https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116420
--- Comment #3 from Jeffrey A. Law <law at gcc dot gnu.org> --- Fun. The DF framework provides us a way to run dataflow problems on sub-graphs. Naturally a bitmap of interesting blocks is passed into those routines. At a confluence point, the DF framework will not mark a block for re-processing if it's not in that set of interesting blocks. When ext-dce sets up that set of interesting blocks it's using the wrong counter. ie, it's using n_basic_blocks rather than last_basic_block. If there are holes in the block indices, some number of blocks won't get marked as interesting. In this case the block needing reprocessing has an index higher than n_basic_blocks. It never gets reprocessed and the newly found live chunks don't propagate further up the CFG -- ultimately resulting in a pseudo appearing to have only the low 8 bits live, when in fact the low 32 bits are actually live. Testing a pretty obvious fix.
