wrapper: introduce `log2u()`

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>