Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches

[Arvind Sankar <nivedita@alum.mit.edu>]

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.

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. 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.

> 
> 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.

> 
> And yes, the positive -fbuiltin-foo would also be quite useful in order
> to get the compiler to recognize a few important functions (memcpy,
> memcmp) while using -ffreestanding (or just plain -fno-builtin) to tell
> it to avoid assuming anything about most std functions - I've worked on
> a VxWorks target where snprintf() didn't have the correct "return what
> would be written" semantics but rather behaved like the kernel's
> non-standard scnprintf(), and who knows what other odd quirks that libc had.
> 
> Rasmus