On Tue, Mar 29, 2022 at 12:13:51PM -0400, Marek Polacek wrote:
> 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>
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> 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.
LGTM.
Jakub
