https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95929
Bug ID: 95929
Summary: Failure to optimize tautological comparisons of
comparisons to a single one
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
int f(int a, int b)
{
return (((b != 0) & (a == 0)) | ((a != 0) & (b == 0)));
}
This can be optimized to `(a != 0) ^ (b != 0)`. I originally found this while
compiling this code :
inline bool nand(bool a, bool b)
{
return !(a && b);
}
int f(int a, int b)
{
return nand(nand(b, nand(a, a)), nand(a, nand(b, b)));
}
Which GCC compiles to the above example, and that LLVM optimizes with the
transformation I gave (strangely, LLVM does not seem to optimize the example at
the top of this bug report to the transformed version if directly given the top
example, which is why I'm giving these details as I'm thinking there could be
some kind of UB weirdness or something like that with the transformations here)
