattern(bool carry,T a,T b,T& out) noexcept
{
T temp{carry+a};
out=temp+b;
return (out < b) | (temp < a);
}
Well. it should be restricted with unsigned_integral.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/103565] GCC emits more assembly than clang for carry flag
2021-12-05 15:41 [Bug tree-optimization/103565] New: GCC emits more assembly than clang for carry flag unlvsur at live dot com
2021-12-05 15:48 ` [Bug tree-optimization/103565] " unlvsur at live dot com
@ 2021-12-06 21:47 ` pinskia at gcc dot gnu.org
2021-12-06 21:49 ` [Bug target/103565] " pinskia at gcc dot gnu.org
2021-12-06 21:54 ` unlvsur at live dot com
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-12-06 21:47 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103565
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The difference is just argument and return register differences (and maybe a
register allocation issue).
That is the extra instructions are:
for add_carry_pattern_test:
movzx edi, dil
mov r8, rcx
xor ecx, ecx
for add_carry_x86_intrinsics:
movzx edi, dil
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug target/103565] GCC emits more assembly than clang for carry flag
2021-12-05 15:41 [Bug tree-optimization/103565] New: GCC emits more assembly than clang for carry flag unlvsur at live dot com
2021-12-05 15:48 ` [Bug tree-optimization/103565] " unlvsur at live dot com
2021-12-06 21:47 ` pinskia at gcc dot gnu.org
@ 2021-12-06 21:49 ` pinskia at gcc dot gnu.org
2021-12-06 21:54 ` unlvsur at live dot com
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-12-06 21:49 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103565
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Severity|normal |enhancement
Component|tree-optimization |target
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The tree level looks good:
_6 = (long long unsigned int) carry_1(D);
_13 = .ADD_OVERFLOW (a_3(D), _6);
temp_7 = REALPART_EXPR <_13>;
_14 = IMAGPART_EXPR <_13>;
_15 = .ADD_OVERFLOW (b_4(D), temp_7);
_8 = REALPART_EXPR <_15>;
_16 = IMAGPART_EXPR <_15>;
*out_5(D) = _8;
_9 = _16 != 0;
_10 = _14 != 0;
_11 = _9 | _10;
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug target/103565] GCC emits more assembly than clang for carry flag
2021-12-05 15:41 [Bug tree-optimization/103565] New: GCC emits more assembly than clang for carry flag unlvsur at live dot com
` (2 preceding siblings ...)
2021-12-06 21:49 ` [Bug target/103565] " pinskia at gcc dot gnu.org
@ 2021-12-06 21:54 ` unlvsur at live dot com
3 siblings, 0 replies; 5+ messages in thread
From: unlvsur at live dot com @ 2021-12-06 21:54 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103565
--- Comment #4 from cqwrteur <unlvsur at live dot com> ---
(In reply to Andrew Pinski from comment #3)
> The tree level looks good:
> _6 = (long long unsigned int) carry_1(D);
> _13 = .ADD_OVERFLOW (a_3(D), _6);
> temp_7 = REALPART_EXPR <_13>;
> _14 = IMAGPART_EXPR <_13>;
> _15 = .ADD_OVERFLOW (b_4(D), temp_7);
> _8 = REALPART_EXPR <_15>;
> _16 = IMAGPART_EXPR <_15>;
> *out_5(D) = _8;
> _9 = _16 != 0;
> _10 = _14 != 0;
> _11 = _9 | _10;
so is that possible to understand the pattern of addcarry in C or C++ for GCC?
clang does that by adding new builtins but Jakub said pattern matching is
better since the compiler can optimize it later.
I think it is totally possible for using pattern matching instead of built-in.
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2021-12-06 21:54 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-12-05 15:41 [Bug tree-optimization/103565] New: GCC emits more assembly than clang for carry flag unlvsur at live dot com
2021-12-05 15:48 ` [Bug tree-optimization/103565] " unlvsur at live dot com
2021-12-06 21:47 ` pinskia at gcc dot gnu.org
2021-12-06 21:49 ` [Bug target/103565] " pinskia at gcc dot gnu.org
2021-12-06 21:54 ` unlvsur at live dot com
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).