[llvm] TargetLibraryInfo: Use pointer index size to determine getSizeTSize(). (PR #118747)

[Owen Anderson via llvm-commits]

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@@ -1465,13 +1465,19 @@ unsigned TargetLibraryInfoImpl::getSizeTSize(const Module &M) const {
 
   // Historically LLVM assume that size_t has same size as intptr_t (hence
   // deriving the size from sizeof(int*) in address space zero). This should
-  // work for most targets. For future consideration: DataLayout also implement
-  // getIndexSizeInBits which might map better to size_t compared to
-  // getPointerSizeInBits. Hard coding address space zero here might be
-  // unfortunate as well. Maybe getDefaultGlobalsAddressSpace() or
-  // getAllocaAddrSpace() is better.
+  // work for most targets. For future consideration: Hard coding address space
+  // zero here might be unfortunate. Maybe getMaxIndexSizeInBits() is better.
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resistor wrote:

It turns out that using `getMaxIndexSizeInBits` is not a better default, as it computes the wrong size for a number of currently supported targets, including i386, x86-64 x32, and aaarch64 x32 (quite possibly others, but those are the ones I detected quickly).

I have updated the comment to reflect this reality.

@s-barannikov In the process I did experiment with per-target overrides in `getSizeTSize` and it worked without a hitch. If there were only a single configuration that needed overriding that would have been the way to go, but as it turned out the default behavior was clearly suboptimal.

https://github.com/llvm/llvm-project/pull/118747