https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116039
--- Comment #3 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jeff Law <l...@gcc.gnu.org>: https://gcc.gnu.org/g:34fb0feca71f763b2fbe832548749666d34a4a76 commit r15-2321-g34fb0feca71f763b2fbe832548749666d34a4a76 Author: Jeff Law <j...@ventanamicro.com> Date: Thu Jul 25 12:32:28 2024 -0600 [PR rtl-optimization/116039] Fix life computation for promoted subregs So this turned out to be a neat little test and while the fuzzer found it on RISC-V, I wouldn't be surprised if the underlying issue is also the root cause of the loongarch issue with ext-dce. The key issue is that if we have something like (set (dest) (any_extend (subreg (source)))) If the subreg object is marked with SUBREG_PROMOTED and the sign/unsigned state matches the any_extend opcode, then combine (and I guess anything using simplify-rtx) may simplify that to (set (dest) (source)) That implies that bits outside the mode of the subreg are actually live and valid. This needs to be accounted for during liveness computation. We have to be careful here though. If we're too conservative about setting additional bits live, then we'll inhibit the desired optimization in the coremark examples. To do a good job we need to know the extension opcode. I'm extremely unhappy with how the use handling works in ext-dce. It mixes different conceptual steps and has horribly complex control flow. It only handles a subset of the unary/binary opcodes, etc etc. It's just damn mess. It's going to need some more noodling around. In the mean time this is a bit hacky in that it depends on non-obvious behavior to know it can get the extension opcode, but I don't want to leave the trunk in a broken state while I figure out the refactoring problem. Bootstrapped and regression tested on x86 and tested on the crosses. Pushing to the trunk. PR rtl-optimization/116039 gcc/ * ext-dce.cc (ext_dce_process_uses): Add some comments about concerns with current code. Mark additional bit groups as live when we have an extension of a suitably promoted subreg. gcc/testsuite * gcc.dg/torture/pr116039.c: New test.
