Further optimize NEON & SSE2 conversion-based rounding stubs
PiperOrigin-RevId: 311283485
diff --git a/src/math/roundne-sse2-cvt.c b/src/math/roundne-sse2-cvt.c
index 2c30600..530fea9 100644
--- a/src/math/roundne-sse2-cvt.c
+++ b/src/math/roundne-sse2-cvt.c
@@ -18,30 +18,27 @@
{
assert(n % (4 * sizeof(float)) == 0);
- // This magic number with a bit representation 0x80000000 serves two purposes:
- // 1. Extract the sign of a floating-point number.
+ // This magic number serves two purposes:
+ // 1. Set the bit corresponding to the sign of a floating-point number in a bitmask.
// 2. Check if the input to CVTPS2DQ (_mm_cvtps_epi32) is out-of-range, which results in 0x80000000 output.
- const __m128 vmagic = _mm_set1_ps(-0.0f);
+ const __m128i vmagic = _mm_set1_epi32(0x80000000);
for (; n != 0; n -= 4 * sizeof(float)) {
const __m128 vx = _mm_load_ps(input);
input += 4;
- // Extract the sign of the input.
- // We need the sign to preserve negative zero value, which would otherwise get lost in FP->INT->FP conversion.
- const __m128 vsignx = _mm_and_ps(vx, vmagic);
// Convert floating-point value x to integer, with default rounding (to nearest-even).
// If x is beyond [-2**31, 2**31-1] range or x is NaN, the result is -2**31 (0x80000000).
const __m128i vintx = _mm_cvtps_epi32(vx);
- // Compute bitmask for out-of-range conversion input.
- // The bitmask is set to all ones when x is out-of-range for CVTPS2DQ, and also when x == -2**31. The latter case
- // is ok, because this x is already an integer, and can be passed to output as is.
- const __m128 vrndmask = _mm_castsi128_ps(_mm_cmpeq_epi32(vintx, _mm_castps_si128(vmagic)));
+ // Compute bitmask for the bits we want to copy from the rounded x. Other bits will be copied from x.
+ // If x is out-of-range for CVTPS2DQ, we want all bits from x.
+ // If x is in-range for CVTPS2DQ, we want all but the sign bit from the rounded x and the sign bit from x.
+ const __m128 vrndmask = _mm_castsi128_ps(_mm_or_si128(vmagic, _mm_cmpeq_epi32(vintx, vmagic)));
// Convert integer back to floating-point.
// We binary OR the result with the sign of x to restore the sign of negative zero.
- const __m128 vrndx = _mm_or_ps(_mm_cvtepi32_ps(vintx), vsignx);
+ const __m128 vrndx = _mm_cvtepi32_ps(vintx);
// Combine x rounded via conversion to integer and the initial x value.
// For -2**31 < x < 2**31, the result is x rounded via conversion to integer.