Fix MIPS64 memcpy regression

[Joseph Myers <joseph at codesourcery dot com>]

The MIPS memcpy optimizations at
<https://sourceware.org/ml/libc-alpha/2015-10/msg00597.html>
introduced a bug causing many string function tests to fail with
segfaults for n32 and n64:

FAIL: string/stratcliff
FAIL: string/test-bcopy
FAIL: string/test-memccpy
FAIL: string/test-memcmp
FAIL: string/test-memcpy
FAIL: string/test-memmove
FAIL: string/test-mempcpy
FAIL: string/test-stpncpy
FAIL: string/test-strncmp
FAIL: string/test-strncpy

(Some failures in other directories could also be caused by this bug.)

The problem is that after the check for whether a word of input is
left that can be copied as a word before moving to byte copies, a load
can occur in the branch delay slot, resulting in a segfault if we are
at the end of a page and the following page is unmapped.  I don't see
how this would have passed the tests as reported in the original patch
posting (different kernel configurations affecting the code setting up
unmapped pages, maybe?), since the tests in question don't appear to
have changed recently.

This patch adds a nop in the delay slot.

Tested with string/ tests for n32.  Will commit subject to full
testing for n32 and n64.

2016-01-21  Joseph Myers  <joseph@codesourcery.com>

	* sysdeps/mips/memcpy.S (MEMCPY_NAME) [USE_DOUBLE]: Avoid word
	load in branch delay slot when less than a word of input left.

diff --git a/sysdeps/mips/memcpy.S b/sysdeps/mips/memcpy.S
index d79e144..9ae0ba6 100644
--- a/sysdeps/mips/memcpy.S
+++ b/sysdeps/mips/memcpy.S
@@ -565,6 +565,7 @@ L(lastw):
 #ifdef USE_DOUBLE
 	andi    t8,a2,3		/* a2 is the remainder past 4 byte chunks.  */
 	beq	t8,a2,L(lastb)
+	nop
 	lw	REG3,0(a1)
 	sw	REG3,0(a0)
 	PTR_ADDIU a0,a0,4

-- 
Joseph S. Myers
joseph@codesourcery.com