https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43644
--- Comment #2 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:d8a6945c6ea22efa4d5e42fe1922d2b27953c8cd commit r14-554-gd8a6945c6ea22efa4d5e42fe1922d2b27953c8cd Author: Roger Sayle <ro...@nextmovesoftware.com> Date: Sun May 7 07:52:15 2023 +0100 Don't call emit_clobber in lower-subreg.cc's resolve_simple_move. Following up on posts/reviews by Segher and Uros, there's some question over why the middle-end's lower subreg pass emits a clobber (of a multi-word register) into the instruction stream before emitting the sequence of moves of the word-sized parts. This clobber interferes with (LRA) register allocation, preventing the multi-word pseudo to remain in the same hard registers. This patch eliminates this (presumably superfluous) clobber and thereby improves register allocation. A concrete example of the observed improvement is PR target/43644. For the test case: __int128 foo(__int128 x, __int128 y) { return x+y; } on x86_64-pc-linux-gnu, gcc -O2 currently generates: foo: movq %rsi, %rax movq %rdi, %r8 movq %rax, %rdi movq %rdx, %rax movq %rcx, %rdx addq %r8, %rax adcq %rdi, %rdx ret with this patch, we now generate the much improved: foo: movq %rdx, %rax movq %rcx, %rdx addq %rdi, %rax adcq %rsi, %rdx ret 2023-05-07 Roger Sayle <ro...@nextmovesoftware.com> gcc/ChangeLog PR target/43644 * lower-subreg.cc (resolve_simple_move): Don't emit a clobber immediately before moving a multi-word register by parts. gcc/testsuite/ChangeLog PR target/43644 * gcc.target/i386/pr43644.c: New test case.
