[PATCH] D54287: [WebAssembly] Implement TargetLowering hooks for vectors

[Heejin Ahn via Phabricator via llvm-commits]

aheejin added a comment.

I think we can have test for each of these functions; each selection should affect code generation of some constructs. For example, for `storeOfVectorConstantIsCheap`, we can have a test that .ll code that stores a vector value to memory will generate a single vector store rather than n scalar stores. Searching for the usages of each function in llvm code might help I think.



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Comment at: lib/Target/WebAssembly/WebAssemblyISelLowering.cpp:526
+  return true;
+}
+
----------------
The explanation of this function says:
> Return true if inserting a scalar into a variable element of an undef vector is more efficiently handled by splatting the scalar instead.

I'm not 100% sure what this means, but to me this sounds like, if you want to insert a single element to an undef vector, it is better to just do splat then inserting a single element.

1. Is my interpretation correct?
2. If so, what should other undef elements be initialized with?
3. Don't we have `replace_lane` too? Why is `splat` faster than `replace_lane`? 

And while investigating current behaviors related to this, I filed a [[ https://bugs.llvm.org/show_bug.cgi?id=39685 | bug ]]. I might be mistaken, so please let me know then.


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Comment at: lib/Target/WebAssembly/WebAssemblyISelLowering.cpp:529
+bool WebAssemblyTargetLowering::isVectorShiftByScalarCheap(Type *Ty) const {
+  // Anything else is actually very expensive.
+  return true;
----------------
Maybe it's better to state that wasm SIMD shift instruction only takes scalar shift amounts, so if it's not a scalar we need an expensive sequence of code.


Repository:
  rL LLVM

https://reviews.llvm.org/D54287