[llvm] [DeadArgElim] Renumber allocsize instead of dropping it (PR #221868)

[Nikita Popov via llvm-commits]

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@@ -839,9 +854,42 @@ bool DeadArgumentEliminationPass::removeDeadStuffFromFunction(Function *F) {
 
   AttributeSet RetAttrs = AttributeSet::get(F->getContext(), RAttrs);
 
-  // Strip allocsize attributes. They might refer to the deleted arguments.
-  AttributeSet FnAttrs =
-      PAL.getFnAttrs().removeAttribute(F->getContext(), Attribute::AllocSize);
+  // allocsize names parameters by index, so deleting an argument ahead of one
+  // renumbers it. surveyFunction() keeps the arguments allocsize points at
+  // alive, so renumber the attribute rather than dropping it; drop it only if
+  // they went away regardless, which leaves the callee no way to report the
+  // size it allocates.
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nikic wrote:

I see, thanks. In that case my next question would be: *Should* call-site allocsize() also prevent the transform? (The question is not rhetorical, I'm not entirely sure... but it seems a bit inconsistent as implemented.)

https://github.com/llvm/llvm-project/pull/221868