[libc-commits] [libc] [libc] implement `strings/ffs` (PR #129892)

[via libc-commits]

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@@ -0,0 +1,17 @@
+//===-- Implementation of ffs ---------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/strings/ffs.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(int, ffs, (int i)) { return __builtin_ffs(i); }
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lntue wrote:

If you follow the godbolt link https://godbolt.org/z/xafvqha4c , `(value == 0) ? 0 : countr_zero(value) + 1` is identical to `__builtin_ffs` in both 0 and unsigned + signed maximum values.

What I really meant is that our current implementations and test cases for `stdc_first_trailing_one` are wrong on both 0 and unsigned maximum values.

https://github.com/llvm/llvm-project/pull/129892