[llvm-dev] [RFC] Refinement of convergent semantics

[Owen Anderson via llvm-dev]

> On Sep 22, 2015, at 2:02 PM, Jingyue Wu <jingyue at google.com> wrote:
> 
> Is LLVM allowed to unroll
> 
> for (int i = 0; i < 4; ++i) {
>   if (i < c) // c is loop invariant
>     convergent();
> }
> 
> to
> 
> if (0 < c)
>   convergent();
> if (1 < c)
>   convergent();
> if (2 < c)
>   convergent();
> if (3 < c)
>   convergent();
> 
> ?
> 
> Is "0 < c" considered "an additional value"? I'd vote no, but one can argue the other way. 

My intuition agrees with you here, but I don’t know how to formalize it.

> One approach (Bjarke's idea) to work around such ambiguities is to define: a program is convergent-correct if everything marked convergent are indeed convergent. Then a transformation is convergent-correct unless it transforms a convergent-correct program to a convergent-incorrect one. However, defining "convergent-correct" involves SIMT concepts which you want to avoid here. 

There are situations where this is over-conservative as well.  Some convergent operations do not require uniformity per se.  For example, a gradient operation in graphics programming models requires that all four threads in a given quad are either all executing or all not executing, though a warp is generally larger than that.  Restricting convergent code motion to only be between uniform control flow points in the program would penalize that use case.

—Owen
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