https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120038
Bug ID: 120038
Summary: Missed combining shr and shrx in collatz_f1()
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: kaelfandrew at gmail dot com
Target Milestone: ---
Created attachment 61258
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=61258&action=edit
C code
collatz_f1() uses shr then tzcnt then shrx.
collatz_f0() uses only tzcnt then shrx.
collatz_f0() speeds up by 10% when I tested it.
Flags are -O3 -march=icelake-client
gcc -v
============================
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-pc-linux-gnu/16/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-pc-linux-gnu
Configured with:
/var/tmp/portage/sys-devel/gcc-16.0.9999/work/gcc-16.0.9999/configure
--host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --prefix=/usr
--bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/16
--includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/16/include
--datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/16
--mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/16/man
--infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/16/info
--with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/16/include/g++-v16
--disable-silent-rules --disable-dependency-tracking
--with-python-dir=/share/gcc-data/x86_64-pc-linux-gnu/16/python
--enable-languages=c,c++ --enable-obsolete --enable-secureplt --disable-werror
--with-system-zlib --enable-nls --without-included-gettext
--disable-libunwind-exceptions --enable-checking=yes,extra
--with-bugurl=https://bugs.gentoo.org/ --with-pkgversion='Gentoo 16.0.9999 p,
commit 05d9c70131be267bfe5b36e663acda8729da6456' --with-gcc-major-version-only
--enable-libstdcxx-time --enable-lto --disable-libstdcxx-pch --enable-shared
--enable-threads=posix --enable-__cxa_atexit --enable-clocale=gnu
--enable-multilib --with-multilib-list=m32,m64 --disable-fixed-point
--enable-targets=all --enable-offload-defaulted
--enable-offload-targets=nvptx-none --enable-libgomp --disable-libssp
--disable-libada --enable-cet --disable-systemtap
--disable-valgrind-annotations --disable-vtable-verify --disable-libvtv
--with-zstd --with-isl --disable-isl-version-check --enable-default-pie
--enable-host-pie --enable-host-bind-now --enable-default-ssp
--disable-fixincludes --with-gxx-libcxx-include-dir=/usr/include/c++/v1
--with-build-config='bootstrap-lto bootstrap-cet'
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 16.0.0 20250423 (experimental)
e9a81addd5b7d018e173fa8d59aafc2f84e41d8b (Gentoo 16.0.9999 p, commit
05d9c70131be267bfe5b36e663acda8729da6456)
=========================================
