TODO [SYCL] Fix circular dependency in byval parameter handling in NVPTXLowerArgs

Author: bratpiorka

llvm/lib/Target/NVPTX/NVPTXLowerArgs.cpp

@@ -529,9 +529,21 @@ void copyByValParam(Function &F, Argument &Arg) {
   // the use of the byval parameter with this alloca instruction.
   AllocA->setAlignment(
       Arg.getParamAlign().value_or(DL.getPrefTypeAlign(StructType)));
-  Arg.replaceAllUsesWith(AllocA);
 
+  // CRITICAL: Must create ArgInParam BEFORE replaceAllUsesWith.
+  // createNVVMInternalAddrspaceWrap needs to use Arg as an operand, but
+  // replaceAllUsesWith(AllocA) would replace ALL uses of Arg (including the
+  // use we're about to create), leading to a circular dependency and crash.
   CallInst *ArgInParam = createNVVMInternalAddrspaceWrap(IRB, Arg);
+  assert(ArgInParam->getArgOperand(0) == &Arg &&
+         "ArgInParam must use original Arg before replacement");
+  
+  Arg.replaceAllUsesWith(AllocA);
+  
+  // Verify ArgInParam still has Arg as operand after replacement (it should,
+  // because replaceAllUsesWith skips the value being replaced itself)
+  assert(ArgInParam->getArgOperand(0) == &Arg &&
+         "ArgInParam must still use original Arg after replaceAllUsesWith");
 
   // Be sure to propagate alignment to this load; LLVM doesn't know that NVPTX
   // addrspacecast preserves alignment.  Since params are constant, this load
@@ -599,10 +611,14 @@ static void handleByValParam(const NVPTXTargetMachine &TM, Argument *Arg) {
         ParamSpaceArg, IRB.getPtrTy(ADDRESS_SPACE_GENERIC),
         Arg->getName() + ".gen");
 
-    Arg->replaceAllUsesWith(GenericArg);
-
-    // Do not replace Arg in the cast to param space
-    ParamSpaceArg->setOperand(0, Arg);
+    // Collect all uses of Arg except the one in ParamSpaceArg, then replace them
+    SmallVector<Use *> UsesToReplace;
+    for (Use &U : Arg->uses()) {
+      if (U.getUser() != ParamSpaceArg)
+        UsesToReplace.push_back(&U);
+    }
+    for (Use *U : UsesToReplace)
+      U->set(GenericArg);
   } else
     copyByValParam(*Func, *Arg);
 }