[llvm] [RFC][DataLayout] Add null pointer value infrastructure (PR #183207)

[Shilei Tian via llvm-commits]

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@@ -95,6 +96,9 @@ class DataLayout {
     bool HasExternalState;
     // Symbolic name of the address space.
     std::string AddrSpaceName;
+    /// The null pointer bit representation for this address space.
+    /// If absent, this address space uses the default null pointer value.
+    std::optional<APInt> NullPtrValue;
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shiltian wrote:

This value has the correct pointer size, so to have it as a `std::optional<APInt>` would be convenient, since we can directly take the value in the query. However, the default one can be indeed changed to `enum class NullPtrValue { Zero, AllOnes}` to avoid using an `APInt(1)` and then do the 0/1 check.

https://github.com/llvm/llvm-project/pull/183207