From d343068e4abc5e43d1ef1d5fed42bf4d7aa8cff4 Mon Sep 17 00:00:00 2001 From: Patrick Steinhardt Date: Wed, 4 Sep 2024 10:53:00 +0200 Subject: [PATCH] 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 Signed-off-by: Junio C Hamano --- bisect.c | 12 +----------- wrapper.h | 18 ++++++++++++++++++ 2 files changed, 19 insertions(+), 11 deletions(-) diff --git a/bisect.c b/bisect.c index 135f94ba09..a7862195a9 100644 --- a/bisect.c +++ b/bisect.c @@ -1130,16 +1130,6 @@ cleanup: return res; } -static inline int log2i(int n) -{ - int log2 = 0; - - for (; n > 1; n >>= 1) - log2++; - - return log2; -} - static inline int exp2i(int n) { return 1 << n; @@ -1162,7 +1152,7 @@ int estimate_bisect_steps(int all) if (all < 3) return 0; - n = log2i(all); + n = log2u(all); e = exp2i(n); x = all - e; diff --git a/wrapper.h b/wrapper.h index 1b2b047ea0..a6b3e1f09e 100644 --- a/wrapper.h +++ b/wrapper.h @@ -140,4 +140,22 @@ int csprng_bytes(void *buf, size_t len); */ uint32_t git_rand(void); +/* Provide log2 of the given `size_t`. */ +static inline unsigned log2u(uintmax_t sz) +{ + unsigned l = 0; + + /* + * Technically this isn't required, but it helps the compiler optimize + * this to a `bsr` instruction. + */ + if (!sz) + return 0; + + for (; sz; sz >>= 1) + l++; + + return l - 1; +} + #endif /* WRAPPER_H */