[llvm-dev] [RFC] Vector Predication (original) (raw)

Simon Moll via llvm-dev llvm-dev at lists.llvm.org
Tue Feb 5 04:26:52 PST 2019

On 2/5/19 12:49 PM, Luke Kenneth Casson Leighton wrote:

Basically what I am saying is that the semantics "request an AVL from the hardware and get an ACTUAL number of elements to be processed" really needs to become part of the API. Ok. We could add this behavior to the EVL contract with an intrinsic.

%EffectiveVL = llvm.evl.setvl(, %RequestedAVL) where vscale would be interpreted as VLMul on RISC-V.

the inner workings of AVL are entirely hidden and opaque to the IR. The IR sets the total explicit number of elements, and It Gets Done.

That would still be ok, if the following invariant holds on RVV:

Given that

   %effectivevl = setvl %reqVL

Will the following invariant hold whenever %avl <= %effectivevl?

   %avl == setvl , %avl

In that case, we could require the VL parameter to be derived from the evl.setvl intrinsic without violating that part of EVL semantics (all elements in the range from [0 to VL) will be processed).

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