[cfe-dev] RFC: do not optimize on basis of __attribute__((nonnull)) in glibc headers

[Chandler Carruth via cfe-dev]

On Tue, Jan 3, 2017 at 3:10 PM Hal Finkel via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

On the one hand, GCC has already started optimizing based on the nonnull
attribute on memcpy, and so that ship has sailed.

I know of a large number of libraries that only behave correctly with
recent GCCs by using the flag Eli mentioned to disable all nonnull
optimizations wholesale. =/

If the standards committees actually change their mind here, I think we
could reasonably enable exactly this flag for Clang and LLVM when built
with a GCC version not implementing the fix.

And even if not, even if we do actually have to fix all of our code to work
in the face of those optimizations, I at least have a large body of users
that aren't sure they will ever finish fixing all of these issues. They
don't have any good way to test and discover *all* of them, only the ones
hit in code paths today. So I'd still like to give them a toolchain that
will protect the places in their software where they erroneously relied on
a guarantee not provided and have no tests or ability to go and fix.

-Chandler
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