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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/102586] [12 Regression] ICE in
 clear_padding_type, at gimple-fold.c:4798 since r12-3433-ga25e0b5e6ac8a77a
Date: Thu, 10 Feb 2022 22:27:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102586

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |redi at gcc dot gnu.org,
                   |                            |rodgertq at gcc dot gnu.org

--- Comment #17 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I guess for the <atomic> purposes restricting __builtin_clear_padding to
trivially copyable types is ok,
http://eel.is/c++draft/atomics.types.generic#general-1.1 requires that.
But we use __builtin_clear_padding or the infrastructure also for:
1) __builtin_bit_cast constant evaluation
2) OpenMP atomics
3) -fauto-var-init=3D
I don't remember the std::bit_cast case right now, OpenMP atomics are about
scalar floats only, 3) will always call it with address of a variable and
therefore know the complete object.

So perhaps go with your patch and diagnose if the builtin is called on
non-trivially copyable type unless it is called with address of a decl?=