[Bug 18916] Thief 2 crashes when bringing up in-game menu

[wine-bugs at winehq.org]

http://bugs.winehq.org/show_bug.cgi?id=18916





--- Comment #12 from Henri Verbeet <hverbeet at gmail.com>  2009-06-16 06:02:38 ---
(In reply to comment #11)
> (In reply to comment #10)
> > I think http://www.winehq.org/pipermail/wine-patches/2009-June/074312.html
> > should fix this.
> 
> I'm a bit troubled by this computation:
> 
>     WORD d15 = source[x] & 0xfffe;
>     DWORD d24 = d15 * 0x100 + (d15 * 0xff80 + 0x3fff80) / 0x7fff00;
> 
> If I multiply the arithmetic out as if this was floating point, this comes to
> 
>    d24 = 256.0078 * d15 + 0.5
> 
> Don't get it.  You're not relying on d15*0xff80 overflowing and doing something
> evil ... are you?  Certainly if the top bit of d15 is set then this overflows
> as an unsigned value ... and then the division, in this inner loop, is not
> going to speed things up.
> 
> Why not just
> 
>     DWORD d24 = (source[x] & 0xfffe) << 8;
> 
> ?  What is the conversion rule that's really being applied here?

"DWORD d24 = (source[x] & 0xfffe) << 8;" would map 0x7fff to 0xfffe00 rather
than 0xffffff. We essentially need to multiply by (0xffffff/0x7fff) and round
correctly, which would result in something like "d24 = (d15 >> 1) * 512.0156 +
0.5", which in turn corresponds to the "d24 = 256.0078 * d15 + 0.5" you got
above.

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