From: Rasmus Villemoes <linux@rasmusvillemoes.dk>
To: Arvind Sankar <nivedita@alum.mit.edu>,
Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: "Nick Desaulniers" <ndesaulniers@google.com>,
"Dávid Bolvanský" <david.bolvansky@gmail.com>,
"Eli Friedman" <efriedma@quicinc.com>,
"Linus Torvalds" <torvalds@linux-foundation.org>,
"H. Peter Anvin" <hpa@zytor.com>,
"Masahiro Yamada" <masahiroy@kernel.org>,
"Andrew Morton" <akpm@linux-foundation.org>,
"Thomas Gleixner" <tglx@linutronix.de>,
"Ingo Molnar" <mingo@redhat.com>,
"Borislav Petkov" <bp@alien8.de>,
"Michal Marek" <michal.lkml@markovi.net>,
"Linux Kbuild mailing list" <linux-kbuild@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
"Kees Cook" <keescook@chromium.org>,
"Tony Luck" <tony.luck@intel.com>,
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"Andy Shevchenko" <andriy.shevchenko@linux.intel.com>,
"Alexandru Ardelean" <alexandru.ardelean@analog.com>,
"Yury Norov" <yury.norov@gmail.com>,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<x86@kernel.org>, "Ard Biesheuvel" <ardb@kernel.org>,
"Paul E . McKenney" <paulmck@kernel.org>,
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"Vamshi K Sthambamkadi" <vamshi.k.sthambamkadi@gmail.com>
Subject: Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches
Date: Fri, 21 Aug 2020 08:45:52 +0200 [thread overview]
Message-ID: <721e6979-e83b-2238-4763-a8cf99b57631@rasmusvillemoes.dk> (raw)
In-Reply-To: <20200820175617.GA604994@rani.riverdale.lan>
On 20/08/2020 19.56, Arvind Sankar wrote:
> On Thu, Aug 20, 2020 at 04:56:02PM +0200, Rasmus Villemoes wrote:
>> On 18/08/2020 23.41, Arvind Sankar wrote:
>>>
>>> Note that -fno-builtin-foo seems to mean slightly different things in
>>> clang and gcc. From experimentation, clang will neither optimize a call
>>> to foo, nor perform an optimization that introduces a call to foo. gcc
>>> will avoid optimizing calls to foo, but it can still generate new calls
>>> to foo while optimizing something else. Which means that
>>> -fno-builtin-{bcmp,stpcpy} only solves things for clang, not gcc. It's
>>> just that gcc doesn't seem to have implemented those optimizations.
>>>
>>
>> I think it's more than that. I've always read gcc's documentation
>>
>> '-fno-builtin'
>> '-fno-builtin-FUNCTION'
>> Don't recognize built-in functions that do not begin with
>> '__builtin_' as prefix. ...
>>
>> GCC normally generates special code to handle certain built-in
>> functions more efficiently; for instance, calls to 'alloca' may
>> become single instructions which adjust the stack directly, and
>> calls to 'memcpy' may become inline copy loops.
>> ...
>>
>> to mean exactly that observed above and nothing more, i.e. that
>> -fno-builtin-foo merely means that gcc stops treating a call of a
>> function named foo to mean a call to a function implementing the
>> standard function by that name (and hence allows it to e.g. replace a
>> memcpy(d, s, 1) by byte load+store). It does not mean to prevent
>> emitting calls to foo, and I don't think it ever will - it's a bit sad
>> that clang has chosen to interpret these options differently.
>
> That documentation is misleading, as it also goes on to say:
> "...nor can you change the behavior of the functions by linking with a
> different library"
> which implies that you _can_ change the behavior if you use the option,
> and which is what your "i.e." is saying as well.
>
> My point is that this is not completely true: in gcc, foo by default is
> defined to be __builtin_foo, and -fno-builtin-foo simply removes this
> definition. So the effect is just that calls to foo in the original
> source will be left alone.
Yes, this is a much better way of putting it. And with -fbuiltin-foo in
effect, the compiler just needs to transform the code in some way as-if
the standard function by that name was called, which it can of course
decide to implement by emitting such a call, but it can also open-code
it - or synthesize it using other std functions.
> But in order for an optimization that introduces a new call to foo to be
> valid, foo _must_ have standard semantics: strchr(s,'\0') is not s +
> strlen(s) unless strlen has standard semantics.
Correct. So I agree that -fno-builtin-strlen should prevent the compiler
from generating calls to strlen() that don't appear in the code.
This is an oversight in
> gcc's optimizations: it converts to s + __builtin_strlen(s), which then
> (normally) becomes s + strlen(s).
>
> Check out this horror: https://godbolt.org/z/a1r9fK
>
> Clang will disable this optimization if -fno-builtin-strlen is
> specified.
>
> Clang's interpretation is more useful for embedded, since you can use
> -fno-builtin-foo and avoid calling __builtin_foo directly, and be
> guaranteed that there will be no calls to foo that you didn't write
> explicitly (outside of memcpy/memset/memcmp). In this case you are free
> to implement foo with non-standard semantics, or avoid implementing it
> altogether, and be reasonably confident that it will all work.
Yeah, except that the list of -fno-builtin-foo one would have to pass is
enourmous, so for targets with a somewhat wonky libc, I'd much rather be
able to do a blanket -fno-builtin, and then manually check their memcpy,
memset and memcmp implementations and opt back in with
-fbuiltin-mem{cpy,set,cmp} so that small constant-size memcpys do get
properly open-coded.
