[Bug rtl-optimization/60043] -fschedule-insns2 breaks anti-dependency
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60043 --- Comment #3 from rguenther at suse dot de rguenther at suse dot de --- On Tue, 4 Feb 2014, abel at gcc dot gnu.org wrote: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60043 Andrey Belevantsev abel at gcc dot gnu.org changed: What|Removed |Added CC||abel at gcc dot gnu.org --- Comment #1 from Andrey Belevantsev abel at gcc dot gnu.org --- I don't follow the reasoning of this example and the original ML thread. The load of *b follows the store to *a, thus the scheduler is checking for the presence of the _true_ dependence between them: gcc/sched-deps.c: 2660 if (true_dependence (XEXP (pending_mem, 0), VOIDmode, t) 2661 ! sched_insns_conditions_mutex_p (insn, 2662 XEXP (pending, 0))) 2663 note_mem_dep (t, XEXP (pending_mem, 0), XEXP (pending, 0), 2664 sched_deps_info-generate_spec_deps 2665 ? BEGIN_DATA | DEP_TRUE : DEP_TRUE); which does not exist because the mems have different alias sets. But you have agreed that TBAA can be used for true dependences in the ML thread, no? What is then required from the scheduler? Yes, TBAA can be used for true dependences - but a true dependence is read-after-write. Here we have an anti-dependence, write-after-read. When the scheduler wants to exchange two mems then it needs to use the predicate that is correct _before_ the transform, not after. Thus in the above code it seems that it does not check whether the write from pending[_mem] is before or after the read in 't' - the used predicate needs to change dependent on the order of the insns (or conservatively assume an anti-dependence and thus disable TBAA).
[Bug rtl-optimization/60043] -fschedule-insns2 breaks anti-dependency
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60043 Richard Biener rguenth at gcc dot gnu.org changed: What|Removed |Added Status|UNCONFIRMED |RESOLVED Resolution|--- |INVALID --- Comment #4 from Richard Biener rguenth at gcc dot gnu.org --- Hmm, oops. The testcase has a true dependence ... ;) Still, isn't the code in sched_analyze_2 wrong? Or are pending_mems all before 't'?
[Bug rtl-optimization/60043] -fschedule-insns2 breaks anti-dependency
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60043 --- Comment #5 from Andrey Belevantsev abel at gcc dot gnu.org --- (In reply to Richard Biener from comment #4) Hmm, oops. The testcase has a true dependence ... ;) Still, isn't the code in sched_analyze_2 wrong? Or are pending_mems all before 't'? Yes (to second question :). The analysis code roughly works like this (sched_analyze is the entry point): - process insns one by one in a bb/ebb doing two things in parallel: o mark reads/writes to registers, writes/reads to mems of this insn in various fields of the deps context; and o find dependencies between this insn reads/writes of registers/memory and previously noted reads/writes of the same register or memory from the deps context. So the pending_mem read/write will always happen before the currently processing insn mem read/write.
[Bug rtl-optimization/60043] -fschedule-insns2 breaks anti-dependency
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60043 Andrey Belevantsev abel at gcc dot gnu.org changed: What|Removed |Added CC||abel at gcc dot gnu.org --- Comment #1 from Andrey Belevantsev abel at gcc dot gnu.org --- I don't follow the reasoning of this example and the original ML thread. The load of *b follows the store to *a, thus the scheduler is checking for the presence of the _true_ dependence between them: gcc/sched-deps.c: 2660 if (true_dependence (XEXP (pending_mem, 0), VOIDmode, t) 2661 ! sched_insns_conditions_mutex_p (insn, 2662 XEXP (pending, 0))) 2663 note_mem_dep (t, XEXP (pending_mem, 0), XEXP (pending, 0), 2664 sched_deps_info-generate_spec_deps 2665 ? BEGIN_DATA | DEP_TRUE : DEP_TRUE); which does not exist because the mems have different alias sets. But you have agreed that TBAA can be used for true dependences in the ML thread, no? What is then required from the scheduler?
[Bug rtl-optimization/60043] -fschedule-insns2 breaks anti-dependency
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60043
--- Comment #2 from Andrey Belevantsev abel at gcc dot gnu.org ---
And indeed, if we change the test case to
int foo (long long *a, short *b, int n)
{
int k = *b + 1000;
*a = (long long) (n * 100);
return k;
}
then we get the desired anti-dependency because alias.c:write_dependence will
dispatch to rtx_refs_may_alias_p with tbaa_p = false. So I don't see an issue
here wrt the ML thread discussion.
