linux-kernel - Re: [PATCH 1/7] cleanup: Introduce DEFINE_ACQUIRE() a CLASS() for conditional locking

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Message-ID: <20250513200619.GJ2023217@ZenIV>
Date: Tue, 13 May 2025 21:06:19 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>, dan.j.williams@...el.com,
	linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Lechner <dlechner@...libre.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Fabio M. De Francesco" <fabio.maria.de.francesco@...ux.intel.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Jonathan Cameron <jonathan.cameron@...wei.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Alison Schofield <alison.schofield@...el.com>,
	Vishal Verma <vishal.l.verma@...el.com>,
	Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH 1/7] cleanup: Introduce DEFINE_ACQUIRE() a CLASS() for
 conditional locking

On Tue, May 13, 2025 at 12:46:29PM -0700, Linus Torvalds wrote:

> Right now IS_ERR_OR_NULL() generates pretty disgusting code, with
> clang doing things like this:
> 
>         testq   %rdi, %rdi
>         sete    %al
>         cmpq    $-4095, %rdi                    # imm = 0xF001
>         setae   %cl
>         orb     %al, %cl
>         je      .LBB3_1
> 
> in order to avoid two jumps, while gcc generates that
> 
>         testq   %rdi, %rdi
>         je      .L189
>         cmpq    $-4096, %rdi
>         ja      .L189
> 
> pattern.

FWIW (unsigned long)v - 1 >= (unsigned long)(-MAX_ERRNO-1) yields
        leaq    -1(%rdi), %rax
	cmpq    $-4097, %rax
	ja      .L4
from gcc and
        leaq    4095(%rdi), %rax
	cmpq    $4095, %rax                     # imm = 0xFFF
	ja      .LBB0_1
from clang...