f08995eefb
GCC currently hides the shift and xor reduction inside a backend specific UNSPEC PARITY, making it invisible to the RTL optimizers until very late during compilation. It is normally reasonable for the middle-end to maintain wider mode representations for as long as possible and split them later, but this only helps if the semantics are visible at the RTL-level (to combine and other passes), but UNSPECs are black boxes, so in this case splitting early (during RTL expansion) is a better strategy. It turns out that that popcount instruction on modern x86_64 processors has (almost) made the integer parity flag in the x86 ALU completely obsolete, especially as POPCOUNT's integer semantics are a much better fit to RTL. The one remaining case where these transistors are useful is where __builtin_parity is immediately tested by a conditional branch, and therefore the result is wanted in a flags register rather than as an integer. This case is captured by two peephole2 optimizations in the attached patch. 2020-06-07 Roger Sayle <roger@nextmovesoftware.com> gcc/ChangeLog: * config/i386/i386.md (paritydi2, paritysi2): Expand reduction via shift and xor to an USPEC PARITY matching a parityhi2_cmp. (paritydi2_cmp, paritysi2_cmp): Delete these define_insn_and_split. (parityhi2, parityqi2): New expanders. (parityhi2_cmp): Implement set parity flag with xorb insn. (parityqi2_cmp): Implement set parity flag with testb insn. New peephole2s to use these insns (UNSPEC PARITY) when appropriate. gcc/testsuite/ChangeLog: * gcc.target/i386/parity-3.c: New test. * gcc.target/i386/parity-4.c: Likewise. * gcc.target/i386/parity-5.c: Likewise. * gcc.target/i386/parity-6.c: Likewise. * gcc.target/i386/parity-7.c: Likewise. * gcc.target/i386/parity-8.c: Likewise. * gcc.target/i386/parity-9.c: Likewise. |
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| INSTALL | ||
| config | ||
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| libatomic | ||
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| libffi | ||
| libgcc | ||
| libgfortran | ||
| libgo | ||
| libgomp | ||
| libhsail-rt | ||
| libiberty | ||
| libitm | ||
| libobjc | ||
| liboffloadmic | ||
| libphobos | ||
| libquadmath | ||
| libsanitizer | ||
| libssp | ||
| libstdc++-v3 | ||
| libvtv | ||
| lto-plugin | ||
| maintainer-scripts | ||
| zlib | ||
| .dir-locals.el | ||
| .gitattributes | ||
| .gitignore | ||
| ABOUT-NLS | ||
| COPYING | ||
| COPYING.LIB | ||
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| COPYING3 | ||
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| ChangeLog | ||
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| MAINTAINERS | ||
| Makefile.def | ||
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| compile | ||
| config-ml.in | ||
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| configure | ||
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| depcomp | ||
| install-sh | ||
| libtool-ldflags | ||
| libtool.m4 | ||
| ltgcc.m4 | ||
| ltmain.sh | ||
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| lt~obsolete.m4 | ||
| missing | ||
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| test-driver | ||
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README
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.