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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) -- links below jump to the message on this page -- 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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