We have an implementation of a function that computes the log2 for an
integer. While we could instead use log2(3P), that involves floating
point numbers and is thus needlessly complex and inefficient.
We're about to add a second caller that wants to compute log2 for a
`size_t`. Let's thus move the function into "wrapper.h" such that it
becomes generally available.
While at it, tweak the implementation a bit:
- The parameter is converted from `int` to `uintmax_t`. This
conversion is safe to do in "bisect.c" because we already check that
the argument is positive.
- The return value is an `unsigned`. It cannot ever be negative, so it
is pointless for it to be a signed integer.
- Loop until `!n` instead of requiring that `n > 1` and then subtract
1 from the result and add a special case for `!sz`. This helps
compilers to generate more efficient code.
Compilers recognize the pattern of this function and optimize
accordingly. On GCC 14.2 x86_64:
log2u(unsigned long):
test rdi, rdi
je .L3
bsr rax, rdi
ret
.L3:
mov eax, -1
ret
Clang 18.1 does not yet recognize the pattern, but starts to do so on
Clang trunk x86_64. The code isn't quite as efficient as the one
generated by GCC, but still manages to optimize away the loop:
log2u(unsigned long):
test rdi, rdi
je .LBB0_1
shr rdi
bsr rcx, rdi
mov eax, 127
cmovne rax, rcx
xor eax, -64
add eax, 65
ret
.LBB0_1:
mov eax, -1
ret
The pattern is also recognized on other platforms like ARM64 GCC 14.2.0,
where we end up using `clz`:
log2u(unsigned long):
clz x2, x0
cmp x0, 0
mov w1, 63
sub w0, w1, w2
csinv w0, w0, wzr, ne
ret
Note that we have a similar function `fastlog2()` in the reftable code.
As that codebase is separate from the Git codebase we do not adapt it to
use the new function.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
remove_or_warn() is only used by entry.c and apply.c, but it is
currently declared and defined in wrapper.{h,c}, so it has a scope much
greater than it needs. This needlessly large scope also causes wrapper.c
to need to include object.h, when this file is largely unconcerned with
Git objects.
Move remove_or_warn() to entry.{h,c}. The file apply.c still has access
to it, since it already includes entry.h for another reason.
Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
When we initially created background maintenance -- with its hourly,
daily, and weekly schedules -- we considered the effects of all clients
launching fetches to the server every hour on the hour. The worry of
DDoSing server hosts was noted, but left as something we would consider
for a future update.
As background maintenance has gained more adoption over the past three
years, our worries about DDoSing the big Git hosts has been unfounded.
Those systems, especially those serving public repositories, are already
resilient to thundering herds of much smaller scale.
However, sometimes organizations spin up specific custom server
infrastructure either in addition to or on top of their Git host. Some
of these technologies are built for a different range of scale, and can
hit concurrency limits sooner. Organizations with such custom
infrastructures are more likely to recommend tools like `scalar` which
furthers their adoption of background maintenance.
To help solve for this, create get_random_minute() as a method to help
Git select a random minute when creating schedules in the future. The
integrations with this method do not yet exist, but will follow in
future changes.
To avoid multiple sources of randomness in the Git codebase, create a
new helper function, git_rand(), that returns a random uint32_t. This is
similar to how rand() returns a random nonnegative value, except it is
based on csprng_bytes() which is cryptographic and will return values
larger than RAND_MAX.
One thing that is important for testability is that we notice when we
are under a test scenario and return a predictable result. The schedules
themselves are not checked for this value, but at least one launchctl
test checks that we do not unnecessarily reboot the schedule if it has
not changed from a previous version.
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
As mentioned in the thread starting at [1], trace2 counters should be
used to count events instead of ad-hoc static variables.
Convert the two fsync static variables to trace2 counters, reducing the
coupling between wrapper.c and the trace2 subsystem. Adjust t/t5351 to
match the trace2 counter output format.
The counters are not per-thread because the ones being replaced also
were not.
[1] https://lore.kernel.org/git/20230627195251.1973421-2-calvinwan@google.com/
Signed-off-by: Beat Bolli <dev+git@drbeat.li>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Since the functions in wrapper.c are widely used across the codebase,
include it by default in git-compat-util.h. A future patch will remove
now unnecessary inclusions of wrapper.h from other files.
Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
