On Sun, May 19, 2019 at 6:16 PM Marc Glisse <marc.gli...@inria.fr> wrote:
>
> Hello,
>
> 2 pieces:
>
> - the first one handles the case where the denominator is negative. It
> doesn't happen often with exact_div, so I don't handle it everywhere, but
> this one looked trivial
>
> - handle the case where a pointer difference is cast to an unsigned type
> before being compared to a constant (I hit this in std::vector). With some
> range info we could probably handle some non-constant cases as well...
>
> The second piece breaks Walloca-13.c (-Walloca-larger-than=100 -O2)
>
> void f (void*);
> void g (int *p, int *q)
> {
> __SIZE_TYPE__ n = (__SIZE_TYPE__)(p - q);
> if (n < 100)
> f (__builtin_alloca (n));
> }
>
> At the time of walloca2, we have
>
> _1 = p_5(D) - q_6(D);
> # RANGE [-2305843009213693952, 2305843009213693951]
> _2 = _1 /[ex] 4;
> # RANGE ~[2305843009213693952, 16140901064495857663]
> n_7 = (long unsigned intD.10) _2;
> _11 = (long unsigned intD.10) _1;
> if (_11 <= 396)
> [...]
> _3 = allocaD.1059 (n_7);
>
> and warn.
That's indeed to complicated relation of _11 to n_7 for
VRP predicate discovery.
> However, DOM3 later produces
>
> _1 = p_5(D) - q_6(D);
> _11 = (long unsigned intD.10) _1;
> if (_11 <= 396)
while _11 vs. _1 works fine.
> [...]
> # RANGE [0, 99] NONZERO 127
> _2 = _1 /[ex] 4;
> # RANGE [0, 99] NONZERO 127
> n_7 = (long unsigned intD.10) _2;
> _3 = allocaD.1059 (n_7);
>
> so I am tempted to say that the walloca2 pass is too early, xfail the
> testcase and file an issue...
Hmm, there's a DOM pass before walloca2 already and moving
walloca2 after loop opts doesn't look like the best thing to do?
I suppose it's not DOM but sinking that does the important transform
here? That is,
Index: gcc/passes.def
===================================================================
--- gcc/passes.def (revision 271395)
+++ gcc/passes.def (working copy)
@@ -241,9 +241,9 @@ along with GCC; see the file COPYING3.
NEXT_PASS (pass_optimize_bswap);
NEXT_PASS (pass_laddress);
NEXT_PASS (pass_lim);
- NEXT_PASS (pass_walloca, false);
NEXT_PASS (pass_pre);
NEXT_PASS (pass_sink_code);
+ NEXT_PASS (pass_walloca, false);
NEXT_PASS (pass_sancov);
NEXT_PASS (pass_asan);
NEXT_PASS (pass_tsan);
fixes it?
> A single_use restriction would also probably fix this testcase, but I
> don't think that's a good idea, the new code is better because the
> division is now in the branch.
OK.
Can you test the pass motion above?
Thanks,
Richard.
> 2019-05-20 Marc Glisse <marc.gli...@inria.fr>
>
> gcc/
> * match.pd (X/[ex]D<Y/[ex]D): Handle negative denominator.
> ((size_t)(A /[ex] B) CMP C): New transformation.
>
> gcc/testsuite/
> * gcc.dg/tree-ssa/cmpexactdiv-3.c: New file.
> * gcc.dg/tree-ssa/cmpexactdiv-4.c: New file.
>
> --
> Marc Glisse
