POV-Ray : Newsgroups : povray.macintosh : Stack Size Testers Wanted : Re: Fwd: Stack Size Testers Wanted Server Time
25 Mar 2023 11:02:29 EDT (-0400)
  Re: Fwd: Stack Size Testers Wanted  
From: clipka
Date: 16 Feb 2019 16:39:11
Message: <5c6882ff$1@news.povray.org>
Am 16.02.2019 um 16:17 schrieb William F Pokorny:

> This leads to me close with being somewhat worried about the suggested 
> test size of 512KB.

The "target" size of 512 kiB is an invariant - that's the per-thread 
stack size we would get on OS X by default.

> We still have some largish stack allocations - the 
> sturm solver when we increased the max order from 15 to 35(why ?) in 
> v3.7 now causes a 600KB plus (??? don't recall exactly) allocation on 
> the stack - at a considerable performance penalty I'll add(1) - no 
> matter the actual incoming equation order. This alone might cause your 
> suggested size of 512KB to fail. Well, except perhaps on windows where 
> presumably splitting still in place.

Where would those 600 kB be allocated?

The only out-of-the-ordinary local variable I see in all of the 
polynomial solver code is `sseq` in `polysolve()`, which is 36 elements 
of type `polynomial`, which in turn consists of 1 integer and 36 
doubles. That's about 10 kB, not 600.

> (1) - My C++ ish attempts to address this have all been really slow or 
> not worked at all when I try to get fancy/fast with raw memory 
> allocations and pointers.

If the variable is local to a function that is guaranteed to never be 
called recursively, the easiest solution to guarantee speedy allocation 
and not hogging the stack is to change the variable declaration to 
`thread_local`. This is essentially the same as `static`, but with each 
thread having its own copy of the variable.

> C and some C++ compilers (IBM's XLC being one) 
> support the needed dynamic structure array-size allocation mechanism 
> with high performance. This a reason - among others - why I'm toying 
> some with taking the common solver code back to straight C. We will 
> soon, I think? - move to a mixed C++/C mode in picking up FreeType 
> which, as I remember, is actually a C library - so perhaps not so crazy.

Veto to that: All of POV-Ray should be valid C++11 code, so that POV-Ray 
can be compiled with only a single C++ compiler. 3rd party libraries are 
another matter; we expect those to be present in binary form (except on 
Windows, where we expect their source code to be compatible with MSVC's 
dialect of C/C++), and the headers of modern C libs are almost 
invariably designed as hybrid C/C++ headers, in the sense that they use 
pre-processor features to behave as either C or C++ code, depending on 
the compiler used.

There is currently only one genuine C source file ihn POV-Ray proper, 
namely `povms.c`, and even that one is designed as a C/C++ hybrid and 
compiled by including it from a `.cpp` file. (The rationale for this C 
file's existence being that it - in theory - allows 3rd party genuine C 
programs to drive the POV-Ray back-end via POVMS.)

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