patch: wire up -mcx16

[Nick Lewycky]

Rafael Espíndola wrote:
> On 4 October 2013 21:04, Nick Lewycky<nlewycky at google.com>  wrote:
>> GCC has an x86 feature flag, "-mcx16" which enables support for CMPXCHG16B.
>> This feature already exists as a feature flag in llvm's x86 backend, but
>> there is no matching flag in clang (it is automatically enabled/disabled by
>> -march=). This patch has an llvm part and a clang part that wires up -mcx16
>> to the existing flag.
>>
>> One note. There is a discrepancy between clang and gcc on -march= k8-sse3,
>> opteron-sse3, athlon64-sse3 and amdfam10, where our X86.td claims those
>> architectures support cmpxchg16b but gcc does not define the relevant macro.
>> I've gone ahead and made clang define the macro in the same cases that gcc
>> does, but have not changed which processors claim to support the instruction
>> in X86.td.
>
> Why not? It is probably better to keep these in sync. In the end, do
> you know what is the authoritative source of which amd cpus support
> this feature?

Exactly. Why not, because I don't have an authoritative source and 
therefore don't know which is right. Or if there's a third option like 
the CPUs do support it but with weird problems (extremely slow?) and gcc 
deliberately doesn't define the macro for those CPUs. Dunno.

> LGTM, but pleas at least open a bug about llvm and gcc not agreeing
> about which cpus have this feature.

Okay, I've filed PR17485 and will land the patch as-is. At least this is 
the safe direction, where we're not advertising support for a feature 
which the backend would support if you used it.

Nick