https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113424
Bug ID: 113424
Summary: lim fails to notice possible aliasing
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: kristerw at gcc dot gnu.org
Target Milestone: ---
The lim pass miscompiles the following C++ program when compiled as -O3 for
x86_64 (note: it works as intended when compiled as a C program)
struct { char elt1; char bits; } *a;
char
bar (char *x, char b)
{
if (0)
next_bit:
return 1;
while (1)
{
if (b)
if (a->bits)
goto next_bit;
*x = b;
if (a->elt1)
return 0;
a = 0;
}
}
The loop lim gets as input looks as following
<bb 3>
if (b_9(D) != 0)
goto <bb 4>;
else
goto <bb 5>;
<bb 4>
a.0_1 = a;
_2 = a.0_1->bits;
if (_2 != 0)
goto <bb 7>;
else
goto <bb 5>;
<bb 5>
*x_10(D) = b_9(D);
a.1_3 = a;
_4 = a.1_3->elt1;
if (_4 != 0)
goto <bb 7>; [5.50%]
else
goto <bb 6>; [94.50%]
<bb 6>
a = 0B;
goto <bb 3>; [100.00%]
The lim pass changes this to load `a` before the loop and uses the same value
of `a` for both accesses in bb4 and bb5, which is not correct as the store
`*x_10(D)` may have modified `a` before the access in bb5.
