Re: [PATCH] doc: Reword the description of -mrelax-cmpxchg-loop [PR 107676]

[Hongyu Wang <wwwhhhyyy333@gmail.com>]

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> When emitting a compare-and-swap loop for @ref{__sync Builtins}
> and @ref{__atomic Builtins} lacking a native instruction, optimize
> for the highly contended case by issuing an atomic load before the
> @code{CMPXCHG} instruction, and using the @code{PAUSE} instruction
> to save CPU power when restarting the loop.

Thanks for the correction, it looks quite clear now! Here is the
updated patch, ok for trunk?

Alexander Monakov via Gcc-patches <gcc-patches@gcc.gnu.org>
于2022年11月15日周二 21:59写道：
>
>
> On Tue, 15 Nov 2022, Jonathan Wakely wrote:
>
> > > How about the following:
> > >
> > > When emitting a compare-and-swap loop for @ref{__sync Builtins}
> > > and @ref{__atomic Builtins} lacking a native instruction, optimize
> > > for the highly contended case by issuing an atomic load before the
> > > @code{CMPXCHG} instruction, and invoke the @code{PAUSE} instruction
> > > when restarting the loop.
> >
> > That's much better, thanks. My only remaining quibble would be that
> > "invoking" an instruction seems only marginally better than running
> > one. Emitting? Issuing? Using? Adding?
>
> Right, it should be 'using'; let me also add 'to save CPU power':
>
> When emitting a compare-and-swap loop for @ref{__sync Builtins}
> and @ref{__atomic Builtins} lacking a native instruction, optimize
> for the highly contended case by issuing an atomic load before the
> @code{CMPXCHG} instruction, and using the @code{PAUSE} instruction
> to save CPU power when restarting the loop.
>
> Alexander

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From e82f3e03115480ac3d055819658a107249932c65 Mon Sep 17 00:00:00 2001
From: Hongyu Wang <hongyu.wang@intel.com>
Date: Tue, 15 Nov 2022 11:16:17 +0800
Subject: [PATCH] doc: Reword the description of -mrelax-cmpxchg-loop [PR
 107676]

gcc/ChangeLog:

	PR target/107676
	* doc/invoke.texi: Reword the description of
	-mrelax-cmpxchg-loop.
---
 gcc/doc/invoke.texi | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 40f667a630a..bcbe3a7e420 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -33805,10 +33805,11 @@ registers.
 
 @item -mrelax-cmpxchg-loop
 @opindex mrelax-cmpxchg-loop
-Relax cmpxchg loop by emitting an early load and compare before cmpxchg,
-execute pause if load value is not expected. This reduces excessive
-cachline bouncing when and works for all atomic logic fetch builtins
-that generates compare and swap loop.
+When emitting a compare-and-swap loop for @ref{__sync Builtins}
+and @ref{__atomic Builtins} lacking a native instruction, optimize
+for the highly contended case by issuing an atomic load before the
+@code{CMPXCHG} instruction, and using the @code{PAUSE} instruction
+to save CPU power when restarting the loop.
 
 @item -mindirect-branch=@var{choice}
 @opindex mindirect-branch
-- 
2.18.1