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Re: [PATCH, i386] RTM support
On 03/05/2012 10:04 AM, Uros Bizjak wrote:
> On Mon, Mar 5, 2012 at 6:12 PM, Andi Kleen <andi@firstfloor.org> wrote:
>> On Mon, Mar 05, 2012 at 03:31:32PM +0400, Kirill Yukhin wrote:
>>> Adding patch
>>
>> I would still remove the "-mrtm" option. I never understood what options
>> for intrinsics are good for. They are just a pain to add to Makefiles,
>> but don't give any benefit.
>
> Without -mrtm, the test will just emit a call to
> __builtin_ia32_xbegin, for instance. You would ge linking error from
> your app.
>
> And the reason to have -mXXX flags is quite obvious. You don't want
> combiner to optimize the sequence of two supported instructions (SSE
> level X, for example) to an unsupported one (SSE level X+N) that
> implements the same functionality. The same goes for integer
> instructions, but less obvious ;)
>
> So, I'm afraid that -mXXX options stay. But you can sprinkle target
> option pragmas around the code now.
-mrtm is a bit different from the other -m options.
E.g. -msse affects the header files in that it controls whether
the __m128 data type is even available.
E.g. -mssse3, -mavx, -mbmi all affect the code generated for any
code written in plain old C using no intrinsics.
But -mrtm *only* affects the availability of certain intrinsics.
One can reasonably argue that you simply don't use those when
you don't want to use them.
That said, -mrtm could easily be tested by the compiler to generate
(or not) inline HTM for implementing -fgnu-tm....
r~