https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111267
--- Comment #11 from GCC 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:86de9b66480b710202a2898cf513db105d8c432f commit r14-8319-g86de9b66480b710202a2898cf513db105d8c432f Author: Roger Sayle <ro...@nextmovesoftware.com> Date: Sun Jan 21 21:22:28 2024 +0000 PR rtl-optimization/111267: Improved forward propagation. This patch resolves PR rtl-optimization/111267 by improving RTL-level forward propagation. This x86_64 code quality regression was caused (exposed) by my changes to improve how x86's (TImode) argument passing is represented at the RTL-level (reducing the use of SUBREGs to catch more optimization opportunities in combine). The pitfall is that the more complex RTL representations expose a limitation in RTL's fwprop pass. At the heart of fwprop, in try_fwprop_subst_pattern, the logic can be summarized as three steps. Step 1 is a heuristic that rejects the propagation attempt if the expression is too complex, step 2 calls the backend's recog to see if the propagated/simplified instruction is recognizable/valid, and step 3 then calls set_src_cost to compare the rtx costs of the replacement vs. the original, and accepts the transformation if the final cost is the same of better. The logic error (or missed optimization opportunity) is that the step 1 heuristic that attempts to predict (second guess) the process is flawed. Ultimately the decision on whether to fwprop or not should depend solely on actual improvement, as measured by RTX costs. Hence the prototype fix in the bugzilla PR removes the heuristic of calling prop.profitable_p entirely, relying entirely on the cost comparison in step 3. Unfortunately, things are a tiny bit more complicated. The cost comparison in fwprop uses the older set_src_cost API and not the newer (preffered) insn_cost API as currently used in combine. This means that the cost improvement comparisons are only done for single_set instructions (more complex PARALLELs etc. aren't supported). Hence we can only rely on skipping step 1 for that subset of instructions actually evaluated by step 3. The other subtlety is that to avoid potential infinite loops in fwprop we should only reply purely on rtx costs when the transformation is obviously an improvement. If the replacement has the same cost as the original, we can use the prop.profitable_p test to preserve the current behavior. Finally, to answer Richard Biener's remaining question about this approach: yes, there is an asymmetry between how patterns are handled and how REG_EQUAL notes are handled. For example, at the moment propagation into notes doesn't use rtx costs at all, and ultimately when fwprop is updated to use insn_cost, this (and recog) obviously isn't applicable to notes. There's no reason the logic need be identical between patterns and notes, and during stage4 we only need update propagation into patterns to fix this P1 regression (notes and use of cost_insn can be done for GCC 15). For Jakub's reduced testcase: struct S { float a, b, c, d; }; int bar (struct S x, struct S y) { return x.b <= y.d && x.c >= y.a; } On x86_64-pc-linux-gnu with -O2 gcc currently generates: bar: movq %xmm2, %rdx movq %xmm3, %rax movq %xmm0, %rsi xchgq %rdx, %rax movq %rsi, %rcx movq %rax, %rsi movq %rdx, %rax shrq $32, %rcx shrq $32, %rax movd %ecx, %xmm4 movd %eax, %xmm0 comiss %xmm4, %xmm0 jb .L6 movd %esi, %xmm0 xorl %eax, %eax comiss %xmm0, %xmm1 setnb %al ret .L6: xorl %eax, %eax ret with this simple patch to fwprop, we now generate: bar: shufps $85, %xmm0, %xmm0 shufps $85, %xmm3, %xmm3 comiss %xmm0, %xmm3 jb .L6 xorl %eax, %eax comiss %xmm2, %xmm1 setnb %al ret .L6: xorl %eax, %eax ret 2024-01-21 Roger Sayle <ro...@nextmovesoftware.com> Richard Biener <rguent...@suse.de> gcc/ChangeLog PR rtl-optimization/111267 * fwprop.cc (fwprop_propagation::profitabe_p): Rename profitable_p method to likely_profitable_p. (try_fwprop_subst_node): Update call to likely_profitable_p. Only bail-out early when !prop.likely_profitable_p for instructions that are not single sets. When comparing costs, bail-out if the cost is unchanged and !prop.likely_profitable_p. gcc/testsuite/ChangeLog PR rtl-optimization/111267 * gcc.target/i386/pr111267.c: New test case.
