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From: "hubicka at kam dot mff.cuni.cz" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug ipa/103830] [12 Regression] null pointer access optimized away
 by removing function call at -Og
Date: Tue, 04 Jan 2022 14:52:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103830

--- Comment #5 from hubicka at kam dot mff.cuni.cz ---
> I think the recent modref change made the function const.
>=20
> And no, we shouldn't DSE any volatile store and generally we don't.  It's
> probably some side-effect of modref that we do.  Using -fno-ipa-pure-const
> "fixes" this bug with -Og:
>=20
>  local analysis of void MyClass::call()/1
>    NULL memory access; terminating BB
> Function is locally const.
> callgraph:
>=20
> so it's caused by the recent change to mitigate path-isolation damage to
> modref.

The change indeed assumes that with -fdelete-null-pointer-checks the
access to NULL is invalid no matter if it is volatile or normal.  I
would expect code having exception handlers at address 0 to be always
built with -fno-delete-null-pointer-checks.

If we want to preserve user defined volaitle NULL, is there way to stick
another flag on the memory accesses synthetised by isolate-paths to mark
them as OK to be optimized this way?=