[PATCH] [X86][FastIsel] Teach how to select scalar integer to float/double conversions.

[Andrea Di Biagio]

In http://reviews.llvm.org/D7698#125598, @qcolombet wrote:

> Hi Andrea,
>
> On a second thought, why do we need to use the AVX variant at all if we never use the first source operand?
>  The bonus point is that would kill all the code related to the implicit def.


Hi Quentin,

The main reason (at least from my point of view) is that we want to avoid mixing legacy SSE instructions and AVX instructions as much as possible. Otherwise, we end up increasing AVX-SSE transition penalties. That's why (basically everywhere in X86FastISel) we always prefer vex encoded instructions over legacy SSE instructions.

I hope this makes sense.
-Andrea


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