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Re: [RFC] PR64703, glibc sysdeps/powerpc/powerpc64/dl-machine.h miscompile


On Fri, Jan 30, 2015 at 10:12:35AM +0100, Richard Biener wrote:
> Ok - without digging into why the above would fail with your patch
> (don't see that - the use in the function call can't be &opdd) - let's
> take a step back and decide whether we want to allow user-created
> function descriptors.  And if we do that if we should rather expose
> this in a more sensible way to GCC, like with using a (target) builtin.
> Say, force you to do
> 
>   int (*f) (int) = __builtin_fdesc (opd.fd_func, opd.fd_toc, opd.fd_aux);
>   return f (3);
> 
> which would allow GCC to even optimize the call to a direct one
> if it (or the target) can fold reads from the __builtin_fdesc argument
> to a function decl.  Similar builtins could allow you to inspect
> a function descriptor.  That way the actual memory operations
> would be hidden from the middle-end.

That would be my preference too.  Constructing the calls this way is so rare
that pessimizing 99.9% of code out there that doesn't ever need this is
IMHO undesirable.

	Jakub


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