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45 lines
1.2 KiB
Go
45 lines
1.2 KiB
Go
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// Copyright 2024 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package bigmod
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// The generic implementation relies on 64x64->128 bit multiplication and
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// 64-bit add-with-carry, which are compiler intrinsics on many architectures.
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// Wasm doesn't support those. Here we implement it with 32x32->64 bit
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// operations, which is more efficient on Wasm.
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// addMulVVW multiplies the multi-word value x by the single-word value y,
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// adding the result to the multi-word value z and returning the final carry.
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// It can be thought of as one row of a pen-and-paper column multiplication.
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func addMulVVW(z, x []uint, y uint) (carry uint) {
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const mask32 = 1<<32 - 1
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y0 := y & mask32
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y1 := y >> 32
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_ = x[len(z)-1] // bounds check elimination hint
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for i, zi := range z {
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xi := x[i]
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x0 := xi & mask32
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x1 := xi >> 32
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z0 := zi & mask32
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z1 := zi >> 32
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c0 := carry & mask32
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c1 := carry >> 32
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w00 := x0*y0 + z0 + c0
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l00 := w00 & mask32
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h00 := w00 >> 32
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w01 := x0*y1 + z1 + h00
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l01 := w01 & mask32
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h01 := w01 >> 32
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w10 := x1*y0 + c1 + l01
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h10 := w10 >> 32
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carry = x1*y1 + h10 + h01
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z[i] = w10<<32 + l00
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}
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return carry
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}
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