ARM: 8745/1: get rid of __memzero()

The __memzero assembly code is almost identical to memset's except for
two orr instructions. The runtime performance of __memset(p, n) and
memset(p, 0, n) is accordingly almost identical.

However, the memset() macro used to guard against a zero length and to
call __memzero at compile time when the fill value is a constant zero
interferes with compiler optimizations.

Arnd found tha the test against a zero length brings up some new
warnings with gcc v8:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82103

And successively rremoving the test against a zero length and the call
to __memzero optimization produces the following kernel sizes for
defconfig with gcc 6:

    text     data     bss       dec       hex  filename
12248142  6278960  413588  18940690   1210312  vmlinux.orig
12244474  6278960  413588  18937022   120f4be  vmlinux.no_zero_test
12239160  6278960  413588  18931708   120dffc  vmlinux.no_memzero

So it is probably not worth keeping __memzero around given that the
compiler can do a better job at inlining trivial memset(p,0,n) on its
own. And the memset code already handles a zero length just fine.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

arch/arm/kernel/armksyms.c

@@ -92,7 +92,6 @@ EXPORT_SYMBOL(__memset64);
 EXPORT_SYMBOL(memcpy);
 EXPORT_SYMBOL(memmove);
 EXPORT_SYMBOL(memchr);
-EXPORT_SYMBOL(__memzero);
 
 EXPORT_SYMBOL(mmioset);
 EXPORT_SYMBOL(mmiocpy);

arch/arm/kernel/head-common.S

@@ -105,8 +105,9 @@ __mmap_switched:
    ARM(	ldmia	r4!, {r0, r1, sp} )
  THUMB(	ldmia	r4!, {r0, r1, r3} )
  THUMB(	mov	sp, r3 )
-	sub	r1, r1, r0
-	bl	__memzero			@ clear .bss
+	sub	r2, r1, r0
+	mov	r1, #0
+	bl	memset				@ clear .bss
 
 	ldmia	r4, {r0, r1, r2, r3}
 	str	r9, [r0]			@ Save processor ID