[Inline][Cloning] Drop incompatible attributes from `NewFunc`

Performing `instSimplify` while cloning is unsafe due to incomplete
remapping (as reported in #87534). Ideally, `instSimplify` ought to
reason on the updated newly-cloned function, after returns have been
rewritten and callee entry basic block / call-site have been fixed up.
This is in contrast to `CloneAndPruneIntoFromInst` behaviour, which
is inherently expected to clone basic blocks, with pruning on top of
– if any –, and not actually fixing up returns / CFG, which should be
up to the Inliner. We may solve this by letting `instSimplify` work on
the newly-cloned function, while maintaining old function attributes,
so as to avoid inconsistencies between the yet-to-be-solved return
type, and new function ret type attributes.

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/CloneFunction.cpp b/llvm/lib/Transforms/Utils/CloneFunction.cpp
index 38cab25..6f6dc63 100644
--- a/llvm/lib/Transforms/Utils/CloneFunction.cpp
+++ b/llvm/lib/Transforms/Utils/CloneFunction.cpp
@@ -18,6 +18,7 @@
 #include "llvm/Analysis/DomTreeUpdater.h"
 #include "llvm/Analysis/InstructionSimplify.h"
 #include "llvm/Analysis/LoopInfo.h"
+#include "llvm/IR/AttributeMask.h"
 #include "llvm/IR/CFG.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DebugInfo.h"
@@ -819,6 +820,14 @@ void llvm::CloneAndPruneIntoFromInst(Function *NewFunc, const Function *OldFunc,
     }
   }
 
+  // Drop all incompatible return attributes that cannot be applied to NewFunc
+  // during cloning, so as to allow instruction simplification to reason on the
+  // old state of the function. The original attributes are restored later.
+  AttributeMask IncompatibleAttrs =
+      AttributeFuncs::typeIncompatible(OldFunc->getReturnType());
+  AttributeList Attrs = NewFunc->getAttributes();
+  NewFunc->removeRetAttrs(IncompatibleAttrs);
+
   // As phi-nodes have been now remapped, allow incremental simplification of
   // newly-cloned instructions.
   const DataLayout &DL = NewFunc->getParent()->getDataLayout();
@@ -849,6 +858,9 @@ void llvm::CloneAndPruneIntoFromInst(Function *NewFunc, const Function *OldFunc,
     }
   }
 
+  // Restore attributes.
+  NewFunc->setAttributes(Attrs);
+
   // Remap debug intrinsic operands now that all values have been mapped.
   // Doing this now (late) preserves use-before-defs in debug intrinsics. If
   // we didn't do this, ValueAsMetadata(use-before-def) operands would be