linux-kernel - RE: objtool warning "uses BP as a scratch register" with clang-9

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Date:   Fri, 30 Aug 2019 16:42:30 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>
CC:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        "Nick Desaulniers" <ndesaulniers@...gle.com>,
        Ilie Halip <ilie.halip@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: RE: objtool warning "uses BP as a scratch register" with clang-9

From: Linus Torvalds
> Sent: 30 August 2019 17:01
> On Fri, Aug 30, 2019 at 8:55 AM David Laight <David.Laight@...lab.com> wrote:
...
> But yeah, in general it's just not obviously safe to turn individual
> accesses into memset/memcpy. In contrast, the reverse is obviously
> fine (and _required_ for any kind of half-way good performance when
> you do small constant-sized memory copies, which is actually a common
> pattern partly because the insane C aliasing rules have taught people
> that it's the _only_ safe pattern in some situations).

I wonder where the actual cutoff is for converting a sequence of writes
of zero into a call to memset()?

If you assume either:
1) cold cache (for memset).
2) branch predictor not set for zeroing a small number of words.
I suspect that it is considerable.

> This is why I think "-ffreestanding" and "-fno-builtin-memcpy" are
> completely broken as-is: they are an all-or-nothing thing, they don't
> understand that it's directional.

Yep, and some of the conversions are a just a PITA.
eg printf("%s", string) => puts(string).

I was also trying to get around the memcpy@...BC4 fubar so I could
compile code that would run on an old system.
I managed everything except the memcpy() calls that gcc emits for
structure copies (it might even do that even for 'freestanding').
It really ought to emit a call to a different symbol that would normally
be aliased to memcpy() (or better a memcpy_words() function).

	David

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