https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109849
--- Comment #16 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jan Hubicka <hubi...@gcc.gnu.org>: https://gcc.gnu.org/g:7b34cacc5735385e7e2855d7c0a6fad60ef4a99b commit r14-1951-g7b34cacc5735385e7e2855d7c0a6fad60ef4a99b Author: Jan Hubicka <j...@suse.cz> Date: Mon Jun 19 18:28:17 2023 +0200 optimize std::max early we currently produce very bad code on loops using std::vector as a stack, since we fail to inline push_back which in turn prevents SRA and we fail to optimize out some store-to-load pairs. I looked into why this function is not inlined and it is inlined by clang. We currently estimate it to 66 instructions and inline limits are 15 at -O2 and 30 at -O3. Clang has similar estimate, but still decides to inline at -O2. I looked into reason why the body is so large and one problem I spotted is the way std::max is implemented by taking and returning reference to the values. const T& max( const T& a, const T& b ); This makes it necessary to store the values to memory and load them later and max is used by code computing new size of vector on resize. We optimize this to MAX_EXPR, but only during late optimizations. I think this is a common enough coding pattern and we ought to make this transparent to early opts and IPA. The following is easist fix that simply adds phiprop pass that turns the PHI of address values into PHI of values so later FRE can propagate values across memory, phiopt discover the MAX_EXPR pattern and DSE remove the memory stores. gcc/ChangeLog: PR tree-optimization/109811 PR tree-optimization/109849 * passes.def: Add phiprop to early optimization passes. * tree-ssa-phiprop.cc: Allow clonning. gcc/testsuite/ChangeLog: PR tree-optimization/109811 PR tree-optimization/109849 * gcc.dg/tree-ssa/phiprop-1.c: New test. * gcc.dg/tree-ssa/pr21463.c: Adjust template.
