[PATCH] D121771: [RISCV] Add basic cost model for vector casting

[Craig Topper via Phabricator via llvm-commits]

craig.topper added inline comments.


================
Comment at: llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp:246
+      case ISD::UINT_TO_FP: {
+        auto Cost =
+            getCastInstrCost(Opcode, DstVTy->getElementType(),
----------------
fakepaper56 wrote:
> craig.topper wrote:
> > Is there any point in getting the scalar cost? Seems like its always 1 and doesn't have a real connection to how the vector ISA works.
> > 
> > I'm also questioning the accuracy of some of the scalar numbers. i8->float requires a pair of shifts to clear the upper bits before doing an iXLen->float conversion for example. So if we were to fix that, it would be wrong for vectors.
> My point to use scalar cost is just it is simple to implement and enough to make vectorizer think vectorizing casting instructions is profitable in most cases.
> 
> I had written a version by vector instruction count. The logic is a little complex and I am not sure when to consider vsetvli and even lmul. I think maybe we should refine it to a table having some magic number like X86 and Arm, so I just provide the simple design now.  
Please add a FIXME to document what is missing.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D121771/new/

https://reviews.llvm.org/D121771