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From: Richard Biener <rguenther@suse.de>
To: Nadav Amit <namit@vmware.com>
Cc: Segher Boessenkool <segher@kernel.crashing.org>,
	Michael Matz <matz@suse.de>, Borislav Petkov <bp@alien8.de>,
	"gcc@gcc.gnu.org" <gcc@gcc.gnu.org>,
	Ingo Molnar <mingo@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	Sam Ravnborg <sam@ravnborg.org>,
	Alok Kataria <akataria@vmware.com>,
	Christopher Li <sparse@chrisli.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"H. Peter Anvin" <hpa@zytor.com>, Jan Beulich <JBeulich@suse.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Juergen Gross <jgross@suse.com>,
	Kate Stewart <kstewart@linuxfoundation.org>,
	Kees Cook <keescook@chromium.org>,
	"linux-sparse@vger.kernel.org" <linux-sparse@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Philippe Ombredanne <pombredanne@nexb.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	"virtualization@lists.linux-foundation.org" 
	<virtualization@lists.linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Chris Zankel <chris@zankel.net>,
	Max Filippov <jcmvbkbc@gmail.com>,
	"linux-xtensa@linux-xtensa.org" <linux-xtensa@linux-xtensa.org>
Subject: Re: PROPOSAL: Extend inline asm syntax with size spec
Date: Thu, 11 Oct 2018 09:04:58 +0200 (CEST)	[thread overview]
Message-ID: <alpine.LSU.2.20.1810110901350.16707@zhemvz.fhfr.qr> (raw)
In-Reply-To: <5ACDEDEA-0E42-45F2-BC80-B96EC2ABC01D@vmware.com>

[-- Attachment #1: Type: text/plain, Size: 2317 bytes --]

On Wed, 10 Oct 2018, Nadav Amit wrote:

> at 7:53 AM, Segher Boessenkool <segher@kernel.crashing.org> wrote:
> 
> > On Mon, Oct 08, 2018 at 11:07:46AM +0200, Richard Biener wrote:
> >> On Mon, 8 Oct 2018, Segher Boessenkool wrote:
> >>> On Sun, Oct 07, 2018 at 03:53:26PM +0000, Michael Matz wrote:
> >>>> On Sun, 7 Oct 2018, Segher Boessenkool wrote:
> >>>>> On Sun, Oct 07, 2018 at 11:18:06AM +0200, Borislav Petkov wrote:
> >>>>>> Now, Richard suggested doing something like:
> >>>>>> 
> >>>>>> 1) inline asm ("...")
> >>>>> 
> >>>>> What would the semantics of this be?
> >>>> 
> >>>> The size of the inline asm wouldn't be counted towards the inliner size 
> >>>> limits (or be counted as "1").
> >>> 
> >>> That sounds like a good option.
> >> 
> >> Yes, I also like it for simplicity.  It also avoids the requirement
> >> of translating the number (in bytes?) given by the user to
> >> "number of GIMPLE instructions" as needed by the inliner.
> > 
> > This patch implements this, for C only so far.  And the syntax is
> > "asm inline", which is more in line with other syntax.
> > 
> > How does this look?
> 
> It looks good to me in general. I have a couple of reservations, but I
> suspect you will not want to address them:
> 
> 1. It is not backward compatible, requiring a C macro to wrap it, as the 
> kernel might be built with different compilers.
> 
> 2. It is specific to asm. I do not have in mind another use case (excluding
> the __builtin_constant_p), but it would be nicer IMHO to have a builtin
> saying “ignore the cost of this statement” for the matter of optimizations.

The only easy possibility that comes to my mid is sth like

__attribute__((always_inline, zero_cost)) foo ()
{
  ... your stmts ...
}

and us, upon inlining, marking the inlined stmts properly.  That would
also work for the asm() case and it would be backwards compatible
(well, you'd get a diagnostic for the unknown zero_cost attribute).

There's the slight complication that if you have, say

 _1 = _2 * 3; // zero-cost
 _4 = _1 * 2;

and optimization ends up combining those to

 _4 = _2 * 6;

then is this stmt supposed to be zero-cost or not?  Compare that to

 _1 = _2 * 3;
 _4 = _1 * 2; // zero-cost

So outside of asm() there are new issues that come up with respect
to expected (cost) semantics.

