[cfe-dev] CodeGen issue with atomic_load_n

[Reid Kleckner]

I think that's this bug here:
http://llvm.org/bugs/show_bug.cgi?id=17306

Probably it's a bug in Clang's builtin IR generation.


On Tue, Nov 19, 2013 at 3:11 AM, Beren Minor <beren.minor at gmail.com> wrote:

> Hello,
>
> I found some strange codegen output when playing with Clang's
> implementation of GCC atomic builtins and would like to know if this is
> some expected behavior or known issue.
>
> The test case is very simple, and the disassembly can be seen here:
> http://bit.ly/18LED7t
>
> It looks to me that these instructions after the load are unnecessary, and
> GCC does not generate them:
>     movl    %eax, -4(%rsp)
>     movl    -4(%rsp), %eax
>
> After investigating, it appears that this is caused by the volatile
> qualifier of the pointer parameter. Without it, the generated code is the
> same as GCC.
>
> It looks like the volatile qualifier is propagated to some temporary
> variable that Clang uses in the LLVM IR, and after storing the atomic load
> result in this temporary, it has to reload it again before returning it.
> Showing the IR with -emit-llvm exhibits clearly this issue.
>
> It seems to me that is is unnecessary to make this temporary volatile,
> isn't it?
> --
> Beren Minor
>
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