[llvm] [NFCI][IR] Add optional DataLayout argument to zero and null value related APIs (PR #183208)

[Shilei Tian via llvm-commits]

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@@ -52,7 +53,10 @@ class Constant : public User {
   Constant(const Constant &) = delete;
 
   /// Return true if this is the value that would be returned by getNullValue.
-  LLVM_ABI bool isNullValue() const;
+  LLVM_ABI bool isNullValue(const DataLayout *DL = nullptr) const;
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shiltian wrote:

> Checking for an all-null-pointer struct/array probably doesn't make a lot of sense (unlike an all-zero struct/array).

Hmm, I wonder what the expected behavior would be if we check `isNullValue` and `isZeroValue` on things like `{ i32, ptr, i32 }`.

> Though after null pointers are not longer always all-zero, I do wonder whether we should be using ConstantAggregateZero for a vector of null pointer, rather than using a ConstantPointerNull with vector type (similar to how we're in the process of switching ConstantFP/ConstantInt to natively support vector splats. This has recently been enabled by default for FP). This has the benefit that we can always represent a null pointer splat, regardless of whether the null pointer is all zero or not.

That is a great idea! I went ahead doing it right now. https://github.com/llvm/llvm-project/pull/195486

https://github.com/llvm/llvm-project/pull/183208