Update to tagged container-libs releases

Signed-off-by: Miloslav Trmač <mitr@redhat.com>
This commit is contained in:
Miloslav Trmač
2025-09-03 17:56:58 +02:00
parent e54634f56a
commit e5cf4ec75e
52 changed files with 1509 additions and 738 deletions

View File

@@ -371,7 +371,31 @@ func ConsumeVarint(b []byte) (v uint64, n int) {
func SizeVarint(v uint64) int {
// This computes 1 + (bits.Len64(v)-1)/7.
// 9/64 is a good enough approximation of 1/7
return int(9*uint32(bits.Len64(v))+64) / 64
//
// The Go compiler can translate the bits.LeadingZeros64 call into the LZCNT
// instruction, which is very fast on CPUs from the last few years. The
// specific way of expressing the calculation matches C++ Protobuf, see
// https://godbolt.org/z/4P3h53oM4 for the C++ code and how gcc/clang
// optimize that function for GOAMD64=v1 and GOAMD64=v3 (-march=haswell).
// By OR'ing v with 1, we guarantee that v is never 0, without changing the
// result of SizeVarint. LZCNT is not defined for 0, meaning the compiler
// needs to add extra instructions to handle that case.
//
// The Go compiler currently (go1.24.4) does not make use of this knowledge.
// This opportunity (removing the XOR instruction, which handles the 0 case)
// results in a small (1%) performance win across CPU architectures.
//
// Independently of avoiding the 0 case, we need the v |= 1 line because
// it allows the Go compiler to eliminate an extra XCHGL barrier.
v |= 1
// It would be clearer to write log2value := 63 - uint32(...), but
// writing uint32(...) ^ 63 is much more efficient (-14% ARM, -20% Intel).
// Proof of identity for our value range [0..63]:
// https://go.dev/play/p/Pdn9hEWYakX
log2value := uint32(bits.LeadingZeros64(v)) ^ 63
return int((log2value*9 + (64 + 9)) / 64)
}
// AppendFixed32 appends v to b as a little-endian uint32.