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Re: [PATCH][4.3] Expand lround inline for x86_64/i?86 SSE math

On Oct 18, 2006, at 10:13, Richard Guenther wrote:

The second patch in the series adds the expander for lround. Nothing spectacular this time.

(For reference, we expand lround to
        movsd   .LC3(%rip), %xmm2
        movapd  %xmm0, %xmm3
        movsd   .LC5(%rip), %xmm1
        andpd   %xmm2, %xmm3
        movapd  %xmm3, %xmm2
        orpd    %xmm1, %xmm2
        movapd  %xmm2, %xmm1
        addsd   %xmm0, %xmm1
        cvttsd2siq      %xmm1, %rax
with .LC3 a sign mask and .LC5 being 0.5)

Bootstrapped and tested on {x86_64,i686}-unknown-linux-gnu.

Considering positive number only for the moment, do I understand correctly that you add 0.5 and then truncate? If so, that is likely incorrect for the predecessor of 0.5 and for all odd floating-point integers with the binary point directly following the mantissa.

If you add 0.5 to the predecessor of 0.5, the mathematical
result will exactly between 1.0 and its predecessor and
be rounded to nearest even, which is 1.0. Similarly

Two numbers to test are (for double precision):

They should be rounded to 0.0 and 4503599627370497.0, but
simply adding 0.5 and truncating will round to 1.0 and


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