diff options
Diffstat (limited to 'src')
| -rw-r--r-- | src/internal/strconv/itoa.go | 42 |
1 files changed, 2 insertions, 40 deletions
diff --git a/src/internal/strconv/itoa.go b/src/internal/strconv/itoa.go index d166731ea5..16c2bfbc99 100644 --- a/src/internal/strconv/itoa.go +++ b/src/internal/strconv/itoa.go @@ -191,10 +191,6 @@ func formatBase10(a []byte, u uint64) int { // On most systems, the uint32 math is faster, but not all. // The decision here is based on benchmarking. itoaPure64 = host64bit && goarch.GOARCH != "amd64" && goarch.GOARCH != "arm64" && goarch.GOARCH != "s390x" - - // 64-bit systems can all use 64-bit div and mod by a constant, - // which the compiler rewrites to use 64x64→128-bit multiplies. - itoaDivMod64 = host64bit // can use 64-bit div/mod by constant ) if itoaPure64 { @@ -218,47 +214,13 @@ func formatBase10(a []byte, u uint64) int { return i } - // Convert 9-digit chunks using 32-bit math. + // Split into 9-digit chunks that fit in uint32s and convert each chunk using 32-bit math. // Most numbers are small, so the comparison u >= 1e9 is usually pure overhead, // so we approximate it by u>>29 != 0, which is usually faster and good enough. i := len(a) for (host64bit && u>>29 != 0) || (!host64bit && (u>>32 != 0 || uint32(u)>>29 != 0)) { var lo uint32 - if itoaDivMod64 { - u, lo = u/1e9, uint32(u%1e9) - } else { - // On 64-bit systems the compiler rewrites the div and mod above - // into a 64x64→128-bit multiply (https://godbolt.org/z/EPnK8zvMK): - // hi, _ := bits.Mul64(u>>1, 0x89705f4136b4a598) - // q := hi >> 28 - // lo = uint32(u - q*1e9) - // u = q - // On 32-bit systems, the compiler invokes a uint64 software divide, - // which is quite slow. We could write the bits.Mul64 code above - // but even that is slower than we'd like, since it calls a software mul64 - // instead of having a hardware instruction to use. - // Instead we inline bits.Mul64 here and change y0/y1 to constants. - // The compiler does use direct 32x32→64-bit multiplies for this code. - // - // For lots more about division by multiplication see Warren, _Hacker's Delight_. - // For a concise overview, see the first two sections of - // https://ridiculousfish.com/blog/posts/labor-of-division-episode-iii.html. - const mask32 = 1<<32 - 1 - x0 := ((u >> 1) & mask32) - x1 := (u >> 1) >> 32 - const y0 = 0x36b4a598 - const y1 = 0x89705f41 - w0 := x0 * y0 - t := x1*y0 + w0>>32 - w1 := t & mask32 - w2 := t >> 32 - w1 += x0 * y1 - hi := x1*y1 + w2 + w1>>32 - q := hi >> 28 - - lo = uint32(u) - uint32(q)*1e9 // uint32(u - q*1e9) but faster - u = q - } + u, lo = u/1e9, uint32(u%1e9) // Convert 9 digits. for range 4 { |