[clang] [llvm] [Clang][CodeGen] Avoid failing downcast when __builtin_prefech's immarg is poison (PR #201623)

[via cfe-commits]

https://github.com/jjppp updated https://github.com/llvm/llvm-project/pull/201623

>From 3d8c661aeff295f5602fcff6c06e41a3630a9c02 Mon Sep 17 00:00:00 2001
From: jpwang <jpwang at smail.nju.edu.cn>
Date: Fri, 5 Jun 2026 00:37:01 +0800
Subject: [PATCH] [Clang][CodeGen] Avoid failing downcast when
 __builtin_prefech's immarg is poison

---
 clang/test/CodeGen/prefetch-poison-rw.c       |  6 +++++
 .../SelectionDAG/SelectionDAGBuilder.cpp      | 27 +++++++++++++------
 2 files changed, 25 insertions(+), 8 deletions(-)
 create mode 100644 clang/test/CodeGen/prefetch-poison-rw.c

diff --git a/clang/test/CodeGen/prefetch-poison-rw.c b/clang/test/CodeGen/prefetch-poison-rw.c
new file mode 100644
index 0000000000000..c13d4bb246d54
--- /dev/null
+++ b/clang/test/CodeGen/prefetch-poison-rw.c
@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -triple x86_64-pc-linux -emit-llvm %s -o - | FileCheck %s
+
+void test_poison_rw(void) {
+  __builtin_prefetch(0, 2 >> 32);
+  // CHECK: call void @llvm.prefetch.p0(ptr null, i32 0, i32 3, i32 1)
+}
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index eca5bb1598ae0..92b0fb7baa0cc 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -7767,17 +7767,28 @@ void SelectionDAGBuilder::visitIntrinsicCall(const CallInst &I,
     return;
   }
   case Intrinsic::prefetch: {
+    auto *DefaultConstantInt =
+        llvm::ConstantInt::get(llvm::Type::getInt32Ty(*DAG.getContext()), 0);
+    auto GetConstantIntOrDefault = [&](Value *Val) {
+      if (auto *Constant = dyn_cast<ConstantInt>(Val)) {
+        return Constant;
+      }
+      // Cases where Val is non-constant should be rejected by Sema.
+      return DefaultConstantInt;
+    };
+
     SDValue Ops[5];
-    unsigned rw = cast<ConstantInt>(I.getArgOperand(1))->getZExtValue();
-    auto Flags = rw == 0 ? MachineMemOperand::MOLoad :MachineMemOperand::MOStore;
+    unsigned rw = GetConstantIntOrDefault(I.getArgOperand(1))->getZExtValue();
+    auto Flags =
+        rw == 0 ? MachineMemOperand::MOLoad : MachineMemOperand::MOStore;
     Ops[0] = DAG.getRoot();
     Ops[1] = getValue(I.getArgOperand(0));
-    Ops[2] = DAG.getTargetConstant(*cast<ConstantInt>(I.getArgOperand(1)), sdl,
-                                   MVT::i32);
-    Ops[3] = DAG.getTargetConstant(*cast<ConstantInt>(I.getArgOperand(2)), sdl,
-                                   MVT::i32);
-    Ops[4] = DAG.getTargetConstant(*cast<ConstantInt>(I.getArgOperand(3)), sdl,
-                                   MVT::i32);
+    Ops[2] = DAG.getTargetConstant(*GetConstantIntOrDefault(I.getArgOperand(1)),
+                                   sdl, MVT::i32);
+    Ops[3] = DAG.getTargetConstant(*GetConstantIntOrDefault(I.getArgOperand(2)),
+                                   sdl, MVT::i32);
+    Ops[4] = DAG.getTargetConstant(*GetConstantIntOrDefault(I.getArgOperand(3)),
+                                   sdl, MVT::i32);
     SDValue Result = DAG.getMemIntrinsicNode(
         ISD::PREFETCH, sdl, DAG.getVTList(MVT::Other), Ops,
         EVT::getIntegerVT(*Context, 8), MachinePointerInfo(I.getArgOperand(0)),