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-useand-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?
Aha I checked in the 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
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.