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Re: [Tinycc-devel] inline assembly and optimization passes

From: Sylvain BERTRAND
Subject: Re: [Tinycc-devel] inline assembly and optimization passes
Date: Thu, 26 Sep 2013 11:33:38 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

> So that was in response to the "if I were implementing a language
> from scratch" bit, right?


> If I was doing that then I'd be looking at an application
> language that would easily integrate with a systems language,
> hence high-end features along with C compatibility.
> If I was going for a new systems language then I'd just take C,
> modify the syntax for pointers & declarations, and maybe modify
> the standard library. Presumably a smaller job than a
> from-scratch language.

I do not agree. I would go C- for system and application. I don't
like my software stack to depend on tons of different languages
(because at the end, it's what we get).

>>> So you want to hook into the TCC parser itself?
>> I said, if this has no obvious blockers, we could use fake targets
>> that would be optimization passes. They would output C code.
> Yeah, I mostly paid attention to the fake-target bit, since
> outputting IL/IC from that seemed like the easiest way into the
> "standard route".

I don't know what "this standard" route is, all I know is ouputing
C code with fake targets to handle some optimization passes seems to
be a good tradeoff to avoid a lot of kludge and to minimize impact on tcc

>> Regarding the unused code elimitation across compilation units, it
>> involves probably the linker. Then the "trick" of the fake target
>> may not be as easy.
> That depends, Eliminating unused functions & variables works like
> that, but it only requires the ability to detect when you're in
> the file-scope instead of a scope contained within a file, and
> take that as a signal to place any additional variables or
> functions into a new section. That and info on what those
> sections need to import are all that you need, and all of that
> should (at least presumably, it's been a while since I poked at
> object file formats) be supported by your ordinary object file
> format. Or, at least, your ordinary library file format.
> Once you have that, it's a matter of creating a linker mode that
> will assign two bits (one for "needed", one for "supplied") in a
> memory block to each of the sections, and starting a search from
> your "root section" (probably the one containing the "main" or
> equivalent function, but possibly a file declaring exports too).
> Every time that you find a dependency, you ensure that it has
> either it's "needed" or "supplied" bit set. Once you've finished
> checking through every section that you ALREADY knew to check,
> you output it, make a new list of sections to check (they'll be
> the ones marked "needed" instead of "supplied"), switch all of
> the "needed" sections to be only "supplied" instead, and start
> the dependencies search again. You only stop once you run out of
> sections that are "needed".
>> We may have to "annotate" the generated C code for the real target
>> to insert the proper information in the object file for the
>> linker. I bet that optimization pass would be kind of the last
>> one.
> Unused function removal works as I stated above: you find a
> starting point, find all of it's dependencies, write out the
> starting point, and recursively check dependencies for new
> dependencies, and write old dependencies out.
> Other forms of unused code removal either never leave the
> compiler (e.g. removing if( 0 ) blocks), or should be left for a
> later date (some things are more foundational than others).
>>  - A compilation unit scoped dead/unused code removal fake target
> Let's worry about unused function removal first, since that
> should be the fastest to implement, okay? Depending on details
> that Grischka would know but I don't, we might need to build a
> parse tree before we can eliminate unused code INSIDE of
> functions, in which case the target will be a "parse tree" output
> of SOME kind, regardless of whether it's assembly-ish, C-ish, or
> something-else-ish.
> Also, by virtue of some of Grischka's comments below, I don't
> think that just sticking the optimizations entirely inside a fake
> target will be enough: we'll need to build a parse tree for
> anything other than the minor stuff, in which case we might as
> well have the fake target be the parse tree instead of making it
> be the actual optimization.
>>  - A C code annotation target which create a dependency tree of
>>    machine code sections for the linker to optimize out or not.
> Allowing individual sections to have their own dependencies will
> do this perfectly fine.

Allright, then a fake target to anotate C code which anotations
will add extra info in generated elf object, seems a good path
for dead code/unused code removal optimization passes, at file
Compilation unit scope passes can output C code without the need
of anotations for the linker, could start from here.


But, I'm more concerned with "variable aliasing" optimization
passes.  As I said before, my general "ground" experience tells
me that a lot C code use many variables to get to the same data.
(I use aliasing a lot to make my code more readable).
And I was wondering how much of the registers/stack space dance
we would be able to avoid with such passes. I feel it will give
a significant performance boost. I may be wrong though.

The goal would be to make tcc reasonably performant on C code in
order to drop gcc (since c++ is now mandatory).


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