https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121344
--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Krister Walfridsson from comment #4)
> Hmm. The discussion in 111280 says it cannot create new basic blocks, but
> the compiler is correctly creating a new basic block for the function in
> this PR when compiled with -O1, -O2, and -O3. So why is -Os different?
It is not generating a new BB at -O1/-O2/-O3; before coming into SCCP the IR
looks like:
```
<bb 2> [local count: 118111600]:
if (b_3(D) != 0)
goto <bb 5>; [89.00%]
else
goto <bb 7>; [11.00%]
<bb 5> [local count: 105119324]:
<bb 3> [local count: 955630224]:
# b_9 = PHI <b_5(6), b_3(D)(5)>
# c_10 = PHI <c_6(6), 0(5)>
b_5 = b_9 >> 1;
c_6 = c_10 + 1;
if (b_5 != 0)
goto <bb 6>; [89.00%]
else
goto <bb 8>; [11.00%]
<bb 8> [local count: 105119324]:
# c_2 = PHI <c_6(3)>
goto <bb 4>; [100.00%]
```
So the check for `b!=0` was there beforehand due to copying of the loop header
and changing it from a while(){} loop into a do{}while(); loop.
In the other one the count is an increment of the CLZ by 1 and already in a do
while loop form.
You run also into the same issue at -O1/-O2/-O3 with:
```
int
foo (unsigned long long b)
{
int c = 0;
do {
b >>= 1;
c++;
}while (b);
return c-1;
}
```
