lib/ppc/fixunstfdi.c

// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.

// uint64_t __fixunstfdi(long double x);
// This file implements the PowerPC 128-bit double-double -> uint64_t conversion

#include "DD.h"
#include <stdint.h>

uint64_t __fixunstfdi(long double input)
{
	const DD x = { .ld = input };
	const doublebits hibits = { .d = x.hi };
	
	const uint32_t highWordMinusOne = (uint32_t)(hibits.x >> 32) - UINT32_C(0x3ff00000);
	
	// If (1.0 - tiny) <= input < 0x1.0p64:
	if (UINT32_C(0x04000000) > highWordMinusOne)
	{
		const int unbiasedHeadExponent = highWordMinusOne >> 20;
		
		uint64_t result = hibits.x & UINT64_C(0x000fffffffffffff); // mantissa(hi)
		result |= UINT64_C(0x0010000000000000); // matissa(hi) with implicit bit
		result <<= 11; // mantissa(hi) left aligned in the int64 field.
		
		// If the tail is non-zero, we need to patch in the tail bits.
		if (0.0 != x.lo)
		{
			const doublebits lobits = { .d = x.lo };
			int64_t tailMantissa = lobits.x & INT64_C(0x000fffffffffffff);
			tailMantissa |= INT64_C(0x0010000000000000);
			
			// At this point we have the mantissa of |tail|
			
			const int64_t negationMask = ((int64_t)(lobits.x)) >> 63;
			tailMantissa = (tailMantissa ^ negationMask) - negationMask;
			
			// Now we have the mantissa of tail as a signed 2s-complement integer
			
			const int biasedTailExponent = (int)(lobits.x >> 52) & 0x7ff;
			
			// Shift the tail mantissa into the right position, accounting for the
			// bias of 11 that we shifted the head mantissa by.
			tailMantissa >>= (unbiasedHeadExponent - (biasedTailExponent - (1023 - 11)));
			
			result += tailMantissa;
		}
		
		result >>= (63 - unbiasedHeadExponent);
		return result;
	}
	
	// Edge cases are handled here, with saturation.
	if (1.0 > x.hi)
		return UINT64_C(0);
	else
		return UINT64_MAX;
}