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Re: [ft-devel] those OVERFLOW_* macros and Fontval b69 (Re: Freetype-dev

From: Werner LEMBERG
Subject: Re: [ft-devel] those OVERFLOW_* macros and Fontval b69 (Re: Freetype-devel Digest, Vol 149, Issue 5
Date: Sun, 04 Jun 2017 22:32:12 +0200 (CEST)

>> Yes and yes.  They are ugly, and I wonder whether I should drop the
>> `OVERFLOW_' part of its names to get `SUB_LONG', `ADD_INT32', etc.
> Please drop OVERFLOW.


> I see them as a red flag to rethink the code.

I disagree, at least partially, since...

> The add and sub overflows should not be happening on 32 bits because
> we limit ourselves to 1Gb bitmaps aka 32768x32768.  Even at 1/256
> pixel resolution there is plenty of space to avoid overflows.

... rendering is probably the *only* part in FreeType where your
assumption is correct, because we work with absolute point coordinates
in an FT_Outline structure.  However, within fonts, almost everything
is stored as coordinate deltas; this means that malformed fonts can
create arbitrarily large values very easily.  Ditto for bytecode.
Checking every addition, subtraction or multiplication for overflow is
something I want to avoid, so we have to tag all operations where
overflow can occur.


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