[clang] [lld] [llvm] [mlir] [IR] Introduce `T<address space>` to `DataLayout` to represent flat address space if a target supports it (PR #108786)

[Shilei Tian via cfe-commits]

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@@ -245,6 +246,7 @@ class DataLayout {
   unsigned getDefaultGlobalsAddressSpace() const {
     return DefaultGlobalsAddrSpace;
   }
+  unsigned getFlatAddressSpace() const { return FlatAddressSpace; }
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shiltian wrote:

It is optional from a target's perspective, but the expectation is indeed that to check against `~0U` because it represents invalid address space. I will put a comment there.

https://github.com/llvm/llvm-project/pull/108786