如何提供 memcpy 的实现

Question

I am trying to write some bare metal code with a memset -style loop in it:

for (int i = 0; i < N; ++i) {
  arr[i] = 0;
}

It is compiled with GCC and GCC is smart enough to turn that into a call to memset() . Unfortunately because it's bare metal I have no memset() (normally in libc) so I get a link error.

 undefined reference to `memset'

It seems like the optimisation that does this transformation is -ftree-loop-distribute-patterns :

Perform loop distribution of patterns that can be code generated with calls to a library. This flag is enabled by default at -O2 and higher, and by -fprofile-use and -fauto-profile .

So one person's solution was to just lower the optimisation level. Not very satisfying.

I also found this really helpful page that explains that -ffreestanding is not enough to get GCC not to do this, and there's basically no option but to provide your own implementations of memcpy , memmove , memset and memcmp . I'm happy to do that, but how?

If I just write memset the compiler will detect the loop inside it and transform it into a call to memset: In fact in the code provided by the CPU vendor I'm using I actually found this comment:

/*
// This is commented out because the assembly code that the compiler generates appears to be
// wrong.  The code would recursively call the memset function and eventually overruns the
// stack space.
void * memset(void *dest, int ch, size_t count)
...

So I assume that is the issue they ran into.

How do I supply a C implementation of memset without the compiler optimising it to a call to itself and without disabling that optimisation?

Answer 1

Aha I checked inthe glibc code and there's a inhibit_loop_to_libcall modifier which sounds like it should do this. It is defined like this :

/* Add the compiler optimization to inhibit loop transformation to library
   calls.  This is used to avoid recursive calls in memset and memmove
   default implementations.  */
#ifdef HAVE_CC_INHIBIT_LOOP_TO_LIBCALL
# define inhibit_loop_to_libcall \
    __attribute__ ((__optimize__ ("-fno-tree-loop-distribute-patterns")))
#else
# define inhibit_loop_to_libcall
#endif

Answer 2

You mention in your question:

It seems like the optimisation that does this transformation is -ftree-loop-distribute-patterns

all you need to do to turn off this optimization is pass -fno-tree-loop-distribute-patterns to the compiler. This turns off the optimization globally.