lib/string: optimized mem* functions

From: Matteo Croce <[email protected]>

Rewrite the generic mem{cpy,move,set} so that memory is accessed with the widest size possible, but without doing unaligned accesses.

This was originally posted as C string functions for RISC-V[1], but as there was no specific RISC-V code, it was proposed for the generic lib/string.c implementation.

Tested on RISC-V and on x86_64 by undefining __HAVE_ARCH_MEM{CPY,SET,MOVE} and HAVE_EFFICIENT_UNALIGNED_ACCESS.

These are the performances of memcpy() and memset() of a RISC-V machine on a 32 mbyte buffer:

memcpy:
original aligned: 75 Mb/s
original unaligned: 75 Mb/s
new aligned: 114 Mb/s
new unaligned: 107 Mb/s

memset:
original aligned: 140 Mb/s
original unaligned: 140 Mb/s
new aligned: 241 Mb/s
new unaligned: 241 Mb/s

The size increase is negligible:

$ scripts/bloat-o-meter vmlinux.orig vmlinux
add/remove: 0/0 grow/shrink: 4/1 up/down: 427/-6 (421)
Function                                     old     new   delta
memcpy                                        29     351    +322
memset                                        29     117     +88
strlcat                                       68      78     +10
strlcpy                                       50      57      +7
memmove                                       56      50      -6
Total: Before=8556964, After=8557385, chg +0.00%

These functions will be used for RISC-V initially.

[1] https://lore.kernel.org/linux-riscv/[email protected]/

Matteo Croce (3):
lib/string: optimized memcpy
lib/string: optimized memmove
lib/string: optimized memset

 lib/string.c | 130 ++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 113 insertions(+), 17 deletions(-)

2.31.1