[llvm][NVPTX] Inform that 'DYNAMIC_STACKALLOC' is unsupported (#74684)

Catch unsupported path early up, and emit error with information.

Motivated by the following threads:
* https://discourse.llvm.org/t/nvptx-problems-with-dynamic-alloca/70745
* #64017

Author: JOE1994

llvm/lib/Target/NVPTX/NVPTXISelLowering.cpp

@@ -37,6 +37,7 @@
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/DiagnosticInfo.h"
 #include "llvm/IR/FPEnv.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/GlobalValue.h"
@@ -639,6 +640,11 @@ NVPTXTargetLowering::NVPTXTargetLowering(const NVPTXTargetMachine &TM,
   setOperationAction(ISD::ConstantFP, MVT::f16, Legal);
   setOperationAction(ISD::ConstantFP, MVT::bf16, Legal);
 
+  // Lowering of DYNAMIC_STACKALLOC is unsupported.
+  // Custom lower to produce an error.
+  setOperationAction(ISD::DYNAMIC_STACKALLOC, MVT::i32, Custom);
+  setOperationAction(ISD::DYNAMIC_STACKALLOC, MVT::i64, Custom);
+
   // TRAP can be lowered to PTX trap
   setOperationAction(ISD::TRAP, MVT::Other, Legal);
 
@@ -2209,6 +2215,18 @@ SDValue NVPTXTargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
   return Chain;
 }
 
+SDValue NVPTXTargetLowering::LowerDYNAMIC_STACKALLOC(SDValue Op,
+                                                     SelectionDAG &DAG) const {
+  const Function &Fn = DAG.getMachineFunction().getFunction();
+
+  DiagnosticInfoUnsupported NoDynamicAlloca(
+      Fn, "dynamic alloca unsupported by NVPTX backend",
+      SDLoc(Op).getDebugLoc());
+  DAG.getContext()->diagnose(NoDynamicAlloca);
+  auto Ops = {DAG.getConstant(0, SDLoc(), Op.getValueType()), Op.getOperand(0)};
+  return DAG.getMergeValues(Ops, SDLoc());
+}
+
 // By default CONCAT_VECTORS is lowered by ExpandVectorBuildThroughStack()
 // (see LegalizeDAG.cpp). This is slow and uses local memory.
 // We use extract/insert/build vector just as what LegalizeOp() does in llvm 2.5
@@ -2700,6 +2718,8 @@ NVPTXTargetLowering::LowerOperation(SDValue Op, SelectionDAG &DAG) const {
   case ISD::SREM:
   case ISD::UREM:
     return LowerVectorArith(Op, DAG);
+  case ISD::DYNAMIC_STACKALLOC:
+    return LowerDYNAMIC_STACKALLOC(Op, DAG);
   default:
     llvm_unreachable("Custom lowering not defined for operation");
   }

llvm/lib/Target/NVPTX/NVPTXISelLowering.h

@@ -513,6 +513,8 @@ class NVPTXTargetLowering : public TargetLowering {
   SDValue LowerCall(CallLoweringInfo &CLI,
                     SmallVectorImpl<SDValue> &InVals) const override;
 
+  SDValue LowerDYNAMIC_STACKALLOC(SDValue Op, SelectionDAG &DAG) const;
+
   std::string
   getPrototype(const DataLayout &DL, Type *, const ArgListTy &,
                const SmallVectorImpl<ISD::OutputArg> &, MaybeAlign retAlignment,

llvm/test/CodeGen/NVPTX/dynamic_stackalloc.ll

@@ -0,0 +1,10 @@
+; RUN: not llc -march=nvptx < %s 2>&1 | FileCheck %s
+; RUN: not llc -march=nvptx64 < %s 2>&1 | FileCheck %s
+
+; CHECK: in function test_dynamic_stackalloc{{.*}}: dynamic alloca unsupported by NVPTX backend
+
+define void @test_dynamic_stackalloc(i64 %n) {
+  %alloca = alloca i32, i64 %n
+  store volatile i32 0, ptr %alloca
+  ret void
+}