[LLVMdev] GCCBuiltin and Intrinsic Mapping

[David A. Greene]

Eli Friedman <eli.friedman at gmail.com> writes:

> On Mon, Sep 13, 2010 at 8:27 AM, David A. Greene <dag at cray.com> wrote:
>> Eli Friedman <eli.friedman at gmail.com> writes:
>>
>>> int_x86_avx_vhadd_pd_xmm doesn't exist on trunk.  Why does it exist on
>>> your branch if the semantics are exactly equivalent to
>>> int_x86_sse3_hadd_pd?  The register allocator can handle converting to
>>> three-address form if the target provides the appropriate hooks.
>>
>> Because in some cases users may want to explicitly use non-VEX encoded
>> instructions.  So we need to differentiate.
>
> Can you give an example of such a scenario?

Simulator validation, for example.

> In answer to your original question, it's probably just a matter of
> messing with the relevant generator in TableGen, relatively
> straightforward.  Your syntax is probably insufficient, though: how
> will the table generator decide which intrinsic to use for
> __builtin_ia32_haddpd coming from a frontend?

These are GCC builtins.  They only get used by the C generating backend
and I use them in debug scenarios.  In those cases I don't really care
which gets chosen.

For "normal" usage the codegen knows exactly what to emit (the actual
machine instruction).

                          -Dave