From: Michael Matz <matz@suse.de>
To: Andi Kleen <ak@linux.intel.com>
Cc: gcc-patches@gcc.gnu.org, Richard Biener <richard.guenther@gmail.com>
Subject: Re: [PATCH v6 1/8] Improve must tail in RTL backend
Date: Mon, 3 Jun 2024 19:02:00 +0200 (CEST) [thread overview]
Message-ID: <e0768313-3fd8-7cea-521c-1e98dc6979b6@suse.de> (raw)
In-Reply-To: <ZloQI7N9qFapKLWr@tassilo>
Hello,
On Fri, 31 May 2024, Andi Kleen wrote:
> > I think the ultimate knowledge if a call can or cannot be implemented as
> > tail-call lies within calls.cc/expand_call: It is inherently
> > target and ABI specific how arguments and returns are layed out, how the
> > stack frame is generated, if arguments are or aren't removed by callers
> > or callees and so on; all of that being knowledge that tree-tailcall
> > doesn't have and doesn't want to have. As such tree-tailcall should
> > not be regarded as ultimate truth, and failures of tree-tailcall to
> > recognize something as tail-callable shouldn't matter.
>
> It's not the ultimate truth, but some of the checks it does are not
> duplicated at expand time nor the backend. So it's one necessary pre
> condition with the current code base.
>
> Yes maybe the checks could be all moved, but that's a much larger
> project.
Hmm. I count six tests in about 25 lines of code in
tree-tailcall.cc:suitable_for_tail_opt_p and suitable_for_tail_call_opt_p.
Are you perhaps worrying about the sibcall discovery itself (i.e. much of
find_tail_calls)? Why would that be needed for musttail? Is that
attribute sometimes applied to calls that aren't in fact sibcall-able?
One thing I'm worried about is the need for a new sibcall pass at O0 just
for sibcall discovery. find_tail_calls isn't cheap, because it computes
live local variables for the whole function, potentially being quadratic.
Ciao,
Michael.
next prev parent reply other threads:[~2024-06-03 17:02 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-21 14:28 Musttail patchkit v6 Andi Kleen
2024-05-21 14:28 ` [PATCH v6 1/8] Improve must tail in RTL backend Andi Kleen
2024-05-29 13:39 ` Michael Matz
2024-05-31 18:00 ` Andi Kleen
2024-06-03 17:02 ` Michael Matz [this message]
2024-06-03 17:17 ` Jakub Jelinek
2024-06-04 13:49 ` Michael Matz
2024-06-03 17:31 ` Andi Kleen
2024-05-21 14:28 ` [PATCH v6 2/8] Add a musttail generic attribute to the c-attribs table Andi Kleen
2024-05-21 14:28 ` [PATCH v6 3/8] C++: Support clang compatible [[musttail]] (PR83324) Andi Kleen
2024-05-21 14:28 ` [PATCH v6 4/8] C: Implement musttail attribute for returns Andi Kleen
2024-05-21 14:28 ` [PATCH v6 5/8] Add tests for C/C++ musttail attributes Andi Kleen
2024-05-21 14:28 ` [PATCH v6 6/8] Enable musttail tail conversion even when not optimizing Andi Kleen
2024-05-21 14:28 ` [PATCH v6 7/8] Give better error messages for musttail Andi Kleen
2024-06-05 4:52 ` Andi Kleen
2024-05-21 14:28 ` [PATCH v6 8/8] Add documentation for musttail attribute Andi Kleen
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