On Fri, Jul 29, 2022 at 11:52 AM Richard Earnshaw <richard.earns...@foss.arm.com> wrote: > > > > On 29/07/2022 08:06, Richard Biener via Gcc-patches wrote: > > On Thu, Jul 28, 2022 at 6:46 PM Richard Earnshaw > > <richard.earns...@foss.arm.com> wrote: > >> > >> [resend with correct subject line] > >> > >> A SET operation that writes memory may have the same value as an earlier > >> store but if the alias sets of the new and earlier store do not conflict > >> then the set is not truly redundant. This can happen, for example, if > >> objects of different types share a stack slot. > >> > >> To fix this we define a new function in cselib that first checks for > >> equality and if that is successful then finds the earlier store in the > >> value history and checks the alias sets. > >> > >> The routine is used in two places elsewhere in the compiler. Firstly > >> in cfgcleanup and secondly in postreload. > > > > I can't comment on the stripping on SUBREGs and friends but it seems > > to be conservative apart from > > > > + if (!flag_strict_aliasing || !MEM_P (dest)) > > + return true; > > > > where if dest is not a MEM but were to contain one we'd miss it. > > Double-checking > > from more RTL literate people appreciated. > > There are very few things that can wrap a MEM in a SET_DEST. I'm pretty > sure that's all of them. It certainly matches the code in > cselib_invalidate_rtx which has to deal with this sort of case. > > > > > + /* Lookup the equivalents to the dest. This is more likely to succeed > > + than looking up the equivalents to the source (for example, when the > > + src is some form of constant). */ > > > > I think the comment is misleading - we _do_ have to lookup the MEM, > > looking up equivalences of a reg or an expression on the RHS isn't > > what we are interested in. > > OK, I'll try to reword it. > > > > > + return alias_sets_conflict_p (MEM_ALIAS_SET (dest), > > + MEM_ALIAS_SET (src_equiv)); > > > > that's not conservative enough - dse.cc has correct boilerplate, we have > > to check both MEM_ALIAS_SET and MEM_EXPR here (the latter only > > if the former load/store has a MEM_EXPR). Note in particular > > using alias_set_subset_of instead of alias_sets_conflict_p. > > > > /* We can only remove the later store if the earlier aliases > > at least all accesses the later one. */ > > && ((MEM_ALIAS_SET (mem) == MEM_ALIAS_SET (s_info->mem) > > || alias_set_subset_of (MEM_ALIAS_SET (mem), > > MEM_ALIAS_SET (s_info->mem))) > > && (!MEM_EXPR (s_info->mem) > > || refs_same_for_tbaa_p (MEM_EXPR (s_info->mem), > > MEM_EXPR (mem))))) > > > > OK, that's an easy enough change. > > > + /* We failed to find a recorded value in the cselib history, so try the > > + source of this set. */ > > + rtx src = SET_SRC (set); > > + while (GET_CODE (src) == SUBREG) > > + src = XEXP (src, 0); > > + > > + if (MEM_P (src) && rtx_equal_for_cselib_1 (dest_addr, XEXP (src, 0), > > + GET_MODE (dest), 0)) > > + return alias_sets_conflict_p (MEM_ALIAS_SET (dest), > > + MEM_ALIAS_SET (src)); > > > > this looks like an odd case to me - wouldn't that only catch things > > like self-assignments, aka *p = *p? So I'd simply drop this fallback. > > It catches the case of *p = *q when p and q have the same value. It did > come up in testing on x86 (when previously I was aborting to make sure > I'd caught everything). We could leave it out as the fallback case in > this instance is to record a conflict, but it's not a path that's likely > to be performance critical and the probability of this being a redundant > store is quite high. I'll update the comment to make this clearer.
Ah OK - if it did actually catch cases then it's fine to keep. Note the alias check needs to be updated the same as above. Richard. > > > R. > > > > > Otherwise it looks OK to me. > > > > Thanks, > > Richard. > > > >> gcc/ChangeLog: > >> * cselib.h (cselib_redundant_set_p): Declare. > >> * cselib.cc: Include alias.h > >> (cselib_redundant_set_p): New function. > >> * cfgcleanup.cc: (mark_effect): Use cselib_redundant_set_p instead > >> of rtx_equal_for_cselib_p. > >> * postreload.c (reload_cse_simplify): Use cselib_redundant_set_p. > >> (reload_cse_noop_set_p): Delete.
