[ENCODEGEN][LLVM] Add support for llvm.readcyclecounter intrinsic

Add support for the @llvm.readcyclecounter intrinsic. This is used to
obtain the current value of the cycle counter register. The intrinsic is
useful to perform micro benchmarks and other small timing scenarios.

Author: marcauberer

tpde-encodegen/src/arm64/Target.cpp

@@ -303,6 +303,8 @@ void EncodingTargetArm64::get_inst_candidates(
     handle_adrp("ADRP");
   }
 
+  case_default("MRS", "MRS");
+
   const auto case_mov_shift = [&](std::string_view mnem_llvm,
                                   std::string_view mnem_disarm) {
     if (std::string_view{Name} == mnem_llvm) {

tpde-encodegen/src/x64/Target.cpp

@@ -760,6 +760,8 @@ void EncodingTargetX64::get_inst_candidates(
   case_default("MOVMSKPSrr", -1, "SSE_MOVMSKPSrr");
   case_default("MOVMSKPDrr", -1, "SSE_MOVMSKPDrr");
 
+  case_default("RDTSC", -1, "RDTSC");
+
   case_default("MFENCE", -1, "MFENCE");
   // clang-format on
 

tpde-llvm/src/LLVMCompilerBase.hpp

@@ -5025,6 +5025,10 @@ bool LLVMCompilerBase<Adaptor, Derived, Config>::compile_intrin(
     this->result_ref(inst).part(0).set_value(std::move(const_ref));
     return true;
   }
+  case llvm::Intrinsic::readcyclecounter: {
+    ValueRef res = this->result_ref(inst);
+    return derived()->encode_readcyclecounter(res.part(0));
+  }
   default: {
     return derived()->handle_intrin(inst);
   }

tpde-llvm/src/encode_template.c

@@ -822,3 +822,5 @@ void prefetch_wl0(void* addr) { __builtin_prefetch(addr, 1, 0); }
 void prefetch_wl1(void* addr) { __builtin_prefetch(addr, 1, 1); }
 void prefetch_wl2(void* addr) { __builtin_prefetch(addr, 1, 2); }
 void prefetch_wl3(void* addr) { __builtin_prefetch(addr, 1, 3); }
+
+u64 TARGET_V1 readcyclecounter(void) { return __builtin_readcyclecounter(); }

tpde-llvm/test/filetest/intrin_readcyclecounter.ll

@@ -0,0 +1,21 @@
+; NOTE: Assertions have been autogenerated by test/update_tpde_llc_test_checks.py UTC_ARGS: --version 5
+; SPDX-FileCopyrightText: 2025 Contributors to TPDE <https://tpde.org>
+;
+; SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+; RUN: tpde-llc --target=x86_64 %s | %objdump | FileCheck %s -check-prefixes=X64
+; RUN: tpde-llc --target=aarch64 %s | %objdump | FileCheck %s -check-prefixes=ARM64
+
+define i64 @readcyclecounter() {
+; X64-LABEL: <readcyclecounter>:
+; X64:         rdtsc
+; X64-NEXT:    shl rdx, 0x20
+; X64-NEXT:    or rax, rdx
+; X64-NEXT:    ret
+;
+; ARM64-LABEL: <readcyclecounter>:
+; ARM64:         mrs x0, CNTVCT_EL0
+; ARM64-NEXT:    ret
+  %res = call i64 @llvm.readcyclecounter()
+  ret i64 %res
+}