Created attachment 50372 [details]
test.c

When compiling for target where the assignment operator

  *dst = *src;

will generate a call to memcpy (for example aarch64), and when also using compile flags

  -ffreestanding -Os -flto

and link flags

  -nodefaultlibs -nostartfiles

the linking will fail with

  undefined reference to `memcpy'

even if memcpy is defined in the source.

The memcpy is optimized away, even though it is used by the = operator. It needs to be declared with __attribute__((used)) to not be optimized away.

What is interesting is that if the memcpy function is used explicitly, the compilation may still fail with -Os, because the code will be inlined.

If memcpy is used explicitly multiple times, it may be generated with symbol name
  memcpy.constprop.0.isra.0
or
  memcpy.isra.0
depending on whether the arguments passed are the same or different.

But the linking fill still fail, because the assignment operator will generate a call to memcpy, and this symbol is not defined, only memcpy.isra.0 or memcpy.constprop.0.isra.0 is.

I discovered this recently while trying to make U-Boot compile with LTO. We have solved it by declaring "memcmp", "memset", "memcpy" and "memmove" with attribute used, since according to GCC man page, calls to this functions may still be done when -nodefaultlibs is used.