// Copyright 2024 The Go Authors. All rights reserved. // Use of this source code is governed by a BSD-style // license that can be found in the LICENSE file. //go:build !purego package bigmod import "unsafe" // The generic implementation relies on 64x64->128 bit multiplication and // 64-bit add-with-carry, which are compiler intrinsics on many architectures. // Wasm doesn't support those. Here we implement it with 32x32->64 bit // operations, which is more efficient on Wasm. func idx(x *uint, i uintptr) *uint { return (*uint)(unsafe.Pointer(uintptr(unsafe.Pointer(x)) + i*8)) } func addMulVVWWasm(z, x *uint, y uint, n uintptr) (carry uint) { const mask32 = 1<<32 - 1 y0 := y & mask32 y1 := y >> 32 for i := range n { xi := *idx(x, i) x0 := xi & mask32 x1 := xi >> 32 zi := *idx(z, i) z0 := zi & mask32 z1 := zi >> 32 c0 := carry & mask32 c1 := carry >> 32 w00 := x0*y0 + z0 + c0 l00 := w00 & mask32 h00 := w00 >> 32 w01 := x0*y1 + z1 + h00 l01 := w01 & mask32 h01 := w01 >> 32 w10 := x1*y0 + c1 + l01 h10 := w10 >> 32 carry = x1*y1 + h10 + h01 *idx(z, i) = w10<<32 + l00 } return carry } func addMulVVW1024(z, x *uint, y uint) (c uint) { return addMulVVWWasm(z, x, y, 1024/_W) } func addMulVVW1536(z, x *uint, y uint) (c uint) { return addMulVVWWasm(z, x, y, 1536/_W) } func addMulVVW2048(z, x *uint, y uint) (c uint) { return addMulVVWWasm(z, x, y, 2048/_W) }