[clang] [CIR] Support x86 builtin rdtsc (PR #180714)

[Aditya Trivedi via cfe-commits]

https://github.com/adit4443ya created https://github.com/llvm/llvm-project/pull/180714

This PR implements CodeGen for rdtsc builtins in CIR upstream.
Towards #167765 

>From 050e5e989e5d677de31edd9fc0413562d2fb6ef9 Mon Sep 17 00:00:00 2001
From: Aditya Trivedi <adit4443ya at gmail.com>
Date: Tue, 10 Feb 2026 10:08:19 +0000
Subject: [PATCH] [CIR] Support x86 builtin rdtsc

---
 clang/lib/CIR/CodeGen/CIRGenBuiltinX86.cpp | 22 ++++++++++++---
 clang/test/CIR/CodeGenBuiltins/X86/rdtsc.c | 31 ++++++++++++++++++++++
 2 files changed, 49 insertions(+), 4 deletions(-)
 create mode 100644 clang/test/CIR/CodeGenBuiltins/X86/rdtsc.c

diff --git a/clang/lib/CIR/CodeGen/CIRGenBuiltinX86.cpp b/clang/lib/CIR/CodeGen/CIRGenBuiltinX86.cpp
index cad80317cb870..4d1cf98791d3a 100644
--- a/clang/lib/CIR/CodeGen/CIRGenBuiltinX86.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenBuiltinX86.cpp
@@ -846,11 +846,25 @@ CIRGenFunction::emitX86BuiltinExpr(unsigned builtinID, const CallExpr *expr) {
   case X86::BI_m_prefetchw:
     return emitPrefetch(*this, builtinID, expr, ops);
   case X86::BI__rdtsc:
+    return builder.emitIntrinsicCallOp(getLoc(expr->getExprLoc()), "x86.rdtsc",
+                                       builder.getUInt64Ty());
   case X86::BI__builtin_ia32_rdtscp: {
-    cgm.errorNYI(expr->getSourceRange(),
-                 std::string("unimplemented X86 builtin call: ") +
-                     getContext().BuiltinInfo.getName(builtinID));
-    return mlir::Value{};
+    mlir::Location loc = getLoc(expr->getExprLoc());
+    mlir::Type i64Ty = builder.getUInt64Ty();
+    mlir::Type i32Ty = builder.getUInt32Ty();
+    mlir::Type structTy = builder.getAnonRecordTy({i64Ty, i32Ty});
+    mlir::Value result =
+        builder.emitIntrinsicCallOp(loc, "x86.rdtscp", structTy);
+
+    // Extract and store processor_id (element 1 of the returned struct)
+    mlir::Value processorId =
+        cir::ExtractMemberOp::create(builder, loc, i32Ty, result, 1);
+    // ops[0] is the address to store the processor ID
+    Address addr(ops[0], CharUnits::fromQuantity(4));
+    builder.createStore(loc, processorId, addr);
+
+    // Return timestamp (element 0 of the returned struct)
+    return cir::ExtractMemberOp::create(builder, loc, i64Ty, result, 0);
   }
   case X86::BI__builtin_ia32_lzcnt_u16:
   case X86::BI__builtin_ia32_lzcnt_u32:
diff --git a/clang/test/CIR/CodeGenBuiltins/X86/rdtsc.c b/clang/test/CIR/CodeGenBuiltins/X86/rdtsc.c
new file mode 100644
index 0000000000000..b7d203cbcda94
--- /dev/null
+++ b/clang/test/CIR/CodeGenBuiltins/X86/rdtsc.c
@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 -x c -ffreestanding -triple x86_64-unknown-linux -fclangir -emit-cir -o %t.cir %s
+// RUN: FileCheck --check-prefix=CIR --input-file=%t.cir %s
+
+// RUN: %clang_cc1 -x c -ffreestanding -triple x86_64-unknown-linux -fclangir -emit-llvm -o %t.ll %s
+// RUN: FileCheck --check-prefix=LLVM --input-file=%t.ll %s
+
+unsigned long long test_rdtsc() {
+  // CIR-LABEL: @test_rdtsc
+  // CIR: {{%.*}} = cir.call_llvm_intrinsic "x86.rdtsc"() : () -> !u64i
+
+  // LLVM-LABEL: @test_rdtsc
+  // LLVM: call i64 @llvm.x86.rdtsc()
+  
+  return __rdtsc();
+}
+
+unsigned long long test_rdtscp(unsigned int *aux) {
+  // CIR-LABEL: @test_rdtscp
+  // CIR: {{%.*}} = cir.call_llvm_intrinsic "x86.rdtscp"() : () -> !cir.record<!anon
+  // CIR: {{%.*}} = cir.extract_member {{%.*}}[1] : !cir.record<!anon
+  // CIR: cir.store {{%.*}}, {{%.*}} : !u32i
+  // CIR: {{%.*}} = cir.extract_member {{%.*}}[0] : !cir.record<!anon
+
+  // LLVM-LABEL: @test_rdtscp
+  // LLVM: call { i64, i32 } @llvm.x86.rdtscp()
+  // LLVM: extractvalue { i64, i32 } {{%.*}}, 1
+  // LLVM: store i32 {{%.*}}, ptr {{%.*}}
+  // LLVM: extractvalue { i64, i32 } {{%.*}}, 0
+  
+  return __builtin_ia32_rdtscp(aux);
+}