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From the CPU's point of view, getting a cache line for writing is more expensive than reading. See Appendix A.2 Spinlock in: https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/ xeon-lock-scaling-analysis-paper.pdf The full compare and swap will grab the cache line exclusive and causes excessive cache line bouncing. The atomic_fetch_{or,xor,and,nand} builtins generates cmpxchg loop under -march=x86-64 like: movl v(%rip), %eax .L2: movl %eax, %ecx movl %eax, %edx orl $1, %ecx lock cmpxchgl %ecx, v(%rip) jne .L2 movl %edx, %eax andl $1, %eax ret To relax above loop, GCC should first emit a normal load, check and jump to .L2 if cmpxchgl may fail. Before jump to .L2, PAUSE should be inserted to yield the CPU to another hyperthread and to save power, so the code is like .L84: movl (%rdi), %ecx movl %eax, %edx orl %esi, %edx cmpl %eax, %ecx jne .L82 lock cmpxchgl %edx, (%rdi) jne .L84 .L82: rep nop jmp .L84 This patch adds corresponding atomic_fetch_op expanders to insert load/ compare and pause for all the atomic logic fetch builtins. Add flag -mrelax-cmpxchg-loop to control whether to generate relaxed loop. gcc/ChangeLog: PR target/103069 * config/i386/i386-expand.c (ix86_expand_atomic_fetch_op_loop): New expand function. * config/i386/i386-options.c (ix86_target_string): Add -mrelax-cmpxchg-loop flag. (ix86_valid_target_attribute_inner_p): Likewise. * config/i386/i386-protos.h (ix86_expand_atomic_fetch_op_loop): New expand function prototype. * config/i386/i386.opt: Add -mrelax-cmpxchg-loop. * config/i386/sync.md (atomic_fetch_<logic><mode>): New expander for SI,HI,QI modes. (atomic_<logic>_fetch<mode>): Likewise. (atomic_fetch_nand<mode>): Likewise. (atomic_nand_fetch<mode>): Likewise. (atomic_fetch_<logic><mode>): New expander for DI,TI modes. (atomic_<logic>_fetch<mode>): Likewise. (atomic_fetch_nand<mode>): Likewise. (atomic_nand_fetch<mode>): Likewise. * doc/invoke.texi: Document -mrelax-cmpxchg-loop. gcc/testsuite/ChangeLog: PR target/103069 * gcc.target/i386/pr103069-1.c: New test. * gcc.target/i386/pr103069-2.c: Ditto. |
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| c++tools | ||
| config | ||
| contrib | ||
| fixincludes | ||
| gcc | ||
| gnattools | ||
| gotools | ||
| include | ||
| INSTALL | ||
| intl | ||
| libada | ||
| libatomic | ||
| libbacktrace | ||
| libcc1 | ||
| libcody | ||
| libcpp | ||
| libdecnumber | ||
| libffi | ||
| libgcc | ||
| libgfortran | ||
| libgo | ||
| libgomp | ||
| libiberty | ||
| libitm | ||
| libobjc | ||
| liboffloadmic | ||
| libphobos | ||
| libquadmath | ||
| libsanitizer | ||
| libssp | ||
| libstdc++-v3 | ||
| libvtv | ||
| lto-plugin | ||
| maintainer-scripts | ||
| zlib | ||
| .dir-locals.el | ||
| .gitattributes | ||
| .gitignore | ||
| ABOUT-NLS | ||
| ar-lib | ||
| ChangeLog | ||
| ChangeLog.jit | ||
| ChangeLog.tree-ssa | ||
| compile | ||
| config-ml.in | ||
| config.guess | ||
| config.rpath | ||
| config.sub | ||
| configure | ||
| configure.ac | ||
| COPYING | ||
| COPYING.LIB | ||
| COPYING.RUNTIME | ||
| COPYING3 | ||
| COPYING3.LIB | ||
| depcomp | ||
| install-sh | ||
| libtool-ldflags | ||
| libtool.m4 | ||
| ltgcc.m4 | ||
| ltmain.sh | ||
| ltoptions.m4 | ||
| ltsugar.m4 | ||
| ltversion.m4 | ||
| lt~obsolete.m4 | ||
| MAINTAINERS | ||
| Makefile.def | ||
| Makefile.in | ||
| Makefile.tpl | ||
| missing | ||
| mkdep | ||
| mkinstalldirs | ||
| move-if-change | ||
| multilib.am | ||
| README | ||
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| test-driver | ||
| ylwrap | ||
This directory contains the GNU Compiler Collection (GCC). The GNU Compiler Collection is free software. See the files whose names start with COPYING for copying permission. The manuals, and some of the runtime libraries, are under different terms; see the individual source files for details. The directory INSTALL contains copies of the installation information as HTML and plain text. The source of this information is gcc/doc/install.texi. The installation information includes details of what is included in the GCC sources and what files GCC installs. See the file gcc/doc/gcc.texi (together with other files that it includes) for usage and porting information. An online readable version of the manual is in the files gcc/doc/gcc.info*. See http://gcc.gnu.org/bugs/ for how to report bugs usefully. Copyright years on GCC source files may be listed using range notation, e.g., 1987-2012, indicating that every year in the range, inclusive, is a copyrightable year that could otherwise be listed individually.