Resolves microsoft#1120

tests/std/test.lst

@@ -164,6 +164,7 @@ tests\GH_000890_pow_template
 tests\GH_000940_missing_valarray_copy
 tests\GH_001010_filesystem_error_encoding
 tests\GH_001017_discrete_distribution_out_of_range
+tests\GH_001103_countl_zero_correctness
 tests\LWG2597_complex_branch_cut
 tests\LWG3018_shared_ptr_function
 tests\P0024R2_parallel_algorithms_adjacent_difference

tests/std/tests/GH_001103_countl_zero_correctness/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_latest_matrix.lst

tests/std/tests/GH_001103_countl_zero_correctness/test.cpp

@@ -0,0 +1,44 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cassert>
+#include <bit>
+
+// Indirectly test countl_zero on old x86/x64 processors by testing private hepler,
+// which is different from the usual branch.
+
+// Since the fallback intrinsic is available on ARM too, don't need to exclude ARM
+
+// Currently need to test not only in C++20 mode,
+// May update to loder C++ if the helper is used internally too, for example in <bitset>
+
+using namespace std;
+
+int main() {
+    assert(_Countl_zero_bsr(static_cast<unsigned char>(0x00)) == 8);
+    assert(_Countl_zero_bsr(static_cast<unsigned char>(0x13)) == 3);
+    assert(_Countl_zero_bsr(static_cast<unsigned char>(0x83)) == 0);
+    assert(_Countl_zero_bsr(static_cast<unsigned char>(0xF8)) == 0);
+
+    assert(_Countl_zero_bsr(static_cast<unsigned short>(0x0000)) == 16);
+    assert(_Countl_zero_bsr(static_cast<unsigned short>(0x0013)) == 11);
+    assert(_Countl_zero_bsr(static_cast<unsigned short>(0x8003)) == 0);
+    assert(_Countl_zero_bsr(static_cast<unsigned short>(0xF008)) == 0);
+
+    assert(_Countl_zero_bsr(static_cast<unsigned short>(0x0000)) == 16);
+    assert(_Countl_zero_bsr(static_cast<unsigned short>(0x0013)) == 11);
+    assert(_Countl_zero_bsr(static_cast<unsigned short>(0x8003)) == 0);
+    assert(_Countl_zero_bsr(static_cast<unsigned short>(0xF008)) == 0);
+
+    assert(_Countl_zero_bsr(static_cast<unsigned long>(0x0000'0000)) == 32);
+    assert(_Countl_zero_bsr(static_cast<unsigned long>(0x0000'0013)) == 27);
+    assert(_Countl_zero_bsr(static_cast<unsigned long>(0x8000'0003)) == 0);
+    assert(_Countl_zero_bsr(static_cast<unsigned long>(0xF000'0008)) == 0);
+
+    assert(_Countl_zero_bsr(static_cast<unsigned long long>(0x0000'0000'0000'0000)) == 64);
+    assert(_Countl_zero_bsr(static_cast<unsigned long long>(0x0000'0000'0000'0013)) == 59);
+    assert(_Countl_zero_bsr(static_cast<unsigned long long>(0x8000'0000'0000'0003)) == 0);
+    assert(_Countl_zero_bsr(static_cast<unsigned long long>(0xF000'0000'0000'0008)) == 0);
+
+    return 0;
+}