linux-kernel - Re: [PATCH] lib/string.c: Disable tree-loop-distribute-patterns

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Message-ID: <20200819030442.GA3396810@rani.riverdale.lan>
Date:   Tue, 18 Aug 2020 23:04:42 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] lib/string.c: Disable tree-loop-distribute-patterns

On Tue, Aug 18, 2020 at 05:44:03PM -0700, Linus Torvalds wrote:
> On Tue, Aug 18, 2020 at 4:43 PM Arvind Sankar <nivedita@...m.mit.edu> wrote:
> >
> > This by itself is insufficient for gcc if the optimization was
> > explicitly enabled by CFLAGS, so also add a flag to explicitly disable
> > it.
> 
> Using -fno-tree-loop-distribute-patterns seems to really be a bit too
> incestuous with internal compiler knowledge.

Fair enough -- you ok with just the -ffreestanding? That's what protects
the memset in arch/x86/boot/compressed/string.c.

I think this is worthwhile to be safe.

> 
> That generic memcpy implementation is horrible anyway. It should never be used.
> 
> So I'd rather see this either removed entirely, ot possibly rewritten
> to be a somewhat proper memcpy implementation, and in the process made
> to not be recognizable by the compiler (possibly by adding a dummy
> barrier() or something like that).
> 
> Looking at the implementation of "strscpy()" in the same file, and
> then comparing that to the ludicrously simplisting "memcpy()", I
> really get the feeling that that memcpy() is not worth having.
> 
>               Linus

I don't think anything actually uses the generic memcpy, and I think
only c6x uses the generic memset. Might be worth optimizing strnlen etc
with the word-at-a-time thing though.