When memcmp(a, b, len) (or equally, bcmp) is called with a phony length
such that a + len < a, the code would malfunction and not compare the
two buffers correctly. While such arguments are illegal (buffers do not
wrap around the end of the address space), it is neverthless conceivable
that people try things like memcmp(a, b, SIZE_MAX) to compare a and b
until the first mismatch, in the knowledge that such a mismatch exists,
expecting memcmp() to stop comparing somewhere around the mismatch.
While memcmp() is usually written to confirm to this assumption, no
version of ISO/IEC 9899 guarantees this behaviour (in contrast to
memchr() for which it is).
Neverthless it appears sensible to at least not grossly misbehave on
phony lengths. This change hardens memcmp() against this case by
comparing at least until the end of the address space if a + len
overflows a 64 bit integer.
Sponsored by: The FreeBSD Foundation
Approved by: mjg (blanket, via IRC)
See also: b2618b651b28fd29e62a4e285f5be09ea30a85d4
MFC after: 1 week
(cherry picked from commit 953b93cf24d8871c62416c9bcfca935f1f1853b6)
This changeset adds a baseline implementation of memcmp and bcmp
for amd64. The same code is used for both functions with conditional
code were the behaviour differs (we need more precise output for the
memcmp case).
FreeBSD documents that memcmp returns the difference between the
mismatching characters. Slightly faster code would be possible could
we relax this requirement to the ISO/IEC 9899:1999 requirement of
merely returning a negative/positive integer or zero.
Performance is better than bionic and glibc, except for long strings
were the two are 13% faster. This could be because they use SSE4
ptest which we cannot use in a baseline kernel.
Sponsored by: The FreeBSD Foundation
Approved by: mjg
Differential Revision: https://reviews.freebsd.org/D41442
Turns out clang converts "memcmp(foo, bar, len) == 0" and similar to
bcmp calls.
Reviewed by: emaste (previous version), jhb (previous version)
Differential Revision: https://reviews.freebsd.org/D34673
This is a tradeoff which saves jumps for smaller sizes while making
the 8-16 range slower (roughly in line with the other cases).
Tested with glibc test suite.
For example size 3 (most common with vfs namecache) (ops/s):
before: 407086026
after: 461391995
The regressed range of 8-16 (with 8 as example):
before: 540850489
after: 461671032
Both are significantly slower than hand-coded loops. See r338963 for
kernel commit.
bcmp differs from memcmp by always returning 1 when a difference is
found, as opposed to going for a value bigger or lower than 0
depending on what it is. This means it can do less work. For now the
code is duplicated and modified. This will get deduplicated after
another round of optimization when memcmp will get a longer-term form.
Both tested with the glibc suite. While the suite does not have a test
for bcmp, I created a wrapper routine which verified that values match
(0 vs 0, 1 vs non-zero).
Reviewed by: kib
Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17336
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.
