[MemoryBuiltins] Add getFreedOperand() function (NFCI)

We currently assume in a number of places that free-like functions
free their first argument. This is true for all hardcoded free-like
functions, but with the new attribute-based design, the freed
argument is supposed to be indicated by the allocptr attribute.

To make sure we handle this correctly once allockind(free) is
respected, add a getFreedOperand() helper which returns the freed
argument, rather than just indicating whether the call frees *some*
argument.

This migrates most but not all users of isFreeCall() to the new
API. The remaining users are a bit more tricky.

1 files changed, 5 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 83e67b1..c1cf1f1 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -493,13 +493,13 @@ bool llvm::wouldInstructionBeTriviallyDead(Instruction *I,
     }
   }
 
-  if (CallInst *CI = isFreeCall(I, TLI))
-    if (Constant *C = dyn_cast<Constant>(CI->getArgOperand(0)))
-      return C->isNullValue() || isa<UndefValue>(C);
-
-  if (auto *Call = dyn_cast<CallBase>(I))
+  if (auto *Call = dyn_cast<CallBase>(I)) {
+    if (Value *FreedOp = getFreedOperand(Call, TLI))
+      if (Constant *C = dyn_cast<Constant>(FreedOp))
+        return C->isNullValue() || isa<UndefValue>(C);
     if (isMathLibCallNoop(Call, TLI))
       return true;
+  }
 
   // Non-volatile atomic loads from constants can be removed.
   if (auto *LI = dyn_cast<LoadInst>(I))