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

[Nick Desaulniers]

On Tue, Aug 18, 2020 at 3:25 PM Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
>
> Another thing that needs to be fixed is that at least lib/string.c needs
> to be compiled with -ffreestanding.
>
> gcc-10 optimizes the generic memset implementation in there into a call
> to memset. Now that's on x86 which doesn't use the generic
> implementation, but this is just waiting to bite us.
>
> https://godbolt.org/z/6EhG15

I'll let you send the patch for that this time.  (It's too bad godbolt
doesn't have newer versions of GCC for cross compilation...cant test
aarch64 gcc-10, for example.)  It would be interesting for sure to see
resulting differences in disassembly observed in lib/string.o with
-ffreestanding.

But, oof, that's not good.  Certainly impressive and powerful loop
idiom recognition, but wouldn't you consider it a bug that this
optimization should probably first check that it's not replacing part
of a loop with a potentially recursive call to itself?

Admittedly, we've had the same shenanigans with memcpy implemented in
terms of calls to __builtin_memcpy being lowered to infinitely
recursive calls...which feels like the same kind of bug.  ("You wanted
infinite recursion in the kexec purgatory image, right?" "No,
compiler, I did not.")  example: https://godbolt.org/z/MzrTaM
(probably should fix this in both implementations; at the least I feel
like Clang's -Winfinite-recursion should try to help us out here).

Feels almost like it may be difficult to provide an implementation of
memset without stepping on a landmine.  One thing I'd be curious about
is whether all of lib/string.c would need -ffreestanding, or if you
could move just memset to its own TU then use -ffreestanding on that.
A free standing environment must always provide a core set of
functions like memset, memcpy, memcmp, memmove, according to
https://gcc.gnu.org/onlinedocs/gcc/Standards.html.  Maybe those four
should be in a separate TU compiled as -ffreestanding, so that they
can never be lowered to calls to themselves (potentially infinitely
recursive)?
-- 
Thanks,
~Nick Desaulniers