[clang] [Clang][CodeGen] Do not set inbounds flag for struct GEP with null base pointers (PR #130734)

[Eli Friedman via cfe-commits]

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@@ -42,6 +42,11 @@ Potentially Breaking Changes
 C/C++ Language Potentially Breaking Changes
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+- Some old-style offsetof idioms like ``((int)(&(((struct S *)0)->field)))`` are treated
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efriedma-quic wrote:

I was thinking something more like:

New LLVM optimizations have been implemented that optimize pointer arithmetic on null pointers more aggressively.  As part of this, clang has implemented a special case for old-style offsetof idioms like ``((int)(&(((struct S *)0)->field)))``, to ensure they are not caught by these optimizations.  It is also possible to use ``-fwrapv-pointer`` or   ``-fno-delete-null-pointer-checks`` to make pointer arithmetic on null pointers well-defined. (#GH130734, #GH130742)

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(Also -fwrapv-pointer doesn't actually work at the moment for the address computation associated with member access.)

https://github.com/llvm/llvm-project/pull/130734