[llvm] [WIP] Extend data layout to add non zero null value for address space. (PR #83109)

[Pierre van Houtryve via llvm-commits]

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@@ -3044,6 +3044,14 @@ as follows:
     ``n32:64`` for PowerPC 64, or ``n8:16:32:64`` for X86-64. Elements of
     this set are considered to support most general arithmetic operations
     efficiently.
+``z[n]:<value>``
+    This specifies the default null value for an address space. ``n`` denotes
+    the address space number, and if not specified, it is considered to be
+    for the unlisted address space. For unlisted address space, the default
+    null value is ``0``. ``value`` denotes the default null value for an
+    address space ``n``. To represent negatives values, prefix ``neg`` is
+    added to ``value``. for e.g., ``z0:neg1`` represents for ``0`` address
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Pierre-vh wrote:

I'm really wondering if we should prevent modifying the 0 address space.
It seems like it's kind of special, for instance it cannot be marked non integral in the data layout. I think it's fair for a pass to assume zero is the null value if (AS == 0) as it's always the case for every target (AFAIK), so I'd enforce it in the DL.

https://github.com/llvm/llvm-project/pull/83109