Re: [PATCH v2 0/9] x86: macrofying inline asm for better compilation

From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Nadav Amit <namit@vmware.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	Alok Kataria <akataria@vmware.com>,
	Christopher Li <sparse@chrisli.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
	Jan Beulich <JBeulich@suse.com>, Juergen Gross <jgross@suse.com>,
	Kate Stewart <kstewart@linuxfoundation.org>,
	Kees Cook <keescook@chromium.org>,
	linux-sparse@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Philippe Ombredanne <pombredanne@nexb.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	virtualization@lists.linux-foundation.org,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v2 0/9] x86: macrofying inline asm for better compilation
Date: Mon, 4 Jun 2018 14:05:52 -0500	[thread overview]
Message-ID: <20180604190552.hm5e6zcabeyxt26u@treble> (raw)
In-Reply-To: <20180604112131.59100-1-namit@vmware.com>

On Mon, Jun 04, 2018 at 04:21:22AM -0700, Nadav Amit wrote:
> This patch-set deals with an interesting yet stupid problem: kernel code
> that does not get inlined despite its simplicity. There are several
> causes for this behavior: "cold" attribute on __init, different function
> optimization levels; conditional constant computations based on
> __builtin_constant_p(); and finally large inline assembly blocks.
> 
> This patch-set deals with the inline assembly problem. I separated these
> patches from the others (that were sent in the RFC) for easier
> inclusion. I also separated the removal of unnecessary new-lines which
> would be sent separately.
> 
> The problem with inline assembly is that inline assembly is often used
> by the kernel for things that are other than code - for example,
> assembly directives and data. GCC however is oblivious to the content of
> the blocks and assumes their cost in space and time is proportional to
> the number of the perceived assembly "instruction", according to the
> number of newlines and semicolons. Alternatives, paravirt and other
> mechanisms are affected, causing code not to be inlined, and degrading
> compilation quality in general.
> 
> The solution that this patch-set carries for this problem is to create
> an assembly macro, and then call it from the inline assembly block.  As
> a result, the compiler sees a single "instruction" and assigns the more
> appropriate cost to the code.
> 
> To avoid uglification of the code, as many noted, the macros are first
> precompiled into an assembly file, which is later assembled together
> with the the C files. This also enables to avoid duplicate
> implementation that was set before for the asm and C code. This can be
> seen in the exception table changes.
> 
> Overall this patch-set slightly increases the kernel size (my build was
> done using my Ubuntu 18.04 config + localyesconfig for the record):
> 
>    text	   data	    bss	    dec	    hex	filename
> 18140829 10224724 2957312 31322865 1ddf2f1 ./vmlinux before
> 18163608 10227348 2957312 31348268 1de562c ./vmlinux after (+0.1%)
> 
> The number of static functions in the image is reduced by 379, but
> actually inlining is even better, which does not always shows in these
> numbers: a function may be inlined causing the calling function not to
> be inlined.
> 
> The Makefile stuff may not be too clean. Ideas for improvements are
> welcome.
> 
> v1->v2:	* Compiling the macros into a separate .s file, improving
> 	  readability (Linus)
> 	* Improving assembly formatting, applying most of the comments
> 	  according to my judgment (Jan)
> 	* Adding exception-table, cpufeature and jump-labels
> 	* Removing new-line cleanup; to be submitted separately

How did you find these issues?  Is there some way to find them
automatically in the future?  Perhaps with a GCC plugin?

-- 
Josh