With the fix for PR33870 we now create _loads_ of VOPs for QTs qmake makefile.cpp
at even -O so that all machines I have either run OOM or with a debug build of
gcc, tree-ssa-sccvn.c:1853 (DFS visiting VUSEs) recurses too deeply and blows
the 8MB stack on x86_64 (and takes too much compile-time).
Which means, we no longer can build QT.
Created attachment 14577 [details]
tree-ssa-sccvn should use a non-recursive DFS algorithm. Though, that is only part of the solution here, I suppose.
Yes, I wonder if we can "cut" the DFS walk somewhere - in this case we have
1000s of stmts with each ~200 VUSEs...
One workaround in this case is to run another forwprop / dce between inlining
and the first alias pass. This get's rid of a lot of pointers and pointed
to temporaries. Still that doesn't address the fundamental problems here.
(but it makes the testcase work nicely within a bound of 600MB peak
With just a forwprop pass after inlining and before salias we miscompile
cp/semantics.c:pop_to_parent_deferring_access_checks() at -O2.
An optimization barrier like
VEC (deferred_access_check,gc) *checks;
checks = (VEC_last (deferred_access, deferred_access_stack)
VEC_pop (deferred_access, deferred_access_stack);
__asm__ __volatile__ ("" : : : "memory");
ptr = VEC_last (deferred_access, deferred_access_stack);
fixes it. Reducing max-aliased-vops to 100 (as with -O1 which also passes)
fixes the problem as well. -O1 -fstrict-aliasing
--param max-aliased-vops=500 (as with -O2) also breaks. Disabling DOM
makes it work again.
http://gcc.gnu.org/ml/gcc-patches/2007-11/msg01046.html fixes the miscompile
of the C++ frontend.
-fno-strict-aliasing makes the testcase consume a lot more memory than
-fstrict-aliasing. Which is why -O is also a lot worse than -O2.
What one can also notice is that we create lots of unpartitionable SFTs for
structures with just _one_ SFT -- we should not need to create SFTs for those
Created attachment 14585 [details]
Subject: Bug 34148
Date: Wed Nov 21 10:16:21 2007
New Revision: 130329
2007-11-21 Richard Guenther <email@example.com>
* tree-ssa-structalias.c (create_variable_info_for): Do not use
field-sensitive PTA for single-element structures.
* tree-ssa-alias.c (create_overlap_variables_for): Do not create
SFTs for single-element structures.