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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/43361] missing uninitialized warning without
 optimization (-O0) (PHI in always_executed basic block)
Date: Tue, 30 Mar 2021 07:10:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D43361

--- Comment #15 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Martin Sebor from comment #14)
> Reconfirmed with GCC 11 and the C test case below:
>=20
> void f (int);
>=20
> int main () {
>         int i;
>         int array[10];
>         for (; i<10; ++i) {             // no warning
>                 f (i);                  // no warning
>                 array [i] =3D i;          // no warning, really hurts
>         }
> }

So what's going "wrong" here is that we figure the BB with the i<10 check
post-dominates the entry block and thus is always executed.  But when
doing warn_uninit on the SSA var we do not consider the value on the
always-executed path from entry (which is uninitialized i_3(D)).

That's for the early pass, the late pass doesn't run at -O0.

Note since we walk BBs in index order which 'i' we warn for is probably
random, I think we'll only warn once since we set TREE_NO_WARNING on the
underlying decl of the SSA name (that's probably misguided - different
SSA defs can have different uninit state).

It might be possible to populate possibly_undefined_names in the BB walk
by visiting PHIs and considering fallthru entries being taken to have this
work out without major restructuring of the pass (a lattice of "undefinedne=
ss"
would probably be more to the point).=