[libcxx-commits] [libcxx] [Ryu, performance] Use _BitScanForward64 more often, by fixing availability detection, avoiding calling _BitScanForward twice (PR #142000)

[Eugene Golushkov via libcxx-commits]

https://github.com/eugenegff created https://github.com/llvm/llvm-project/pull/142000

Use our private _BitScanForward64 for non-MSVC (in src/include/ryu/ryu.h).
Use MSVC _BitScanForward64 on _M_AMD64 and _M_ARM64, but not on the _M_ARM. 
Remove erroneous public #define _LIBCPP_HAS_BITSCAN64 (should be defined for _M_ARM64 but not for _M_ARM).

>From 200751c7de73c138c4aae647edb2a35577ff58fa Mon Sep 17 00:00:00 2001
From: Eugene Golushkov <e.golushkov at nospam-gmail.com>
Date: Thu, 29 May 2025 19:56:48 +0200
Subject: [PATCH] [Ryu, performance] Use our private _BitScanForward64 for
 non-MSVC (in src/include/ryu/ryu.h). Use MSVC _BitScanForward64 on _M_AMD64
 and _M_ARM64, but not on the _M_ARM. Remove erroneous public #define
 _LIBCPP_HAS_BITSCAN64 (should be defined for _M_ARM64 but not for _M_ARM).

---
 libcxx/include/__config         | 6 ------
 libcxx/include/__cxx03/__config | 3 ---
 libcxx/src/ryu/d2s.cpp          | 2 +-
 3 files changed, 1 insertion(+), 10 deletions(-)

diff --git a/libcxx/include/__config b/libcxx/include/__config
index 110450f6e9c51..316800681ec3f 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -222,15 +222,9 @@ _LIBCPP_HARDENING_MODE_DEBUG
 #    if defined(_MSC_VER) && !defined(__MINGW32__)
 #      define _LIBCPP_MSVCRT // Using Microsoft's C Runtime library
 #    endif
-#    if (defined(_M_AMD64) || defined(__x86_64__)) || (defined(_M_ARM) || defined(__arm__))
-#      define _LIBCPP_HAS_BITSCAN64 1
-#    else
-#      define _LIBCPP_HAS_BITSCAN64 0
-#    endif
 #    define _LIBCPP_HAS_OPEN_WITH_WCHAR 1
 #  else
 #    define _LIBCPP_HAS_OPEN_WITH_WCHAR 0
-#    define _LIBCPP_HAS_BITSCAN64 0
 #  endif // defined(_WIN32)
 
 #  if defined(_AIX) && !defined(__64BIT__)
diff --git a/libcxx/include/__cxx03/__config b/libcxx/include/__cxx03/__config
index ef47327d96355..4dac5964ff917 100644
--- a/libcxx/include/__cxx03/__config
+++ b/libcxx/include/__cxx03/__config
@@ -229,9 +229,6 @@ _LIBCPP_HARDENING_MODE_DEBUG
 #    if defined(_MSC_VER) && !defined(__MINGW32__)
 #      define _LIBCPP_MSVCRT // Using Microsoft's C Runtime library
 #    endif
-#    if (defined(_M_AMD64) || defined(__x86_64__)) || (defined(_M_ARM) || defined(__arm__))
-#      define _LIBCPP_HAS_BITSCAN64
-#    endif
 #    define _LIBCPP_HAS_OPEN_WITH_WCHAR
 #  endif // defined(_WIN32)
 
diff --git a/libcxx/src/ryu/d2s.cpp b/libcxx/src/ryu/d2s.cpp
index c0d11107f880b..0cab0a2ba6d62 100644
--- a/libcxx/src/ryu/d2s.cpp
+++ b/libcxx/src/ryu/d2s.cpp
@@ -479,7 +479,7 @@ struct __floating_decimal_64 {
           36893488u, 7378697u, 1475739u, 295147u, 59029u, 11805u, 2361u, 472u, 94u, 18u, 3u };
 
         unsigned long _Trailing_zero_bits;
-#if _LIBCPP_HAS_BITSCAN64
+#if !defined(_MSC_VER) || defined(_M_AMD64) || defined(_M_ARM64) // we have own _BitScanForward64 for non-MSVC
         (void) _BitScanForward64(&_Trailing_zero_bits, __v.__mantissa); // __v.__mantissa is guaranteed nonzero
 #else // ^^^ 64-bit ^^^ / vvv 32-bit vvv
         const uint32_t _Low_mantissa = static_cast<uint32_t>(__v.__mantissa);