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Re: [ft-devel] Experimental: v38 interpreter with minimal backwards comp

From: Werner LEMBERG
Subject: Re: [ft-devel] Experimental: v38 interpreter with minimal backwards compatibility mode and linear advance widths
Date: Sat, 12 Mar 2016 07:59:59 +0100 (CET)

>> Mhmm.  This is too hars harsh.  I would rather simply stop
>> interpretation of delta instructions but let everything else work.
>> For example, IUP[xy] could be called within conditionals, and it
>> would be bad if the following `EIF' doesn't get interpreted,
>> causing unwarranted execution failures.
> Should execution errors be avoided in general?


> I'm throwing an error in ALIGNRP after IUP[x] and IUP[y]

This is definitely not correct, since ALIGNRP after IUP[xy] *is* valid

> Should I instead define a new "error" that simply stops execution
> and returns success?


> Or, how about doing
>   if ( exc->backwards_compatibility && exc->iupx_called &&
>   exc->iupy_called )
>     goto LNo_Error_;
> at the top of the instruction execution loop? Just pretend
> everything is okay?

No.  I think the correct solution is to `mute' all instructions that
change the rasterization result, but those instructions should be
executed nevertheless.

> I mean, is there any point in continuing if DELTAP/SHPIXes are
> denied anyway?

Of course!  Just think of setting or modifying CVT values, for


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