On 3/8/19 4:40 AM, Richard Biener wrote: > On Fri, Mar 8, 2019 at 1:34 AM Bill Schmidt <wschm...@linux.ibm.com> wrote: >> Hi, >> >> We recently discovered a problem in swap optimization where the du- and >> ud-chains >> were getting corrupted after a preliminary modification phase and prior to >> the >> main body of the pass. The fix for this is to rebuild the chains between >> phases. > It looks expensive - is it possible to keep them up-to-date instead?
That's what I've been doing up till now. There appears to be a problem with rescanning and the DF_DU_CHAIN and DF_UD_CHAIN dataflow problems. Whether I use df_process_deferred_rescans or df_insn_rescan_all, I see a problem where now and then a use-def chain gets lost. As I looked into it further, I found this comment on df_insn_rescan_all: /* Rescan all of the insns in the function. Note that the artificial uses and defs are not touched. This function will destroy def-use or use-def chains. */ So if that's accurate, it appears that the rescan machinery is not compatible with du-/ud-chains, which is consistent with my experience. This is the second time we've encountered this issue. It doesn't come up often, but when it does, it's a real head-scratcher to debug. If an expert in the df code thinks the above comment is erroneous, then there is at least a subtle bug somewhere in the df code that would be good to fix. But seeing the comment, I felt warned off completely from rescan. This at least allows us to fix the problem that the OpenCV community found. Thanks, Bill > >> Bootstrapped and tested on powerpc64le-unknown-linux-gnu with no regressions. >> I've not included a test case because the problem tends to get lost in >> reduction, >> and may shift over time anyway. Is this okay for trunk, and eventual >> backport >> to 8 and 7? >> >> Thanks! >> >> Bill >> >> >> 2019-03-07 Bill Schmidt <wschm...@linux.ibm.com> >> >> * config/rs6000/rs6000-p8swap.c (rs6000_analyze_swaps): Rebuild >> ud- and du-chains between phases. >> >> >> Index: gcc/config/rs6000/rs6000-p8swap.c >> =================================================================== >> --- gcc/config/rs6000/rs6000-p8swap.c (revision 269471) >> +++ gcc/config/rs6000/rs6000-p8swap.c (working copy) >> @@ -2316,7 +2316,14 @@ rs6000_analyze_swaps (function *fun) >> >> /* Pre-pass to recombine lvx and stvx patterns so we don't lose info. */ >> recombine_lvx_stvx_patterns (fun); >> + >> + /* Rebuild ud- and du-chains. */ >> + df_remove_problem (df_chain); >> df_process_deferred_rescans (); >> + df_set_flags (DF_RD_PRUNE_DEAD_DEFS); >> + df_chain_add_problem (DF_DU_CHAIN | DF_UD_CHAIN); >> + df_analyze (); >> + df_set_flags (DF_DEFER_INSN_RESCAN); >> >> /* Allocate structure to represent webs of insns. */ >> insn_entry = XCNEWVEC (swap_web_entry, get_max_uid ()); >>
