[llvm] [llvm] Fix behavior of llvm.objectsize in presence of negative / large offset (PR #115504)

[Harald van Dijk via llvm-commits]

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@@ -580,6 +585,11 @@ bool llvm::getObjectSize(const Value *Ptr, uint64_t &Size, const DataLayout &DL,
   if (!Data.bothKnown())
     return false;
 
+  // We could compute an over-approximation in that situation, may be if
+  // Opts.EvalMode == Max, but let's bee conservative and bail out.
+  if (Data.Offset.isNegative())
+    return false;
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hvdijk wrote:

I think your updated version still has the same potential problem. At the LLVM IR level, out of bounds pointers are not necessarily UB, it depends on whether a `getelemenptr` has the `inbounds` flag. In LLVM IR, it is possible to do the equivalent of `__builtin_object_size(array - 2 + 2, 0)` while keeping behaviour defined. To have this work, `computeImpl` needs to return the real `OffsetSpan` even for out of bounds pointers.

https://github.com/llvm/llvm-project/pull/115504