https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120560
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2025-06-05
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
We optimize away everything, leading to
main:
.LFB0:
.cfi_startproc
.cfi_endproc
.LFE0:
.size main, .-main
.ident "GCC: (GNU) 16.0.0 20250605 (experimental) [gcc-clean2
afd06421b34]"
.section .note.GNU-stack,"",@progbits
aka
int main ()
{
<bb 2> [count: 0]:
__builtin_unreachable ();
The interesting thing is that we have
Estimating # of iterations of loop 1
Estimating # of iterations of loop 1
Loop 1 iterates at most 0 times.
Loop 1 likely iterates at most 0 times.
;; Guessed iterations of loop 1 is 99.999992. New upper bound 0.
;; Scaling loop 1 with scale 1.0% (guessed) to reach upper bound 0
;; Not updating exit probability of loop 1; it has no single exit
t.c:1:5: optimized: loop turned into non-loop; it never loops
Latch of last iteration was marked by __builtin_unreachable ().
but the input here is just
int main ()
{
<bb 2> [local count: 10631108]:
<bb 3> [local count: 1073741824]:
<bb 4> [local count: 1073741824]:
goto <bb 3>; [100.00%]
but with an upper bound for the number of iterations of zero.
We have eliminated the exit in EVRP, it's input was
<bb 2> :
<bb 3> :
# b_2 = PHI <2(2), b_8(5)>
_1 = 1 - b_2;
if (_1 > 6)
goto <bb 4>; [INV]
else
goto <bb 5>; [INV]
<bb 4> :
// predicted unlikely by early return (on trees) predictor.
return 0;
<bb 5> :
b_8 = b_2 + -2147483648;
goto <bb 3>; [INV]
which I think is OK.
During EVRP we do SCEV / niter analysis and there it gets fishy, so this
is likely another case of SCEV messing up overflow:
Analyzing # of iterations of loop 1
exit condition [-1, + , -2147483648(OVF)](no_overflow) <= 6
bounds on difference of bases: 7 ... 7
failed
Induction variable (int) -1 + -2147483648(OVF) * iteration does not wrap in
statement _1 = 1 - b_2;
in loop 1.
Statement _1 = 1 - b_2;
is executed at most 0(OVF) (bounded by 0) + 1 times in loop 1.
this is the IV for _1 and that looks wrong?
