[PATCH] D153963: [InstCombine] Fold binop of select and cast of select condition

[Antonio Frighetto via Phabricator via llvm-commits]

antoniofrighetto added a comment.

In D153963#4482931 <https://reviews.llvm.org/D153963#4482931>, @goldstein.w.n wrote:

> So I'm going to propose something slightly different for accomplishing this patch.
> Instead of making a combine for binops, I think you can just arbitrarily
> constant propagate any usage of `cond` to the true/false arms.

I guess with `TrueVal->Operands()` we actually mean the (unique) user of `TrueVal`? I'm not exactly following how we would benefit from this, where the operands of the select are Values in most of our use cases, here's just an example:

  define i64 @src(i1 %c) {
    %sel = select i1 %c, i64 64, i64 1
    %ext = zext i1 %c to i64
    %add = add i64 %sel, %ext
    ret i64 %add
  }

Conversely, wouldn't the proposed change be trying to catch the following orthogonal test case?

  define i64 @src(i1 %c, i64 %a, i64 %b, i64 %d) {
    %ext = zext i1 %c to i64
    %add = add i64 %a, %ext
    %sub = sub i64 %b, %ext
    %sel = select i1 %c, i64 %add, i64 %sub
    ret i64 %sel
  }

Where `%add` and `%sub` may be optimized respectively with `%1 = add i64 %a, 1` and  `%2 = sub i64 %b, 0`.


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