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* [Bug c/94086] New: Missed optimization when converting a bitfield to an integer on x86-64
@ 2020-03-07 19:33 larrowe.semaj11 at gmail dot com
2020-03-08 4:30 ` [Bug tree-optimization/94086] " pinskia at gcc dot gnu.org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: larrowe.semaj11 at gmail dot com @ 2020-03-07 19:33 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94086
Bug ID: 94086
Summary: Missed optimization when converting a bitfield to an
integer on x86-64
Product: gcc
Version: 9.2.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: larrowe.semaj11 at gmail dot com
Target Milestone: ---
When converting a structure containing bitfields to an unsigned 16-bit integer,
inefficient code is generated.
Minimal testcase:
typedef struct half
{
unsigned short mantissa:10;
unsigned short exponent:5;
unsigned short sign:1;
} half;
unsigned short from_half(half h)
{
return h.mantissa | h.exponent << 10 | h.sign << 15;
}
To compile and place output on `stdout`:
gcc -O3 -S test.c -o -
Assembly output:
movl %edi, %edx
movl %edi, %eax
andw $1023, %di
shrw $15, %dx
andl $31744, %eax
movzbl %dl, %edx
sall $15, %edx
orl %edx, %eax
orl %edi, %eax
ret
This could be optimized into:
movl %edi, %eax
retq
This behavior does not occur when `unsigned short` is being converted into
`half` in a similar manner.
`gcc -v`:
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-pc-linux-gnu/9.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with:
/var/tmp/portage/sys-devel/gcc-9.2.0-r3/work/gcc-9.2.0/configure
--host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --prefix=/usr
--bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/9.2.0
--includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/9.2.0/include
--datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/9.2.0
--mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/9.2.0/man
--infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/9.2.0/info
--with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/9.2.0/include/g++-v9
--with-python-dir=/share/gcc-data/x86_64-pc-linux-gnu/9.2.0/python
--enable-objc-gc --enable-languages=c,c++,d,go,objc,obj-c++,fortran
--enable-obsolete --enable-secureplt --disable-werror --with-system-zlib
--enable-nls --without-included-gettext --enable-checking=release
--with-bugurl=https://bugs.gentoo.org/ --with-pkgversion='Gentoo 9.2.0-r3 p4'
--disable-esp --enable-libstdcxx-time --with-build-config=bootstrap-lto
--enable-shared --enable-threads=posix --enable-__cxa_atexit
--enable-clocale=gnu --enable-multilib --with-multilib-list=m32,m64
--disable-altivec --disable-fixed-point --enable-targets=all --enable-libgomp
--disable-libmudflap --disable-libssp --disable-systemtap
--enable-vtable-verify --enable-lto --with-isl --disable-isl-version-check
--enable-default-pie --enable-default-ssp
Thread model: posix
gcc version 9.2.0 (Gentoo 9.2.0-r3 p4)
`-Wall -Wextra` reports nothing.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/94086] Missed optimization when converting a bitfield to an integer on x86-64
2020-03-07 19:33 [Bug c/94086] New: Missed optimization when converting a bitfield to an integer on x86-64 larrowe.semaj11 at gmail dot com
@ 2020-03-08 4:30 ` pinskia at gcc dot gnu.org
2020-03-08 19:05 ` alex_lop at walla dot com
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2020-03-08 4:30 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94086
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |pinskia at gcc dot gnu.org
Severity|normal |enhancement
Component|c |tree-optimization
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/94086] Missed optimization when converting a bitfield to an integer on x86-64
2020-03-07 19:33 [Bug c/94086] New: Missed optimization when converting a bitfield to an integer on x86-64 larrowe.semaj11 at gmail dot com
2020-03-08 4:30 ` [Bug tree-optimization/94086] " pinskia at gcc dot gnu.org
@ 2020-03-08 19:05 ` alex_lop at walla dot com
2020-03-09 9:40 ` rguenth at gcc dot gnu.org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: alex_lop at walla dot com @ 2020-03-08 19:05 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94086
alex_lop at walla dot com changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |alex_lop at walla dot com
--- Comment #1 from alex_lop at walla dot com ---
Relevant discussion on stackoverflow:
https://stackoverflow.com/q/60580591/5218277
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/94086] Missed optimization when converting a bitfield to an integer on x86-64
2020-03-07 19:33 [Bug c/94086] New: Missed optimization when converting a bitfield to an integer on x86-64 larrowe.semaj11 at gmail dot com
2020-03-08 4:30 ` [Bug tree-optimization/94086] " pinskia at gcc dot gnu.org
2020-03-08 19:05 ` alex_lop at walla dot com
@ 2020-03-09 9:40 ` rguenth at gcc dot gnu.org
2020-05-11 5:21 ` egallager at gcc dot gnu.org
2021-08-22 0:29 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2020-03-09 9:40 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94086
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
Last reconfirmed| |2020-03-09
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed. This is another task for a combined bswap/store-merging where the
bswap tracking would need to be extended to cover bits.
Also part of the reason for the missed optimization is that on GIMPLE we
think 'half' is memory but in reality it is in a register.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/94086] Missed optimization when converting a bitfield to an integer on x86-64
2020-03-07 19:33 [Bug c/94086] New: Missed optimization when converting a bitfield to an integer on x86-64 larrowe.semaj11 at gmail dot com
` (2 preceding siblings ...)
2020-03-09 9:40 ` rguenth at gcc dot gnu.org
@ 2020-05-11 5:21 ` egallager at gcc dot gnu.org
2021-08-22 0:29 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: egallager at gcc dot gnu.org @ 2020-05-11 5:21 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94086
Eric Gallager <egallager at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |egallager at gcc dot gnu.org
--- Comment #3 from Eric Gallager <egallager at gcc dot gnu.org> ---
Possibly related to and/or a dup of bug 83784?
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/94086] Missed optimization when converting a bitfield to an integer on x86-64
2020-03-07 19:33 [Bug c/94086] New: Missed optimization when converting a bitfield to an integer on x86-64 larrowe.semaj11 at gmail dot com
` (3 preceding siblings ...)
2020-05-11 5:21 ` egallager at gcc dot gnu.org
@ 2021-08-22 0:29 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-08-22 0:29 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94086
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The testcase in comment #0 in GCC11+ GCC can optimize it to just a move
followed by a return. Note the optimization only happens on the RTL level
even.
I have not looked into what allowed this to be optimized yet.
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2021-08-22 0:29 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
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2020-03-07 19:33 [Bug c/94086] New: Missed optimization when converting a bitfield to an integer on x86-64 larrowe.semaj11 at gmail dot com
2020-03-08 4:30 ` [Bug tree-optimization/94086] " pinskia at gcc dot gnu.org
2020-03-08 19:05 ` alex_lop at walla dot com
2020-03-09 9:40 ` rguenth at gcc dot gnu.org
2020-05-11 5:21 ` egallager at gcc dot gnu.org
2021-08-22 0:29 ` pinskia at gcc dot gnu.org
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