https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97756
--- Comment #14 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:0a140730c970870a5125beb1114f6c01679a040e commit r14-5385-g0a140730c970870a5125beb1114f6c01679a040e Author: Roger Sayle <ro...@nextmovesoftware.com> Date: Mon Nov 13 09:05:16 2023 +0000 i386: Improve reg pressure of double word right shift then truncate. This patch improves register pressure during reload, inspired by PR 97756. Normally, a double-word right-shift by a constant produces a double-word result, the highpart of which is dead when followed by a truncation. The dead code calculating the high part gets cleaned up post-reload, so the issue isn't normally visible, except for the increased register pressure during reload, sometimes leading to odd register assignments. Providing a post-reload splitter, which clobbers a single wordmode result register instead of a doubleword result register, helps (a bit). An example demonstrating this effect is: unsigned long foo (__uint128_t n) { unsigned long a = n & MASK60; unsigned long b = (n >> 60); b = b & MASK60; unsigned long c = (n >> 120); return a+b+c; } which currently with -O2 generates (13 instructions): foo: movabsq $1152921504606846975, %rcx xchgq %rdi, %rsi movq %rsi, %rax shrdq $60, %rdi, %rax movq %rax, %rdx movq %rsi, %rax movq %rdi, %rsi andq %rcx, %rax shrq $56, %rsi andq %rcx, %rdx addq %rsi, %rax addq %rdx, %rax ret with this patch, we generate one less mov (12 instructions): foo: movabsq $1152921504606846975, %rcx xchgq %rdi, %rsi movq %rdi, %rdx movq %rsi, %rax movq %rdi, %rsi shrdq $60, %rdi, %rdx andq %rcx, %rax shrq $56, %rsi addq %rsi, %rax andq %rcx, %rdx addq %rdx, %rax ret The significant difference is easier to see via diff: < shrdq $60, %rdi, %rax < movq %rax, %rdx --- > shrdq $60, %rdi, %rdx Admittedly a single "mov" isn't much of a saving on modern architectures, but as demonstrated by the PR, people still track the number of them. 2023-11-13 Roger Sayle <ro...@nextmovesoftware.com> gcc/ChangeLog * config/i386/i386.md (<insn><dwi>3_doubleword_lowpart): New define_insn_and_split to optimize register usage of doubleword right shifts followed by truncation.
