x86/bitops: Remove unused __sw_hweight64() assembly implementation on x86-32

Header cleanups in the fast-headers tree highlighted that we have an
unused assembly implementation for __sw_hweight64():

    WARNING: modpost: EXPORT symbol "__sw_hweight64" [vmlinux] version ...

__arch_hweight64() on x86-32 is defined in the
arch/x86/include/asm/arch_hweight.h header as an inline, using
__arch_hweight32():

  #ifdef CONFIG_X86_32
  static inline unsigned long __arch_hweight64(__u64 w)
  {
          return  __arch_hweight32((u32)w) +
                  __arch_hweight32((u32)(w >> 32));
  }

*But* there's also a __sw_hweight64() assembly implementation:

  arch/x86/lib/hweight.S

  SYM_FUNC_START(__sw_hweight64)
  #ifdef CONFIG_X86_64
  ...
  #else /* CONFIG_X86_32 */
        /* We're getting an u64 arg in (%eax,%edx): unsigned long hweight64(__u64 w) */
        pushl   %ecx

        call    __sw_hweight32
        movl    %eax, %ecx                      # stash away result
        movl    %edx, %eax                      # second part of input
        call    __sw_hweight32
        addl    %ecx, %eax                      # result

        popl    %ecx
        ret
  #endif

But this __sw_hweight64 assembly implementation is unused - and it's
essentially doing the same thing that the inline wrapper does.

Remove the assembly version and add a comment about it.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org

Author: ingomolnar

arch/x86/lib/hweight.S

@@ -36,8 +36,12 @@ SYM_FUNC_START(__sw_hweight32)
 SYM_FUNC_END(__sw_hweight32)
 EXPORT_SYMBOL(__sw_hweight32)
 
-SYM_FUNC_START(__sw_hweight64)
+/*
+ * No 32-bit variant, because it's implemented as an inline wrapper
+ * on top of __arch_hweight32():
+ */
 #ifdef CONFIG_X86_64
+SYM_FUNC_START(__sw_hweight64)
 	pushq   %rdi
 	pushq   %rdx
 
@@ -66,18 +70,6 @@ SYM_FUNC_START(__sw_hweight64)
 	popq    %rdx
 	popq    %rdi
 	RET
-#else /* CONFIG_X86_32 */
-	/* We're getting an u64 arg in (%eax,%edx): unsigned long hweight64(__u64 w) */
-	pushl   %ecx
-
-	call    __sw_hweight32
-	movl    %eax, %ecx                      # stash away result
-	movl    %edx, %eax                      # second part of input
-	call    __sw_hweight32
-	addl    %ecx, %eax                      # result
-
-	popl    %ecx
-	RET
-#endif
 SYM_FUNC_END(__sw_hweight64)
 EXPORT_SYMBOL(__sw_hweight64)
+#endif