[C++ PATCH] Don't redefine __* builtins (PR c++/84724)

Jason Merrill jason@redhat.com
Thu Mar 8 21:06:00 GMT 2018

On Thu, Mar 8, 2018 at 1:01 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> The C FE just warns and doesn't override builtins, but C++ FE
> on say int __builtin_trap (); will override the builtin, so later
> builtin_decl_explicit will return the bogus user function which e.g.
> doesn't have any merged attributes, can have different arguments or
> return type etc.
> This patch prevents that; generally the builtins we want to override
> are the DECL_ANTICIPATED which we handle properly earlier.
> The testcase in the PR ICEs not because of the argument mismatch (there is
> none in this case) or return value mismatch (because nothing cares about the
> return value), but because the new decl lacks noreturn attribute and GCC
> relies on builtin_decl_explicit (BUILT_IN_TRAP) to be really noreturn.
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> Or shall we error on these, or ignore the name checks and just
> if (DECL_BUILT_IN (olddecl)) return NULL_TREE;, something else?

Seems like returning NULL_TREE means that we overload the built-in,
which also seems undesirable; what if we return olddecl unmodified?
It would also be good to improve the diagnostic to say that we're
ignoring the declaration.  Perhaps as a permerror.


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