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// Copyright 2020 Google LLC
//
// This source code is licensed under the BSD-style license found in the
// LICENSE file in the root directory of this source tree.
#include <assert.h>
#include <stddef.h>
#include <stdint.h>
#include <wasm_simd128.h>
#include <xnnpack/math-stubs.h>
void xnn_math_f32_roundd__wasmsimd_addsub(
size_t n,
const float* input,
float* output)
{
assert(n % (4 * sizeof(float)) == 0);
// Mask for the sign bit of a floating-point number.
const v128_t vsign_mask = wasm_i32x4_splat(INT32_C(0x80000000));
// Addition of this number to a floating-point number x cause rounding of the result to an integer. Then this magic
// number is subtracted back from the result to get original x rounded to integer. This trick works only for
// 0 <= x < 2**24, but all numbers in 2**23 <= x < 2**24 range are integers, so we can further restrict it to
// 0 <= x < 2**23. Then the upper bound of the validity interval is conveniently the same as the magic number.
const v128_t vmagic_number = wasm_f32x4_splat(0x1.000000p+23f);
// Unit constant to decrement results rounded "wrong way" (i.e. up) in the round-to-nearest-even operation.
const v128_t vone = wasm_f32x4_splat(1.0f);
for (; n != 0; n -= 4 * sizeof(float)) {
const v128_t vx = wasm_v128_load(input);
input += 4;
// The rounding trick works only for x >= 0, so we compute absolute value of x, round it, and restore the sign in
// the end. This method works for round-to-nearest-even because it is an odd function.
const v128_t vabsx = wasm_v128_andnot(vx, vsign_mask);
// Compute bitmask for the bits we want to copy from x. Other bits will be copied from the rounded abs(x).
// If abs(x) < 2**23 or x is NaN, we want the sign bit from x and the rest from the rounded abs(x).
// Otherwise (abs(x) >= 2**23), we want all bits from x.
const v128_t vrndmask = wasm_v128_or(vsign_mask, wasm_f32x4_ge(vabsx, vmagic_number));
// Addition-subtraction trick with the magic number to cause rounding to integer for abs(x).
// Note: the result is valid only for 0 <= abs(x) < 2**23.
// Note: addition-subtraction implicitly converts SNaN inputs to QNaNs.
const v128_t vrndabsx = wasm_f32x4_sub(wasm_f32x4_add(vabsx, vmagic_number), vmagic_number);
// Combine abs(x) rounded via addition-subtraction trick and the input x value.
// For abs(x) < 2**23, the result is abs(x) rounded via addition-subtraction trick with the sign of x.
// For NaN inputs, the result is x converted to QNaN as a side-effect of addition-subtraction.
// For abs(x) >= 2**23, the result is x itself.
const v128_t vrndx = wasm_v128_bitselect(vx, vrndabsx, vrndmask);
// Adjust x rounded towards nearest-even to get x rounded down.
// Note: subtraction implicitly converts SNaN inputs to QNaNs.
const v128_t vy = wasm_f32x4_sub(vrndx, wasm_v128_and(wasm_f32x4_gt(vrndx, vx), vone));
wasm_v128_store(output, vy);
output += 4;
}
}