[llvm] [llvm] Fix __builtin_object_size interaction between Negative Offset … (PR #111827)

[via llvm-commits]

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@@ -686,10 +686,23 @@ ObjectSizeOffsetVisitor::ObjectSizeOffsetVisitor(const DataLayout &DL,
 
 SizeOffsetAPInt ObjectSizeOffsetVisitor::compute(Value *V) {
   InstructionsVisited = 0;
-  return computeImpl(V);
+  OffsetSpan Span = computeImpl(V);
+
+  // In ExactSizeFromOffset mode, we don't care about the Before Field, so allow
+  // us to overwrite it if needs be.
+  if (Span.knownAfter() && !Span.knownBefore() &&
+      Options.EvalMode == ObjectSizeOpts::Mode::ExactSizeFromOffset)
+    Span.Before = APInt::getZero(Span.After.getBitWidth());
+
+  if (!Span.bothKnown())
+    return {};
+  if (Span.Before.isNegative() || Span.After.isNegative())
+    return {};
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serge-sans-paille wrote:

As you proposed at some point, I just removed the check and checked it was still passing the (now well enhanced) test suite, it's indeed the case. Way to go!

https://github.com/llvm/llvm-project/pull/111827