On Tue, Jan 8, 2013 at 5:16 PM, Jeff Law <l...@redhat.com> wrote: > On 01/08/2013 04:26 AM, Richard Biener wrote: > >> >> The issue is VRP - when you remove unreachable blocks you lose the >> conditional statement as it is no longer necessary and thus the predicate >> you can derive value-ranges from. > > Understood. Perhaps we could eliminate them after the first VRP pass, but > before the second. That ought to give us the majority of the benefit of > seeing the conditional and propagating based on the conditional, but also > give us the benefit of eliminating the branch generating straight-line code. > > Clearly it needs more investigation, but I think it's worth exploring.
Sure - especially if we eventually move to preserve value-range information across passes. __builtin_constant_p is a similar thing - it guards one reachable and one unreachable path but we forcefully remove it only during the late fold-all-builtins pass. Richard. > Jeff
