https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106245
--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Roger Sayle <sa...@gcc.gnu.org>: https://gcc.gnu.org/g:e28869670c9879fe7c67caf6cc11af202509ef78 commit r14-4810-ge28869670c9879fe7c67caf6cc11af202509ef78 Author: Roger Sayle <ro...@nextmovesoftware.com> Date: Sat Oct 21 00:06:02 2023 +0100 PR 106245: Split (x<<31)>>31 as -(x&1) in i386.md This patch is the backend piece of a solution to PRs 101955 and 106245, that adds a define_insn_and_split to the i386 backend, to perform sign extension of a single (least significant) bit using and $1 then neg. Previously, (x<<31)>>31 would be generated as sall $31, %eax // 3 bytes sarl $31, %eax // 3 bytes with this patch the backend now generates: andl $1, %eax // 3 bytes negl %eax // 2 bytes Not only is this smaller in size, but microbenchmarking confirms that it's a performance win on both Intel and AMD; Intel sees only a 2% improvement (perhaps just a size effect), but AMD sees a 7% win. 2023-10-21 Roger Sayle <ro...@nextmovesoftware.com> Uros Bizjak <ubiz...@gmail.com> gcc/ChangeLog PR middle-end/101955 PR tree-optimization/106245 * config/i386/i386.md (*extv<mode>_1_0): New define_insn_and_split. gcc/testsuite/ChangeLog PR middle-end/101955 PR tree-optimization/106245 * gcc.target/i386/pr106245-2.c: New test case. * gcc.target/i386/pr106245-3.c: New 32-bit test case. * gcc.target/i386/pr106245-4.c: New 64-bit test case. * gcc.target/i386/pr106245-5.c: Likewise.
