Re: [PATCH] [RFC] Initialization of unused function parameters

From: Peter Zijlstra <peterz@infradead.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Alexander Potapenko <glider@google.com>,
	Evgenii Stepanov <eugenis@google.com>,
	Kees Cook <keescook@chromium.org>, Marco Elver <elver@google.com>,
	Nathan Chancellor <nathan@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Vitaly Buka <vitalybuka@google.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-toolchains <linux-toolchains@vger.kernel.org>
Subject: Re: [PATCH] [RFC] Initialization of unused function parameters
Date: Wed, 15 Jun 2022 00:27:17 +0200	[thread overview]
Message-ID: <YqkLRSBaeb6U1ngW@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAKwvOd=C1GC09zobqLK0higE3xPpghWBbB4xujyC8-QDyd_DTQ@mail.gmail.com>

On Tue, Jun 14, 2022 at 11:08:33AM -0700, Nick Desaulniers wrote:

> One thing I'm curious about; if you have an aggregate in C (struct or
> array) and don't fully initialize the whole object, just
> members/sub-objects, but only use those I assume that's not UB? (Which
> is what my maybe_init example does).  I think that's fine.

Perf does something like that. We only explicitly initialize the first
cacheline of a structure because that's *always* used. Any other field
will only be initialized on request.

At the time (and i've not benchmarked this in a good few years) that was
a significant performance win.