[Bug c++/85746] Premature evaluation of __builtin_constant_p?
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85746 Marc Glisse changed: What|Removed |Added Status|NEW |RESOLVED Resolution|--- |FIXED --- Comment #10 from Marc Glisse --- Fixed in gcc-10.
[Bug c++/85746] Premature evaluation of __builtin_constant_p?
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85746 --- Comment #9 from Marc Glisse --- Author: glisse Date: Tue Oct 22 14:42:38 2019 New Revision: 277292 URL: https://gcc.gnu.org/viewcvs?rev=277292=gcc=rev Log: PR c++/85746: Don't fold __builtin_constant_p prematurely 2019-10-22 Marc Glisse gcc/cp/ * constexpr.c (cxx_eval_builtin_function_call): Only set force_folding_builtin_constant_p if manifestly_const_eval. gcc/testsuite/ * g++.dg/pr85746.C: New file. Added: trunk/gcc/testsuite/g++.dg/pr85746.C Modified: trunk/gcc/cp/ChangeLog trunk/gcc/cp/constexpr.c trunk/gcc/testsuite/ChangeLog
[Bug c++/85746] Premature evaluation of __builtin_constant_p?
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85746 --- Comment #8 from rsandifo at gcc dot gnu.org --- (In reply to Marc Glisse from comment #7) > (In reply to Marc Glisse from comment #6) > > && xi.val[0] <= (HOST_WIDE_INT) ((unsigned HOST_WIDE_INT) > > HOST_WIDE_INT_MAX >> shift)) > > The issue occurs with xi.val[0] == -9223372036854775808 (lshift_large > returns a result of length 2 for that). I don't know if the code mishandles > this case, or if such a number is not supposed to exist in the first place, > but that does seem like a bug. Yeah, looks like this should have been an unsigned HOST_WIDE_INT comparison instead, i.e. casting xi.val[0] rather than the shift result.
[Bug c++/85746] Premature evaluation of __builtin_constant_p?
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85746 Marc Glisse changed: What|Removed |Added CC||rsandifo at gcc dot gnu.org --- Comment #7 from Marc Glisse --- (In reply to Marc Glisse from comment #6) > && xi.val[0] <= (HOST_WIDE_INT) ((unsigned HOST_WIDE_INT) > HOST_WIDE_INT_MAX >> shift)) The issue occurs with xi.val[0] == -9223372036854775808 (lshift_large returns a result of length 2 for that). I don't know if the code mishandles this case, or if such a number is not supposed to exist in the first place, but that does seem like a bug.
[Bug c++/85746] Premature evaluation of __builtin_constant_p?
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85746
--- Comment #6 from Marc Glisse ---
The problematic file seems to be tree-dfa.o, and the only difference I see in
the .original dump is the use of __builtin_constant_p in wi::lshift (3
occurrences because of templates)
- if (precision <= 64)
+ if (__builtin_constant_p (xi.D.16865.D.16740.precision > 64) != 0 &&
xi.D.16865.D.16740.precision > 64 ? ((__builtin_constant_p (shift <= 62) != 0
&& shift <= 62) && xi.D.16865.D.16740.len == 1) && *NON_LVALUE_EXPR
<= (const long int) (9223372036854775807 >> shift) :
precision <= 64)
which corresponds to the use of STATIC_CONSTANT_P in wide-int.h
if (STATIC_CONSTANT_P (xi.precision > HOST_BITS_PER_WIDE_INT)
? (STATIC_CONSTANT_P (shift < HOST_BITS_PER_WIDE_INT - 1)
&& xi.len == 1
&& xi.val[0] <= (HOST_WIDE_INT) ((unsigned HOST_WIDE_INT)
HOST_WIDE_INT_MAX >> shift))
: precision <= HOST_BITS_PER_WIDE_INT)
{
val[0] = xi.ulow () << shift;
result.set_len (1);
}
[Bug c++/85746] Premature evaluation of __builtin_constant_p?
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85746 --- Comment #5 from Marc Glisse --- The patch breaks 3 tests on a bootstrapped compiler: c-c++-common/Warray-bounds-2.c c-c++-common/ubsan/ptr-overflow-sanitization-1.c gcc.dg/Warray-bounds-30.c The last one in particular shows that the compiler is miscompiled (it works with the stage 1 compiler but breaks with stage 2 or 3). From the dumps, the compilation seems to diverge in FRE1, so the miscompiled function might be in a function called from FRE. I find it suspicious that just refusing to fold __builtin_constant_p can cause this. The 2 explanations I can think of are: - latent bug uncovered by the change - a variable that needs to be statically initialized (order of initialization?) and ends up dynamically initialized because of the change
[Bug c++/85746] Premature evaluation of __builtin_constant_p?
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85746 --- Comment #4 from Marc Glisse --- Created attachment 46566 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46566=edit untested patch to fold _bcp only when necessary
[Bug c++/85746] Premature evaluation of __builtin_constant_p?
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85746 --- Comment #3 from Marc Glisse --- (In reply to Jakub Jelinek from comment #2) > For different versions there is the > http://gcc.gnu.org/ml/gcc-patches/2018-03/msg00355.html > patch. Time to ping that one? ;-) (I don't have a particular opinion about how __builtin_early_constant_p should behave)
[Bug c++/85746] Premature evaluation of __builtin_constant_p?
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85746 Jakub Jelinek changed: What|Removed |Added CC||jakub at gcc dot gnu.org --- Comment #2 from Jakub Jelinek --- For different versions there is the http://gcc.gnu.org/ml/gcc-patches/2018-03/msg00355.html patch.
[Bug c++/85746] Premature evaluation of __builtin_constant_p?
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85746 Richard Biener changed: What|Removed |Added Keywords||missed-optimization Status|UNCONFIRMED |NEW Last reconfirmed||2018-05-11 Ever confirmed|0 |1 --- Comment #1 from Richard Biener --- Confirmed.
