linux-kernel - Re: [PATCH 02/10] compiler.h: add is_const() as a replacement of __is

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Message-ID: <aeb1c27d320800dcd8375771022e11e8c27b0d1a.camel@gwdg.de>
Date: Sun, 8 Dec 2024 10:18:40 +0100
From: Martin Uecker <muecker@...g.de>
To: Linus Torvalds <torvalds@...ux-foundation.org>
CC: David Laight <David.Laight@...lab.com>, Vincent Mailhol
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Subject: Re: [PATCH 02/10] compiler.h: add is_const() as a replacement of
 __is_constexpr()

Am Samstag, dem 07.12.2024 um 17:58 -0800 schrieb Linus Torvalds:
> On Sat, 7 Dec 2024 at 15:52, Martin Uecker <muecker@...g.de> wrote:
> > 
> > Can you point me to some horror stories?
> 
> So the main issues tended to be about various static verification tools.
> 
> Ranging from things like the stackleak plugin for gcc, where handling
> VLA's and alloca() (which are pretty much the same thing with
> different syntax) was just very much added complexity, to perhaps
> overly simplistic tools that literally just check the stack usage by
> parsing "objdump --disassemble" output and then building up
> approximate "this is the combined deepest stack" call chain
> approximations.
> 
> And even in the *basic* infrastructure like gcc itself, VLA's simply
> made -Wframe-larger-than= just simply not work.
> 
> I also have this memory of bad code generation (again, this is 5=
> years ago, so take this with a pinch of salt: dim memories), where gcc
> wouldn't end up re-using VLA stack slots, so VLA's made the frame
> bigger for that reason or something like that.
> 
> We explicitly use "-fconserve-stack" to get gcc to reuse spill slots,
> because gcc has been known to sometimes makes insanely piggish stack
> frames when it just creates a spill slot for *everything*, even if the
> spills aren't live at the same time (think big functions with lots of
> case statements).
> 
> We also had several cases of the VLA's just being silly, when a simple
> constant-sized allocation just worked fine and didn't generate
> pointless extra code.
> 
> Pretty much none of the code core actually ever wanted VLA's, so the
> end result was that we had these bad patterns mainly in random drivers
> etc. Don't do that.

Thanks. This confirms that the tooling around VLAs is rather poor.


Martin