Best way to load a 64-bit integer to a double prec

问题:

What is the best/fastest way to load a 64-bit integer value in an xmm SSE2 register in 32-bit mode?

In 64-bit mode, cvtsi2sd can be used, but in 32-bit mode, it supports only 32-bit integers.

So far I haven't found much beyond:

• use fild, fstp to stack then movsd to xmm register
• load the high 32-bit portion, multiply by 2^32, add the low 32-bit

First solution is slow, second solution might introduce precision loss (edit: and it is slow anyway, since the low 32 bit have to be converted as unsigned...)

Any better approach?

回答1:

Your second option can be made to work, though it's a little unwieldy. I'll assume that your 64-bit number is initially in edx:eax.

cvtsi2sd xmm0, edx              // high part * 2**-32
mulsd    xmm0, [2**32 from mem] // high part
movsd    xmm2, [2**52 from mem]
movd     xmm1, eax
orpd     xmm1, xmm2             // (double)(2*52 + low part as unsigned)
subsd    xmm1, xmm2             // (double)(low part as unsigned)
addsd    xmm0, xmm1             // (double)(high part + low part as unsigned)

All of the operations except for possibly the final one are exact, so this is correctly rounded. It should be noted that this conversion produces -0.0 when the input is 0 and the mxcsr is set to round-to-minus-infinity. This would need to be addressed if it were being used in a runtime library for a compiler aiming to provide IEEE-754 conformance, but is not an issue for most usage.