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>

diff --git a/bisect.c b/bisect.c
index 135f94ba097ae07b5984d820e02eb50400271024..a7862195a90009ecc0a6e445d81d04ed8b966cb4 100644 (file)
--- 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 1b2b047ea069272052b13371c31ca06924231cc9..a6b3e1f09ecdef72fdcc4a234df7a176c4daded6 100644 (file)
--- 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 */