-fhosted and -ffreestanding Compiles Flags

[marcrittinghaus]

While building a cpp application, I noticed that I always get the warning that fhosted is not supported for C++.

I checked where we set the parameter and found the following in unikraft/Makefile.uk:94

COMPFLAGS-y += -fhosted -ffreestanding -fno-tree-loop-distribute-patterns

From my understanding -fhosted, which is the default, means that the compiler can expect the standard library to be present and have the standard-conforming implementation. So, it is possible for the compiler to optimize a memset by replacing it with inline ASM instructions.

An application like this:

char buf[1024];

int main(int argc, char *argv[])
{
        memset(buf, 0xff, sizeof(buf));
        return 0;
}

could be compiled to this with -O3:

  10d170:       ba 80 2d 19 00          mov    edx,0x192d80               # Address of buf
  10d175:       48 c7 c0 ff ff ff ff    mov    rax,0xffffffffffffffff     # Value to set
  10d17c:       b9 80 00 00 00          mov    ecx,0x80                   # Length to set
  10d181:       48 89 d7                mov    rdi,rdx
  10d184:       f3 48 ab                rep stos QWORD PTR es:[rdi],rax   # Perform memset
  10d187:       31 c0                   xor    eax,eax                    # retval = 0
  10d189:       c3                      ret

However, currently for Unikraft this will compile to:

  10d110:       55                      push   rbp
  10d111:       ba 00 04 00 00          mov    edx,0x400                  # Length in bytes
  10d116:       be ff 00 00 00          mov    esi,0xff                   # Value to set
  10d11b:       bf 80 3d 19 00          mov    edi,0x193d80               # Address of buf
  10d120:       31 c0                   xor    eax,eax
  10d122:       48 89 e5                mov    rbp,rsp
  10d125:       e8 26 53 06 00          call   172450 <memset>            # Perform memset
  10d12a:       31 c0                   xor    eax,eax
  10d12c:       5d                      pop    rbp
  10d12d:       c3                      ret

This is due to the -ffreestanding parameter that will let the compiler assume that there is no standard library and thus functions like memset are more or less regular functions. Optimizing them is obviously not possible in this case.

The line is introduced by commit 86fef08ac53, which states it introduces the parameters to avoid a recursive call bug for GCC 10.

I was wondering, if (1) the consequences of this where clear, and (2) why both parameters where specified because -ffreestanding takes precedence over -fhosted, thus potentially preventing a lot of optimizations. @vladandrew You were reviewing this. Can you remember any details?

After discussing the topic also with @mschlumpp, my suggestion is to use -ffreestanding only for nolibc to be safe with non-standard behavior but remove it for musl and newlib. Also -fhosted seems redundant as it is the default for C (if not freestanding) and causes warnings for cpp files.

For a more thorough discussion of hosted vs. freestanding also look here.

[vladandrew]

Hi Marc,

I remember we had to do this for a quick fix during the AE process for EuroSys, it was related to a bug in DPDK. I agree with your research, we should use -freestanding only for nolibc.

[razvand]

This is now discussed in #740 , let's move the discussion there.