https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117414
Bug ID: 117414
Summary: missing predicated VN due to combining if statements
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: pinskia at gcc dot gnu.org
Target Milestone: ---
Take the following two functions:
```
void foo();
int f(int *aaa, int j, int t)
{
int b = *aaa;
if (b == 0 || t == 1)
return 0;
for(int i = 0; i < j; i++)
{
if (!*aaa) foo();
}
}
void foo();
int f1(int *aaa, int j, int t)
{
int b = *aaa;
if (b == 0)
return 0;
if (t == 1)
return 0;
for(int i = 0; i < j; i++)
{
if (!*aaa) foo();
}
}
```
They should produce the same code but currently does not. The second one works
as predicated VN is able to see that `!*aaa` is never true. While the second
does not.
The combining of the if statements loses information for the predication about
*aaa.
This is what is causing gcc.dg/tree-ssa/pr111456-1.c to fail after allowing for
slightly more ifcombine as the predicated fre (handled during pre in that case)
is not happening any more. I suspect this was the original missed optimization,
I just worked around it by optimizing `a == 0 & ((short)a) >= 0` into `a == 0`
which then gets the predication as needed and it is also why
`--param=logical-op-non-short-circuit=0` worked originally too.