Richard.

  parent reply	other threads:[~2018-10-11  7:05 UTC|newest]

Thread overview: 116+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-03 21:30 [PATCH v9 00/10] x86: macrofying inline asm Nadav Amit
2018-10-03 21:30 ` [PATCH v9 01/10] xtensa: defining LINKER_SCRIPT for the linker script Nadav Amit
2018-10-04 10:00   ` [tip:x86/build] kbuild/arch/xtensa: Define " tip-bot for Nadav Amit
2018-10-03 21:30 ` [PATCH v9 02/10] Makefile: Prepare for using macros for inline asm Nadav Amit
2018-10-04 10:01   ` [tip:x86/build] kbuild/Makefile: Prepare for using macros in inline assembly code to work around asm() related GCC inlining bugs tip-bot for Nadav Amit
2018-11-06 18:57   ` [PATCH v9 02/10] Makefile: Prepare for using macros for inline asm Logan Gunthorpe
2018-11-06 19:18     ` Nadav Amit
2018-11-06 20:01       ` Logan Gunthorpe
2018-11-07 18:01         ` Nadav Amit
2018-11-07 18:53           ` Logan Gunthorpe
2018-11-07 18:56             ` Nadav Amit
2018-11-07 21:43               ` Logan Gunthorpe
2018-11-07 21:50                 ` hpa
2018-11-08  6:18                   ` Nadav Amit
2018-11-08 17:14                     ` Logan Gunthorpe
2018-11-08 19:54                       ` Nadav Amit
2018-11-08 20:00                         ` Logan Gunthorpe
2018-11-08 20:18                           ` Nadav Amit
2018-11-10 22:04                             ` Nadav Amit
2018-11-13  4:56                               ` Logan Gunthorpe
2018-10-03 21:30 ` [PATCH v9 03/10] x86: objtool: use asm macro for better compiler decisions Nadav Amit
2018-10-04 10:02   ` [tip:x86/build] x86/objtool: Use asm macros to work around GCC inlining bugs tip-bot for Nadav Amit
2018-10-03 21:30 ` [PATCH v9 04/10] x86: refcount: prevent gcc distortions Nadav Amit
2018-10-04  7:57   ` Ingo Molnar
2018-10-04  8:33     ` Ingo Molnar
2018-10-04  8:40       ` hpa
2018-10-04  8:56         ` Ingo Molnar
2018-10-04  8:56         ` Nadav Amit
2018-10-04  9:02           ` hpa
2018-10-04  9:16             ` Ingo Molnar
2018-10-04 19:33               ` H. Peter Anvin
2018-10-04 20:05                 ` Nadav Amit
2018-10-04 20:08                   ` H. Peter Anvin
2018-10-04 20:29                 ` Andy Lutomirski
2018-10-04 23:11                   ` H. Peter Anvin
2018-10-06  1:40                 ` Rasmus Villemoes
2018-10-04  9:12           ` Ingo Molnar
2018-10-04  9:17             ` hpa
2018-10-04  9:30             ` Nadav Amit
2018-10-04  9:45               ` Ingo Molnar
2018-10-04 10:23                 ` Nadav Amit
2018-10-05  9:31                   ` Ingo Molnar
2018-10-05 11:20                     ` Borislav Petkov
2018-10-05 12:52                       ` Ingo Molnar
2018-10-05 20:27                     ` [PATCH 0/3] Macrofying inline asm rebased Nadav Amit
2018-10-05 20:27                       ` [PATCH 1/3] x86/extable: Macrofy inline assembly code to work around GCC inlining bugs Nadav Amit
2018-10-06 14:42                         ` [tip:x86/build] " tip-bot for Nadav Amit
2018-10-05 20:27                       ` [PATCH 2/3] x86/cpufeature: " Nadav Amit
2018-10-06 14:43                         ` [tip:x86/build] " tip-bot for Nadav Amit
2018-10-05 20:27                       ` [PATCH 3/3] x86/jump-labels: " Nadav Amit
2018-10-06 14:44                         ` [tip:x86/build] " tip-bot for Nadav Amit
2018-10-08  2:17                     ` [PATCH v9 04/10] x86: refcount: prevent gcc distortions Nadav Amit
2018-10-04  8:40     ` Nadav Amit
2018-10-04  9:01       ` Ingo Molnar
2018-10-04 10:02   ` [tip:x86/build] x86/refcount: Work around GCC inlining bug tip-bot for Nadav Amit
