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Subject: Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches
To:     Nick Desaulniers <ndesaulniers@google.com>
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        Thomas Gleixner <tglx@linutronix.de>,
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From:   "H. Peter Anvin" <hpa@zytor.com>
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Date:   Tue, 18 Aug 2020 12:02:32 -0700
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On 2020-08-18 10:56, Nick Desaulniers wrote:
>>
>> The problem here is twofold:
>>
>> 1. The user would be expected to know what kind of the optimizations the
>> compiler can do on what function, which is private knowledge to the
>> compiler.
>>
>> 2. The only way to override -fno-builtin is by a header file with macros
>> overriding the function names with __builtin, but that doesn't tell the
>> compiler proper anything about the execution environment.
>>
>> So the Right Thing is for the compiler authors to change the way
>> -ffreestanding works.
> 
> Sir, this is an Arby's
> 
> There are things all across the compilation landscape that make we
> want to pontificate or even throw a tantrum in an Arby's.  Would I?
> Well, no, I'm just trying to flip burgers or shovel the elephant
> sh...or w/e they do at Arby's (I've never actually been; I detest
> roast beef).
> 
> Would it be interesting to have a way of opting in, as you describe,
> such that your compiler knew exactly what kind of embedded environment
> it was targeting?  Maybe, but I'd argue that opting out is just the
> other side of the same coin. Heads, I win; tails, you lose. That the
> opt in or opt out list is shorter for a given project is not
> particularly interesting.  Should we change the semantics of a fairly
> commonly used compiler flag that multiple toolchains are in agreement
> of, then fix all of the breakage in all of the code that relied on
> those semantics?  I'm afraid that ship may have already
> sailed...probably 20 or 30 years ago.
> 
>> -ffreestanding means, by definition, that there
>> are no library calls (other than libgcc or whatever else is supplied
>> with the compiler) that the compiler can call. That is currently an
>> all-or-nothing choice, or at least one choice per C standard implemented.
> 
> Yes?
> 

I'm not saying "change the semantics", nor am I saying that playing
whack-a-mole *for a limited time* is unreasonable. But I would like to go back
to the compiler authors and get them to implement such a #pragma: "this
freestanding implementation *does* support *this specific library function*,
and you are free to call it." The only way we can get what we really need from
the compilers is by speaking up and requesting it, and we have done so very
successfully recently; further back we tended to get a lot of
language-lawyering, but these days both the gcc and the clang teams have been
wonderfully responsive.

	-hpa