linux-kernel - Re: [PATCH v1] x86/mm/pat: (un)track_pfn

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Message-ID: <20250408025129.GA1168636@ax162>
Date: Mon, 7 Apr 2025 19:51:29 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: Dan Carpenter <dan.carpenter@...aro.org>,
	David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, x86@...nel.org,
	kernel test robot <lkp@...el.com>,
	Dan Carpenter <error27@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Rik van Riel <riel@...riel.com>, "H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v1] x86/mm/pat: (un)track_pfn_copy() fix + improvements

On Mon, Apr 07, 2025 at 08:59:44PM +0200, Ingo Molnar wrote:
> Good, although why is this compiler option named so weirdly in Clang:
> 
>   CC_AUTO_VAR_INIT_ZERO_ENABLER := -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
> 
> Hopefully it is named thusly because Clang has adopted GCC's 
> -ftrivial-auto-var-init=zero?

Clang did -ftrivial-auto-var-init first, where the original author added
both pattern and zero but intended to remove zero once pattern has been
optimized enough compared to zero (if I remember and understand
correctly), so the "enabler" flag was added to try and make that clear.
Eventually, Kees leveraged both Linus's stated desire for initializing
stack variables [1] and GCC 12 landing -ftrivial-auto-var-init=zero
without a separate enable option to deprecate the "enabler" flag in
clang 16 [2] and remove it altogether in clang 18 [3].

[1]: https://lore.kernel.org/CAHk-=wgTM+cN7zyUZacGQDv3DuuoA4LORNPWgb1Y_Z1p4iedNQ@mail.gmail.com/
[2]: https://github.com/llvm/llvm-project/commit/aef03c9b3bed5cef5a1940774b80128aefcb4095
[3]: https://github.com/llvm/llvm-project/commit/00e54d04ae2802d498741097d4b83e898bc99c5b

Cheers,
Nathan