linux-kernel - Re: clang and drm issue: objtool warnings from clang build

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Message-ID: <zizdtmyqqprnlgy2rzvvmqzhganxpuxqqkudhwakianlapl6zp@zucf2xtoabez>
Date: Mon, 28 Apr 2025 12:54:02 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Bill Wendling <morbo@...gle.com>, 
	Nathan Chancellor <nathan@...nel.org>, Harry Wentland <harry.wentland@....com>, 
	Leo Li <sunpeng.li@....com>, Alex Deucher <alexander.deucher@....com>, 
	Christian König <christian.koenig@....com>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, Nick Desaulniers <nick.desaulniers+lkml@...il.com>, 
	Justin Stitt <justinstitt@...gle.com>, the arch/x86 maintainers <x86@...nel.org>, 
	dri-devel <dri-devel@...ts.freedesktop.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, 
	llvm@...ts.linux.dev
Subject: Re: clang and drm issue: objtool warnings from clang build

On Mon, Apr 28, 2025 at 12:34:27PM -0700, Linus Torvalds wrote:
> Honestly, the least wrong thing is to just NOT HAVE THE CHECK FOR ZERO AT ALL.
> 
> IOW, just generate the divide instruction.
> 
> I can almost guarantee that that will actually then generate the best
> code too, because you'll probably just end up sharing the divide
> instruction will all the *normal* cases.
> 
> So the best model is to literally remove that pointless and stupid "is
> this a divide by zero" code. It's pointless and stupid because it
> literally just makes for more work both for the compiler AND it
> generates worse code.

BTW, I've noticed Clang also generates UB for negative shift values.  I
assume we'd want it to stop checking for those as well.

-- 
Josh