https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107172
--- Comment #51 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:69185294f322dd53d4e1592115014c5488302e2e commit r14-1405-g69185294f322dd53d4e1592115014c5488302e2e Author: Roger Sayle <ro...@nextmovesoftware.com> Date: Tue May 30 14:40:50 2023 +0100 PR target/107172: Avoid "unusual" MODE_CC comparisons in simplify-rtx.cc I believe that a better (or supplementary) fix to PR target/107172 is to avoid producing incorrect (but valid) RTL in simplify_const_relational_operation when presented with questionable (obviously invalid) expressions, such as those produced during combine. Just as with the "first do no harm" clause with the Hippocratic Oath, simplify-rtx (probably) shouldn't unintentionally transform invalid RTL expressions, into incorrect (non-equivalent) but valid RTL that may be inappropriately recognized by recog. In this specific case, many GCC backends represent their flags register via MODE_CC, whose representation is intentionally "opaque" to the middle-end. The only use of MODE_CC comprehensible to the middle-end's RTL optimizers is relational comparisons between the result of a COMPARE rtx (op0) and zero (op1). Any other uses of MODE_CC should be left alone, and some might argue indicate representational issues in the backend. In practice, CPUs occasionally have numerous instructions that affect the flags register(s) other than comparisons [AVR's setc, powerpc's mtcrf, x86's clc, stc and cmc and x86_64's ptest that sets C and Z flags in non-obvious ways, c.f. PR target/109973]. Currently care has to be taken, wrapping these in UNSPEC, to avoid combine inappropriately merging flags setters with flags consumers (such as conditional jumps). It's safer to teach simplify_const_relational_operation not to modify expressions that it doesn't understand/recognize. 2023-05-30 Roger Sayle <ro...@nextmovesoftware.com> gcc/ChangeLog PR target/107172 * simplify-rtx.cc (simplify_const_relational_operation): Return early if we have a MODE_CC comparison that isn't a COMPARE against const0_rtx.
