linux-kernel - Re: [PATCH v6 9/9] drm/amd/display: Mark dc_fixpt_from

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Message-ID: <05cdb3b4c9bddf25f7b839229b635d2dec5140a4.camel@xry111.site>
Date: Sat, 21 Dec 2024 15:40:12 +0800
From: Xi Ruoyao <xry111@...111.site>
To: Nathan Chancellor <nathan@...nel.org>, Peter Zijlstra
 <peterz@...radead.org>
Cc: Tiezhu Yang <yangtiezhu@...ngson.cn>, Alex Deucher
 <alexdeucher@...il.com>,  Josh Poimboeuf <jpoimboe@...nel.org>, Huacai Chen
 <chenhuacai@...nel.org>, loongarch@...ts.linux.dev, 
	amd-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org, Nick
 Desaulniers	 <ndesaulniers@...gle.com>, llvm@...ts.linux.dev
Subject: Re: [PATCH v6 9/9] drm/amd/display: Mark dc_fixpt_from_fraction()
 noinline

On Fri, 2024-12-20 at 15:34 -0700, Nathan Chancellor wrote:
> > Now, the thing is, these ASSERT()s are checking for divide-by-zero, I
> > suspect clang figured that out and invokes UB on us and just stops
> > code-gen.
> 
> Yeah, I think your analysis is spot on, as this was introduced by a
> change in clang from a few months ago according to my bisect:
> 
> https://github.com/llvm/llvm-project/commit/37932643abab699e8bb1def08b7eb4eae7ff1448
> 
> Since the ASSERT does not do anything to prevent the divide by zero (it
> just flags it with WARN_ON) and the rest of the code doesn't either, I
> assume that the codegen stops as soon as it encounters the unreachable
> that change created from the path where divide by zero would occur via
> 
>   dc_fixpt_recip() ->
>     dc_fixpt_from_fraction() ->
>       complete_integer_division_u64() ->
>         div64_u64_rem()
> 
> Shouldn't callers of division functions harden them against dividing by
> zero?

Yes I think it'd be the correct solution.

-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University