https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105198
Bug ID: 105198
Summary: Wrong code for C loop (GCC 12 -O2, GCC 11 -O3)
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: tomas.kalibera at gmail dot com
Target Milestone: ---
Created attachment 52770
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52770&action=edit
Reproducible example to compile and execute (see comment in file)
It seems that GCC produces wrong code for function next_set found in R package
MASS (traced down by Brian Ripley):
static void next_set(int *x, int n, int k)
{
int i, j, tmp;
j = k - 1;
tmp = x[j]++;
while(j > 0 && x[j] >= n - (k - 1 -j)) tmp = ++x[--j];
for(i = j+1; i < k; i++) x[i] = ++tmp;
}
The attached standalone example reproduces the problem on a slightly modified
and instrumented variant of the function, for one specific input, see comments
in the code.
Correct output (GCC 12 -O1, and can be seen from the C code):
n == 5, k == 3, x == 0 1 4
tmp == 2, j == 1, x == 0 2 5
Incorrect but seen on x86_64/Linux (GCC 12 -O2, GCC 11 -O3):
n == 5, k == 3, x == 0 1 4
tmp == 4, j == 2, x == 0 1 5
One can modify the example to get slightly simpler native code via writing to
the array in "main" via volatile variables.
It works with GCC 10 -O3.
