https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69619
Bug ID: 69619
Summary: [6 Regression] compilation doesn't terminate during
CCMP expansion
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Keywords: compile-time-hog, memory-hog
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ktkachov at gcc dot gnu.org
Target Milestone: ---
Target: aarch64
Testcase:
int a, b, c, d;
int e[1];
void
fn1 ()
{
int *f = &d;
c = 6;
for (; c; c--)
{
b = 0;
for (; b <= 5; b++)
{
short g = e[(b + 2) * 9 + c];
*f = *f == a && e[(b + 2) * 9 + c];
}
}
}
Given -O3 for aarch64 GCC doesn't terminate compilation and seems to keep
eating more and more memory. The testcase does contain undefined behaviour and
GCC warns about it:
mycrash.c: In function 'fn1':
mycrash.c:13:22: warning: iteration 1 invokes undefined behavior
[-Waggressive-loop-optimizations]
short g = e[(b + 2) * 9 + c];
~^~~~~~~~~~~~~~~~~
mycrash.c:11:7: note: within this loop
for (; b <= 5; b++)
but GCC shouldn't go into an infinite loop. At -O2 the testcase compiles
instantaneously.
Interrupting compilation in gdb and dumping the callstack shows that it's
recursing deeply into the ccmp expansion code. After a few seconds of
compilation I see a stack frame more than 400 levels deep with expand_ccmp_expr
appearing periodically in there, though I don't know if that's the ccmp expand
code's fault or the tree optimisers' fault.
I can provide the tree dumps if needed, though they should be easy to reproduce
with a recent aarch64 compiler
