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.
