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[v2] x86/crc32: use builtins to improve code generation

Message ID CAGG=3QVkd9Vb9a=pQ=KwhKzGJXaS+6Mk5K+JtBqamj15MzT9mQ@mail.gmail.com
State New
Headers show
Series [v2] x86/crc32: use builtins to improve code generation | expand

Commit Message

Bill Wendling Feb. 27, 2025, 11:47 p.m. UTC 
For both gcc and clang, crc32 builtins generate better code than the
inline asm. GCC improves, removing unneeded "mov" instructions. Clang
does the same and unrolls the loops. GCC has no changes on i386, but
Clang's code generation is vastly improved, due to Clang's "rm"
constraint issue.

The number of cycles improved by ~0.1% for GCC and ~1% for Clang, which
is expected because of the "rm" issue. However, Clang's performance is
better than GCC's by ~1.5%, most likely due to loop unrolling.

Link: https://github.com/llvm/llvm-project/issues/20571#issuecomment-2649330009
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: x86@kernel.org
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Eric Biggers <ebiggers@kernel.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <nick.desaulniers+lkml@gmail.com>
Cc: Justin Stitt <justinstitt@google.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-crypto@vger.kernel.org
Cc: llvm@lists.linux.dev
Signed-off-by: Bill Wendling <morbo@google.com>
---
v2 - Limited range of '-mcrc32' usage to single file.
   - Use a function instead of macros.
---
 arch/x86/lib/Makefile     |  2 ++
 arch/x86/lib/crc32-glue.c | 15 ++++++++-------
 2 files changed, 10 insertions(+), 7 deletions(-)
diff mbox series

Patch

diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
index 8a59c61624c2..1251f611ce3d 100644
--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -14,6 +14,8 @@  ifdef CONFIG_KCSAN
 CFLAGS_REMOVE_delay.o = $(CC_FLAGS_FTRACE)
 endif

+CFLAGS_crc32-glue.o := -mcrc32
+
 inat_tables_script = $(srctree)/arch/x86/tools/gen-insn-attr-x86.awk
 inat_tables_maps = $(srctree)/arch/x86/lib/x86-opcode-map.txt
 quiet_cmd_inat_tables = GEN     $@
diff --git a/arch/x86/lib/crc32-glue.c b/arch/x86/lib/crc32-glue.c
index 2dd18a886ded..fc70462ae2c1 100644
--- a/arch/x86/lib/crc32-glue.c
+++ b/arch/x86/lib/crc32-glue.c
@@ -47,11 +47,12 @@  u32 crc32_le_arch(u32 crc, const u8 *p, size_t len)
 }
 EXPORT_SYMBOL(crc32_le_arch);

-#ifdef CONFIG_X86_64
-#define CRC32_INST "crc32q %1, %q0"
-#else
-#define CRC32_INST "crc32l %1, %0"
-#endif
+static unsigned long crc32_ul(u32 crc, unsigned long p)
+{
+       if (IS_ENABLED(CONFIG_X86_64))
+               return __builtin_ia32_crc32di(crc, p);
+       return __builtin_ia32_crc32si(crc, p);
+}

 /*
  * Use carryless multiply version of crc32c when buffer size is >= 512 to
@@ -78,10 +79,10 @@  u32 crc32c_le_arch(u32 crc, const u8 *p, size_t len)

        for (num_longs = len / sizeof(unsigned long);
             num_longs != 0; num_longs--, p += sizeof(unsigned long))
-               asm(CRC32_INST : "+r" (crc) : "rm" (*(unsigned long *)p));
+               crc = crc32_ul(crc,  *(unsigned long *)p);

        for (len %= sizeof(unsigned long); len; len--, p++)
-               asm("crc32b %1, %0" : "+r" (crc) : "rm" (*p));
+               crc = __builtin_ia32_crc32qi(crc, *p);

        return crc;
 }