Re: [ft-devel] FT_MulFix assembly

[Miles Bader]

James Cloos <address@hidden> writes:
>>> The C version does away-from-zero rounding.
>
> MB> Do you have test cases that show this?  I tried using random inputs,
> MB> but even up to billions of iterations, I can't seem to find a set of
> MB> inputs where my function yields different results from yours.
>
> The C version saves the two signs, takes the absolute values,
> multiplies, scales and then sets the sign.
>
> When I tested, I used dd(1) to generate a quarter-gig file from urandom
> (I used a fixed file so that it would be reproducable), mmap(2)ed that
> to an int[], and went through two at a time.  The C and my initial asm
> versions produced different results whenever the second int was -1 (ie
> 0xFFFFFFFF) and the first matched: (a > 0 && (a & 0xFFFF == 0x8000)).
>
> In other words, multiplying something like 7.5 by -1/65536.
>
> An example of that test's output was:
>
>   7AFA8000, FFFFFFFF, FFFF8505, FFFF8506, 0
>
> In that example, FFFF8505 is what the C version generates.

Hm, are you sure that's not backwards?  When I tried the git C version[*],
as well as your most recent FT_MulFix_x86_64, it returned 0xFFFF8506...


The following C version:

   typedef signed int FT_Int;
   typedef signed long FT_Long;
   typedef signed long FT_Int64; /* on x86-64 */

   FT_Long
   FT_MulFix_C_new( FT_Long  a,
                    FT_Long  b )
   {
     return (((FT_Int64)a * (FT_Int64)b) + 0x8000) >> 16;
   }

... generates this code:

        imulq   %rsi, %rdi
        leaq    32768(%rdi), %rax
        sarq    $16, %rax


It seems to yield exactly the same results as the offical C version[*],
both for your test case:

   $ ./t 0x7AFA8000 0xFFFFFFFF
   0x7afa8000 x 0xffffffff =>
       C: 0x7fff8506
   C_new: 0x7fff8506
     asm: 0x7fff8506

... and also for <billions of random inputs>.


Is there something I'm missing...?

[*] Fetched from:
http://git.savannah.gnu.org/cgit/freetype/freetype2.git/tree/src/base/ftcalc.c


Thanks,

-Miles

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