[PATCH][CodeGen, DAG-combine, fast-math] Disable some fast-math transformations in DAG-combiner after legalization

[Nadav Rotem]

LGTM. 

On Mar 25, 2013, at 3:31 PM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:

> Hi, There:
> 
>    The attached patch disable some unsafe-fp-math DAG-combine transformation after legalization.
> For instance, following transformation is disabled:
>     x + x + x => 3.0f * x;
> 
>   The problem of these transformations is that it introduces a FP constant, which
> following Instruction-Selection pass cannot handle.
> 
>    This patch is stress-tested by running SingleSource/MultiSource with
> EnableUnsafeFPMath (@include/llvm/CodeGen/CommandFlags.h) being hard-coded to true.
> 
> rdar://13445387
> 
> Thanks
> Shuxin
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