[PATCH][CodeGen] Fix a bug in bitcast optimization of DAGCombiner

[Jiangning Liu]

BTW, this patch is to fix https://llvm.org/bugs/show_bug.cgi?id=23065 .

Thanks,
-Jiangning

2015-03-31 17:47 GMT+08:00 Jiangning Liu <liujiangning1 at gmail.com>:

> Hi,
>
> The small patch attached is to fix a bug in bitcast optimization of
> DAGCombiner.
>
> Originally, the code tries to optimize bitcast with build_vector input to
> be a scalar_to_vector, if only the bitcast and build_vector can meet some
> conditions. In particular, it thought "ThisVal.zext(SrcBitSize) == OpVal"
> is one of the requirements. This requirement means, if zero extension of a
> narrow element value is equal to the original wide element value, the
> optimization would be allowed. Unfortunately this is incorrect, because the
> semantic of scalar_to_vector is the top 1 to N-1 elements are undef rather
> than zero. So no matter we use zero_extension or sign_extension, this
> transformation would be always invalid. The patch attached disables this
> optimization.
>
> Thanks,
> -Jiangning
>
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