Re: [PATCH 1/2] compare-elim: Test the successor's live-in set for the flags register

On 8/9/2026 1:25 PM, [email protected] wrote:
> From: Kyrylo Tkachov <[email protected]>
>
> find_comparisons_in_bb looks for a flags value that is live out of BB and
> live into a successor that is not part of the extended basic block.  The
> edge walk computes DEST but then queries the live-in set of BB rather than
> of DEST, so the answer says nothing about the successor being examined.
> The pass can therefore leave missing_uses clear for a comparison whose
> flags really are used in a join block.
>
> The test read DF_LIVE_BB_INFO (dest)->in when the pass was added in
> r169131.  r179749 mechanically converted the accessors and changed DEST to
> BB at the same time.  Restore the original test.
>
> The condition is only reachable when the flags register is live across a
> basic block boundary, which is why this has gone unnoticed.
>
> Bootstrapped and tested on aarch64-none-linux-gnu.
> Ok for trunk?
> Thanks,
> Kyrill
>
> gcc/ChangeLog:
>
> 	* compare-elim.cc (find_comparisons_in_bb): Test the live-in set
> 	of the successor block rather than of the current block.
OK.   Guessing the testcase in patch #2 of the series actually tests this?

Jeff