d343068e4a
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>
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