From: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Date: Wed, 11 Jun 2025 22:14:41 +0000 (+0200)
Subject: bswap.h: remove optimized x86 version of bswap32/64
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=13c2acd2873397f52f07827a7c1fb663bb8f4d14;p=thirdparty%2Fgit.git

bswap.h: remove optimized x86 version of bswap32/64

On x86 the bswap32/64 macro is implemenated based on the x86 opcode wich
performs the required shifting in just one opcode.
The other CPUs fallback to the generic shifting as implemented by
default_swab32() and default_bswap64() if needed.

I've been looking at how good a compiler is at recognizing the default
shift and emitting an optimized operation:
- x86, arm64 msvc v19.20
  default_swab32() optimized
  default_bswap64() shifts
  _byteswap_uint64() otimized

- x86, arm64 msvc v19.37
  default_swab32() optimized
  default_bswap64() optimized
  _byteswap_uint64() otimized

- arm64, gcc-4.9.4: optimized
- x86-64, gcc-4.4.7: shifts
- x86-64, gcc-4.5.3: optimized
- x86-64, clang-3.0: optimized

Given that gcc-4.5 and clang-3.0 are fairly old, any recent compiler
should recognize the shift.

Remove the optimized x86 version and rely on the compiler.

Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---

diff --git a/compat/bswap.h b/compat/bswap.h
index aeef304f67..ed00f6d1d5 100644
--- a/compat/bswap.h
+++ b/compat/bswap.h
@@ -35,46 +35,7 @@ static inline uint64_t default_bswap64(uint64_t val)
 #undef bswap32
 #undef bswap64
 
-#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
-
-#define bswap32 git_bswap32
-static inline uint32_t git_bswap32(uint32_t x)
-{
-	uint32_t result;
-	if (__builtin_constant_p(x))
-		result = default_swab32(x);
-	else
-		__asm__("bswap %0" : "=r" (result) : "0" (x));
-	return result;
-}
-
-#define bswap64 git_bswap64
-#if defined(__x86_64__)
-static inline uint64_t git_bswap64(uint64_t x)
-{
-	uint64_t result;
-	if (__builtin_constant_p(x))
-		result = default_bswap64(x);
-	else
-		__asm__("bswap %q0" : "=r" (result) : "0" (x));
-	return result;
-}
-#else
-static inline uint64_t git_bswap64(uint64_t x)
-{
-	union { uint64_t i64; uint32_t i32[2]; } tmp, result;
-	if (__builtin_constant_p(x))
-		result.i64 = default_bswap64(x);
-	else {
-		tmp.i64 = x;
-		result.i32[0] = git_bswap32(tmp.i32[1]);
-		result.i32[1] = git_bswap32(tmp.i32[0]);
-	}
-	return result.i64;
-}
-#endif
-
-#elif defined(_MSC_VER) && (defined(_M_IX86) || defined(_M_X64) || defined(_M_ARM64))
+#if defined(_MSC_VER) && (defined(_M_IX86) || defined(_M_X64) || defined(_M_ARM64))
 
 #include <stdlib.h>