On Jan 16, 2013, Jakub Jelinek <ja...@redhat.com> wrote: > On Wed, Jan 16, 2013 at 08:28:59AM -0200, Alexandre Oliva wrote: >> PR49888 introduced clobber_overlapping_mems to detach VALUEs (and >> variables bound to them) from MEMs as the MEMs are modified. This >> turned out to be quite expensive, particularly the computation of >> canonical addresses passed to alias dependency. >> >> This patch introduces caching of the canonical addresses to alleviate >> this expensive operation. We cache mappings that apply to the entire >> function, from equivalences recorded in the global cselib table, and >> mappings that apply only to the local basic block. This cut down 2% of >> a full regstrap cycle, and about 1% of the time it took to build target >> libraries for C, C++ and Java.
> Can you safely cache the canon addresses already during vt_initialize > (when cselib_* is still processing new insns, cselib VALUEs contain > REGs and MEMs that are flushed at the end of processing the current bb > in vt_initialize)? No. AFAICT we only call the address canonicalization function after collecting all MOps, when the cselib table is fully constructed and cleaned up from any local information, retaining only the global equivalences. > Also, what effects (if any) does the patch have on the > .debug_info/.debug_loc size and coverage? It shouldn't have any, since it's just caching results that would have been recomputed over and over. However, there's a possibility of being “lucky” and recording an equivalence in the cache whose path would later be removed from a dynamic set (say, if an incoming VALUE is reset and re-bound within a block; I'm not sure this ever actually happens). In this case, these retained equivalences might enable alias analysis to figure out that two memory refs do not overlap, and so one can be retained in a dynamic equivalence list when we process a MOp that modifies the other. Or something ;-) It shouldn't really make any difference, just speed things up a bit. Paraphrasing Knuth, “I proved it, but I didn't test it” ;-) -- Alexandre Oliva, freedom fighter http://FSFLA.org/~lxoliva/ You must be the change you wish to see in the world. -- Gandhi Be Free! -- http://FSFLA.org/ FSF Latin America board member Free Software Evangelist Red Hat Brazil Compiler Engineer
