[LLVMdev] [cfe-dev] [RFC] add Function Attribute to disable optimization

[Eli Friedman]

On Mon, Jun 17, 2013 at 3:32 PM, Jeffrey Walton <noloader at gmail.com> wrote:

> On Mon, Jun 17, 2013 at 6:23 PM, Sean Silva <silvas at purdue.edu> wrote:
> > On Mon, Jun 17, 2013 at 10:29 AM, Jeffrey Walton <noloader at gmail.com>
> wrote:
> >> First is to ensure dead-writes are not removed. For example, a
> >> function that zeroizes or wipes memory is subject to removal during
> >> optimization. I often have to look at program's disassembly to ensure
> >> the memset is not removed by the optimizer.
> >
> > Appropriate use of `volatile` is probably sufficient for this use case.
> That brings up a good point. As I understand it, volatile is
> essentially implementation defined. What is Clang/LLVM's
> interpretation?
>
> Here's what I know. Microsoft's interpretation allows me to use
> volatile for the situation under MSVC++ [1]. GCC's interpretation of
> volatile is for memory mapped hardware, so it does not allow me to use
> the qualifier to tame the optimizer [2].
>
>
clang doesn't treat volatile loads/stores as aquire/release barriers, if
that's what you're asking.

Actually, if you look at the 2012 version of the docs "
http://msdn.microsoft.com/en-us/library/vstudio/12a04hfd(v=vs.110).aspx",
you can see Microsoft was forced to change its own rules so as to not
completely screw over performance on non-X86 platforms.

-Eli
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