2018-10-03 21:30 ` [PATCH v9 05/10] x86: alternatives: macrofy locks for better inlining Nadav Amit
2018-10-04 10:03   ` [tip:x86/build] x86/alternatives: Macrofy lock prefixes to work around GCC inlining bugs tip-bot for Nadav Amit
2018-10-03 21:30 ` [PATCH v9 06/10] x86: bug: prevent gcc distortions Nadav Amit
2018-10-04 10:03   ` [tip:x86/build] x86/bug: Macrofy the BUG table section handling, to work around GCC inlining bugs tip-bot for Nadav Amit
2018-10-03 21:30 ` [PATCH v9 07/10] x86: prevent inline distortion by paravirt ops Nadav Amit
2018-10-04 10:04   ` [tip:x86/build] x86/paravirt: Work around GCC inlining bugs when compiling " tip-bot for Nadav Amit
2018-10-03 21:30 ` [PATCH v9 08/10] x86: extable: use macros instead of inline assembly Nadav Amit
2018-10-03 21:30 ` [PATCH v9 09/10] x86: cpufeature: " Nadav Amit
2018-10-03 21:31 ` [PATCH v9 10/10] x86: jump-labels: " Nadav Amit
2018-10-07  9:18 ` PROPOSAL: Extend inline asm syntax with size spec Borislav Petkov
     [not found]   ` <20181007132228.GJ29268@gate.crashing.org>
2018-10-07 14:13     ` Borislav Petkov
2018-10-07 15:14       ` Segher Boessenkool
2018-10-08  5:58         ` Ingo Molnar
2018-10-08  7:53           ` Segher Boessenkool
2018-10-07 15:53     ` Michael Matz
2018-10-08  6:13       ` Ingo Molnar
2018-10-08  8:18         ` Segher Boessenkool
2018-10-08  7:31       ` Segher Boessenkool
2018-10-08  9:07         ` Richard Biener
2018-10-08 10:02           ` Segher Boessenkool
2018-10-09 14:53           ` Segher Boessenkool
2018-10-10  6:35             ` Ingo Molnar
2018-10-10  7:12             ` Richard Biener
2018-10-10  7:22               ` Ingo Molnar
2018-10-10  8:03                 ` Segher Boessenkool
2018-10-10  8:19                   ` Borislav Petkov
2018-10-10  8:35                     ` Richard Biener
2018-10-10 18:54                     ` Segher Boessenkool
2018-10-10 19:14                       ` Borislav Petkov
2018-10-13 19:33                         ` Borislav Petkov
2018-10-13 21:14                           ` Alexander Monakov
2018-10-13 21:30                             ` Borislav Petkov
2018-10-25 10:24                           ` Borislav Petkov
2018-10-31 12:55                           ` Peter Zijlstra
2018-10-31 13:11                             ` Peter Zijlstra
2018-10-31 16:31                             ` Segher Boessenkool
2018-11-01  5:20                             ` Joe Perches
2018-11-01  9:01                               ` Peter Zijlstra
2018-11-01  9:20                                 ` Joe Perches
2018-11-01 11:15                                   ` Peter Zijlstra
2018-12-27  4:47                             ` Masahiro Yamada
2018-10-10 10:29                   ` Richard Biener
2018-10-10  7:53               ` Segher Boessenkool
2018-10-10 16:31             ` Nadav Amit
2018-10-10 19:21               ` Segher Boessenkool
2018-10-11  7:04               ` Richard Biener [this message]
2018-11-29 11:46             ` Masahiro Yamada
2018-11-29 12:25               ` Segher Boessenkool
2018-11-30  9:06                 ` Boris Petkov
2018-11-30 13:16                   ` Segher Boessenkool
2018-12-10  8:16                     ` Masahiro Yamada
2018-11-29 13:07               ` Borislav Petkov
2018-11-29 13:09                 ` Richard Biener
2018-11-29 13:16                   ` Borislav Petkov
2018-11-29 13:24                     ` Richard Biener
2018-10-08 16:24       ` David Laight
2018-10-07 16:09   ` Nadav Amit
2018-10-07 16:46     ` Richard Biener
2018-10-07 19:06       ` Nadav Amit
2018-10-07 19:52         ` Jeff Law
2018-10-08  7:46         ` Richard Biener

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