AudioStream: Vectorize volume application

pcsx2/Host/AudioStream.cpp

@@ -324,12 +324,34 @@ void AudioStream::ReadFrames(SampleType* samples, u32 num_frames)
 
 	if (m_volume != 100)
 	{
-		const s32 volume_mult = static_cast<s32>((static_cast<float>(m_volume) / 100.0f) * 32768.0f);
-
 		u32 num_samples = num_frames * m_output_channels;
+
+		const u32 aligned_samples = Common::AlignDownPow2(num_samples, 8);
+		num_samples -= aligned_samples;
+
+		const float volume_mult = static_cast<float>(m_volume) / 100.0f;
+		const GSVector4 volume_multv = GSVector4(volume_mult);
+		const SampleType* const aligned_samples_end = samples + aligned_samples;
+		for (; samples != aligned_samples_end; samples += 8)
+		{
+			GSVector4i iv = GSVector4i::load<false>(samples); // [0, 1, 2, 3, 4, 5, 6, 7]
+			GSVector4i iv1 = iv.upl16(iv); // [0, 0, 1, 1, 2, 2, 3, 3]
+			GSVector4i iv2 = iv.uph16(iv); // [4, 4, 5, 5, 6, 6, 7, 7]
+			iv1 = iv1.sra32<16>(); // [0, 1, 2, 3]
+			iv2 = iv2.sra32<16>(); // [4, 5, 6, 7]
+			GSVector4 fv1 = GSVector4(iv1); // [f0, f1, f2, f3]
+			GSVector4 fv2 = GSVector4(iv2); // [f4, f5, f6, f7]
+			fv1 = fv1 * volume_multv; // [f0, f1, f2, f3]
+			fv2 = fv2 * volume_multv; // [f4, f5, f6, f7]
+			iv1 = GSVector4i(fv1); // [0, 1, 2, 3]
+			iv2 = GSVector4i(fv2); // [4, 5, 6, 7]
+			iv = iv1.ps32(iv2); // [0, 1, 2, 3, 4, 5, 6, 7]
+			GSVector4i::store<false>(samples, iv);
+		}
+
 		while (num_samples > 0)
 		{
-			*samples = static_cast<s16>(std::clamp((static_cast<s32>(*samples) * volume_mult) >> 15, -0x7fff, 0x7fff));
+			*samples = static_cast<s16>(std::clamp(static_cast<float>(*samples) * volume_mult, -32768.0f, 32767.0f));
 			samples++;
 			num_samples--;
 		}