https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103597
--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The trunk branch has been updated by Marek Polacek <mpola...@gcc.gnu.org>: https://gcc.gnu.org/g:d886a5248e66ab911391af18bf955beb87ee8461 commit r12-7899-gd886a5248e66ab911391af18bf955beb87ee8461 Author: Marek Polacek <pola...@redhat.com> Date: Mon Mar 28 18:19:20 2022 -0400 gimple: Wrong -Wimplicit-fallthrough with if(1) [PR103597] This patch fixes a wrong -Wimplicit-fallthrough warning for case 0: if (1) // wrong may fallthrough return 0; case 1: which in .gimple looks like <D.1981>: // case 0 if (1 != 0) goto <D.1985>; else goto <D.1986>; <D.1985>: D.1987 = 0; // predicted unlikely by early return (on trees) predictor. return D.1987; <D.1986>: // dead <D.1982>: // case 1 and the warning thinks that <D.1986>: falls through to <D.1982>:. It does not know that <D.1986> is effectively a dead label, only reachable through fallthrough from previous instructions, never jumped to. To that effect, Jakub introduced UNUSED_LABEL_P, which is set on such dead labels. collect_fallthrough_labels has code to deal with cases like case 2: if (e != 10) i++; // this may fallthru, warn else return 44; case 3: which collects labels that may fall through. Here it sees the "goto <D.1990>;" at the end of the then branch and so when the warning reaches ... <D.1990>: // from if-then <D.1984>: // case 3 it knows it should warn about the possible fallthrough. But an UNUSED_LABEL_P is not a label that can fallthrough like that, so it should ignore those. However, we still want to warn about this: case 0: if (1) n++; // falls through case 1: so collect_fallthrough_labels needs to return the "n = n + 1;" statement, rather than the dead label. Co-authored-by: Jakub Jelinek <ja...@redhat.com> PR middle-end/103597 gcc/ChangeLog: * gimplify.cc (collect_fallthrough_labels): Don't push UNUSED_LABEL_Ps into labels. Maybe set prev to the statement preceding UNUSED_LABEL_P. (gimplify_cond_expr): Set UNUSED_LABEL_P. * tree.h (UNUSED_LABEL_P): New. gcc/testsuite/ChangeLog: * c-c++-common/Wimplicit-fallthrough-39.c: New test.