The advice in gcc's documentation of just #definining memcpy() to
__builtin_memcpy() doesn't work in the real world (for example it breaks
C++ code that uses std::memcpy(...)).
>> Thinking out load, it would be useful if both compilers grew
>>
>> -fassume-provided-std-foo
>>
>> and
>>
>> -fno-assume-provided-std-foo
>>
>> options to tell the compiler that a function named foo with standard
>> semantics can be assumed (or not) to be provided by the execution
>> environment; i.e. one half of what -f(no-)builtin-foo apparently does
>> for clang currently.
>
> Not following: -fno-assume-provided-std-foo sounds like it would have
> exactly the same semantics as Clang's -fno-builtin-foo, except maybe in
> addition it should cause the compiler to error on seeing __builtin_foo
> if it can't implement that without calling foo.
Yeah, I think you've convinced me there's no use for a separate option
to prevent inventing calls to foo() - I was mostly thinking of it as a
way to avoid having to provide each and every libc function that may
have been half-way standardized at some point. But if one doesn't
provide, say, bcmp, the code base certainly doesn't use bcmp itself, so
one doesn't lose anything by just using -fno-builtin-bcmp; there are no
explicit bcmp() uses that fail to get optimized.
Rasmus
next prev parent reply other threads:[~2020-08-21 6:46 UTC|newest]
Thread overview: 68+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-17 22:02 [PATCH 0/4] -ffreestanding/-fno-builtin-* patches Nick Desaulniers
2020-08-17 22:02 ` [PATCH 1/4] Makefile: add -fno-builtin-stpcpy Nick Desaulniers
2020-08-17 22:31 ` H. Peter Anvin
2020-08-17 23:36 ` Nick Desaulniers
2020-08-18 19:21 ` Kees Cook
2020-08-18 7:10 ` Ard Biesheuvel
2020-08-18 7:25 ` Greg KH
2020-08-18 7:29 ` Ard Biesheuvel
2020-08-18 7:34 ` Greg KH
2020-08-18 19:23 ` Kees Cook
2020-08-17 22:02 ` [PATCH 2/4] Revert "lib/string.c: implement a basic bcmp" Nick Desaulniers
2020-08-18 5:44 ` Nathan Chancellor
2020-08-18 18:00 ` Nick Desaulniers
2020-08-18 19:24 ` Kees Cook
2020-08-17 22:02 ` [PATCH 3/4] x86/boot: use -fno-builtin-bcmp Nick Desaulniers
2020-08-18 19:24 ` Kees Cook
2020-08-17 22:02 ` [PATCH 4/4] x86: don't build CONFIG_X86_32 as -ffreestanding Nick Desaulniers
2020-08-18 19:24 ` Kees Cook
2021-01-07 0:27 ` Fangrui Song
2022-04-07 15:34 ` [tip: x86/build] x86/build: Don't " tip-bot2 for Nick Desaulniers
2022-04-07 17:01 ` Nick Desaulniers
2022-04-07 22:28 ` Borislav Petkov
2020-08-17 22:44 ` [PATCH 0/4] -ffreestanding/-fno-builtin-* patches H. Peter Anvin
2020-08-18 17:56 ` Nick Desaulniers
2020-08-18 19:02 ` H. Peter Anvin
2020-08-18 19:13 ` Linus Torvalds
2020-08-18 19:25 ` Nick Desaulniers
2020-08-18 19:58 ` Nick Desaulniers
2020-08-19 12:19 ` Clement Courbet
2020-08-18 20:24 ` Arvind Sankar
2020-08-18 20:27 ` Nick Desaulniers
2020-08-18 20:58 ` Nick Desaulniers
2020-08-18 21:41 ` Arvind Sankar
2020-08-18 21:51 ` Dávid Bolvanský
2020-08-18 21:59 ` Nick Desaulniers
2020-08-18 22:05 ` Dávid Bolvanský
2020-08-18 23:22 ` Nick Desaulniers
2020-08-20 14:56 ` Rasmus Villemoes
2020-08-20 17:56 ` Arvind Sankar
2020-08-20 18:05 ` Dávid Bolvanský
2020-08-20 23:33 ` Linus Torvalds
2020-08-21 17:29 ` Arvind Sankar
2020-08-21 17:54 ` Linus Torvalds
2020-08-21 18:02 ` Linus Torvalds
2020-08-21 19:14 ` Arvind Sankar
2020-08-21 19:23 ` Linus Torvalds
2020-08-21 19:57 ` Arvind Sankar
2020-08-21 20:03 ` Peter Zijlstra
2020-08-21 21:39 ` Linus Torvalds
2020-08-22 0:12 ` Nick Desaulniers
2020-08-22 12:20 ` David Laight
2020-08-21 6:45 ` Rasmus Villemoes [this message]
2020-08-24 15:57 ` Masahiro Yamada
2020-08-24 17:34 ` Arvind Sankar
2020-08-25 7:10 ` Nick Desaulniers
2020-08-25 7:31 ` Nick Desaulniers
2020-08-25 12:28 ` Masahiro Yamada
2020-08-25 14:02 ` Nick Desaulniers
2020-08-26 13:28 ` Masahiro Yamada
2020-08-18 21:53 ` David Laight
2020-08-20 22:41 ` H. Peter Anvin
2020-08-20 23:17 ` Arvind Sankar
2020-08-18 19:35 ` Nick Desaulniers
2020-08-18 22:25 ` Arvind Sankar
2020-08-18 22:59 ` Nick Desaulniers
2020-08-18 23:51 ` Arvind Sankar
2020-08-19 0:20 ` Arvind Sankar
2020-08-19 8:26 ` David Laight
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