Instead of jumping to locations which store the exact number of bytes,
use displacement to move the destination.
In particular the following clears an area between 8-16 (inclusive)
branch-free:
movq %r10,(%rdi)
movq %r10,-8(%rdi,%rcx)
For instance for rcx of 10 the second line is rdi + 10 - 8 = rdi + 2.
Writing 8 bytes starting at that offset overlaps with 6 bytes written
previously and writes 2 new, giving 10 in total.
Provides a nice win for smaller stores. Other ones are erratic depending
on the microarchitecture.
General idea taken from NetBSD (restricted use of the trick) and bionic
string functions (use for various ranges like in this patch).
Reviewed by: kib (previous version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17660
- tidy up memset to have rax set earlier for small sizes
- finish the tail in memset with an overlapping store
- align memset buffers to 16 bytes before using rep stos
Sponsored by: The FreeBSD Foundation
See r339205 for details.
An unused ERMS support is retained in the macro. It will be activated
after ifunc support lands.
Reviewed by: kib
Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17405
This is a depessimization, see r334537 for an explanation. Routines
remain significantly slower than they have to be.
bzero was removed from the kernel but remains in libc. Macroify to
accommodate differences to memset (no return value, always setting to 0).
The bzero.S file is left in place due to libc build magic which pulls in
a C variant if a matching .S file is missing.
Reviewed by: kib
Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D17355
is used to set the ELF size attribute for functions. It isn't normally
critical but some things can make use of it (gdb for stack traces).
Valgrind needs it so I'm adding it in. The problem is present on all
branches and on both i386 and amd64.
