[Bug tree-optimization/36602] memset should be optimized into an empty CONSTRUCTOR

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36602

--- Comment #5 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-06-06 
14:11:07 UTC ---
Created attachment 27563
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27563
proposed patch

Not as easy when the initialized struct contains padding ...

The following testcase is reduced from a fail of
gcc.target/x86_64/abi/test_struct_returning.c
where SRA messes up information in the padding.  With s = {} being
a memset SRA would not do anything.

The question in general would be whether memcpy/memset affect padding
in a well-defined way, thus, if memset (&s, 0, sizeof (s)) is equivalent
to s = {} and if memcpy (&s, &r, sizeof (s)) is equivalent to s = r.
We at least assume that in the way we currently fold memcpy.


struct S { char c; long long i; };

struct S __attribute__((noinline,noclone))
foo (void)
{
  struct S s = {};
  s.c = 42;
  return s;
}

void __attribute__((noinline,noclone))
keepit (struct S *s)
{
  __builtin_memset (s, -1, sizeof (struct S));
}

void __attribute__((noinline,noclone))
clobberstack ()
{
  struct S s;
  keepit (&s);
}

int main ()
{
  struct S s1, s2 = { 42, 0 };
  clobberstack();
  s1 = foo();
  if (__builtin_memcmp (&s1, &s2, sizeof (struct S)) != 0)
    __builtin_abort ();
  return 0;
}