[llvm] [TableGen] Use PrintFatalError in ParseTreePattern for proper error handling of llvm-tblgen (PR #161417)

[Prerona Chaudhuri via llvm-commits]

https://github.com/pchaudhuri-nv updated https://github.com/llvm/llvm-project/pull/161417

>From 8ec45c99561b1417f2b135beb665d56d6a0e7f46 Mon Sep 17 00:00:00 2001
From: pchaudhuri-nv <prerona.chaudhuri16 at vit.edu>
Date: Tue, 30 Sep 2025 18:30:56 +0000
Subject: [PATCH] fic

---
 llvm/test/TableGen/invalid_typecast_assert.td | 51 +++++++++++++++++++
 .../TableGen/Common/CodeGenDAGPatterns.cpp    | 20 ++++++--
 2 files changed, 68 insertions(+), 3 deletions(-)
 create mode 100644 llvm/test/TableGen/invalid_typecast_assert.td

diff --git a/llvm/test/TableGen/invalid_typecast_assert.td b/llvm/test/TableGen/invalid_typecast_assert.td
new file mode 100644
index 0000000000000..9fa65801bb6df
--- /dev/null
+++ b/llvm/test/TableGen/invalid_typecast_assert.td
@@ -0,0 +1,51 @@
+// The test incorrect classifies the pattern as a typecast and it ends up with
+// typecast DAG with zero arguements, leading to llvm-tblgen crash.
+// This test will check if the error is gracefully handled without any crash.
+ 
+// RUN: not llvm-tblgen -gen-dag-isel -I %p/../../include %s 2>&1 | FileCheck -check-prefix=ERROR-CHK %s
+
+include "llvm/Target/Target.td"
+
+class MyReg<string n>
+  : Register<n> {
+  let Namespace = "MyTarget";
+}
+
+def X0 : MyReg<"x0">;
+def X1 : MyReg<"x1">;
+
+def XRegs : RegisterClass<"MyTarget", [i64], 64, (add X0, X1)>;
+
+class TestInstruction : Instruction {
+  let Size = 2;
+  let Namespace = "MyTarget";
+  let hasSideEffects = false;
+  let hasExtraSrcRegAllocReq = false;
+  let hasExtraDefRegAllocReq = false;
+
+  field bits<16> Inst;
+  bits<3> dst;
+  bits<3> src;
+  bits<3> opcode;
+
+  let Inst{2-0} = dst;
+  let Inst{5-3} = src;
+  let Inst{7-5} = opcode;
+}
+
+def MY_LOAD : TestInstruction {
+  let OutOperandList = (outs XRegs:$dst);
+  let InOperandList = (ins ptr_rc:$ptr);
+  let AsmString = "my_load $dst, $ptr";
+  let opcode = 0;
+}
+
+def : Pat<
+  (i64 (load (iPTR:$src))),
+  (MY_LOAD $val, $src)
+>;
+
+def MyTargetISA : InstrInfo;
+def MyTarget : Target { let InstructionSet = MyTargetISA; }
+
+// ERROR-CHK: error: This type cast has zero arguments. It takes only one operand!
diff --git a/llvm/utils/TableGen/Common/CodeGenDAGPatterns.cpp b/llvm/utils/TableGen/Common/CodeGenDAGPatterns.cpp
index 75bea77faba42..acf57ea2ee5dd 100644
--- a/llvm/utils/TableGen/Common/CodeGenDAGPatterns.cpp
+++ b/llvm/utils/TableGen/Common/CodeGenDAGPatterns.cpp
@@ -3012,7 +3012,15 @@ TreePatternNodePtr TreePattern::ParseTreePattern(const Init *TheInit,
     return nullptr;
   }
 
-  auto ParseCastOperand = [this](const DagInit *Dag, StringRef OpName) {
+  auto ParseCastOperand =
+      [this](const DagInit *Dag,
+             StringRef OpName) -> std::optional<TreePatternNodePtr> {
+    if (Dag->getNumArgs() == 0) {
+      PrintFatalError(
+          "This type cast has zero arguments. It takes only one operand!");
+      return std::nullopt;
+    }
+
     if (Dag->getNumArgs() != 1)
       error("Type cast only takes one operand!");
 
@@ -3025,7 +3033,10 @@ TreePatternNodePtr TreePattern::ParseTreePattern(const Init *TheInit,
   if (const ListInit *LI = dyn_cast<ListInit>(Dag->getOperator())) {
     // If the operator is a list (of value types), then this must be "type cast"
     // of a leaf node with multiple results.
-    TreePatternNodePtr New = ParseCastOperand(Dag, OpName);
+    auto MaybeNew = ParseCastOperand(Dag, OpName);
+    if (!MaybeNew)
+      return nullptr;
+    TreePatternNodePtr New = *MaybeNew;
 
     size_t NumTypes = New->getNumTypes();
     if (LI->empty() || LI->size() != NumTypes)
@@ -3050,7 +3061,10 @@ TreePatternNodePtr TreePattern::ParseTreePattern(const Init *TheInit,
   if (Operator->isSubClassOf("ValueType")) {
     // If the operator is a ValueType, then this must be "type cast" of a leaf
     // node.
-    TreePatternNodePtr New = ParseCastOperand(Dag, OpName);
+    auto MaybeNew = ParseCastOperand(Dag, OpName);
+    if (!MaybeNew)
+      return nullptr;
+    TreePatternNodePtr New = *MaybeNew;
 
     if (New->getNumTypes() != 1)
       error("ValueType cast can only have one type!");