[llvm-dev] Folding zext from i1 into PHI nodes with only zwo incoming values.

[Björn Steinbrink via llvm-dev]

Hi Sanjay,

2017-01-30 22:22 GMT+01:00 Sanjay Patel <spatel at rotateright.com>:

> My minimal patch idea is to ease the restriction in ShouldChangeType
> because i1 is special. I tried the patch below, and it works on the
> example...and nothing in 'make check' failed. :)
>

Yeah, that would work for me as well, I just wasn't sure about the
implications that has. Simply making FoldPHIArgOpIntoPHI act like
FoldPHIArgZextsIntoPHI seemed like the safer option to me, but I wanted
feedback on it before creating a PR. Do you want to go ahead with that
minimal approach and create a PR yourself?

Björn


> Index: lib/Transforms/InstCombine/InstructionCombining.cpp
> ===================================================================
> --- lib/Transforms/InstCombine/InstructionCombining.cpp    (revision
> 293485)
> +++ lib/Transforms/InstCombine/InstructionCombining.cpp    (working copy)
> @@ -88,12 +88,12 @@
>
>  /// Return true if it is desirable to convert an integer computation from
> a
>  /// given bit width to a new bit width.
> -/// We don't want to convert from a legal to an illegal type for example
> or from
> -/// a smaller to a larger illegal type.
> +/// We don't want to convert from a legal to an illegal type or from a
> smaller
> +/// to a larger illegal type. Width of '1' is always treated as a legal
> type.
>  bool InstCombiner::ShouldChangeType(unsigned FromWidth,
>                                      unsigned ToWidth) const {
> -  bool FromLegal = DL.isLegalInteger(FromWidth);
> -  bool ToLegal = DL.isLegalInteger(ToWidth);
> +  bool FromLegal = FromWidth == 1 ? true : DL.isLegalInteger(FromWidth);
> +  bool ToLegal = ToWidth == 1 ? true : DL.isLegalInteger(ToWidth);
>
>    // If this is a legal integer from type, and the result would be an
> illegal
>    // type, don't do the transformation.
> @@ -110,7 +110,7 @@
>
>  /// Return true if it is desirable to convert a computation from 'From'
> to 'To'.
>  /// We don't want to convert from a legal to an illegal type for example
> or from
> -/// a smaller to a larger illegal type.
> +/// a smaller to a larger illegal type. i1 is always treated as a legal
> type.
>  bool InstCombiner::ShouldChangeType(Type *From, Type *To) const {
>    assert(From->isIntegerTy() && To->isIntegerTy());
>
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