[Bug target/39102] gcc generating multiple stack stores in optimised code

[Bug target/39102] gcc generating multiple stack stores in optimised code

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39102

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-01-19
     Ever Confirmed|0                           |1
           Severity|normal                      |enhancement

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-01-19 06:20:02 UTC ---
Confirmed, I think this is the standard passing via registers a struct who's
mode is BLKmode.

[Bug target/39102] gcc generating multiple stack stores in optimised code

[jens.maurer at gmx dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39102

--- Comment #2 from Jens Maurer <jens.maurer at gmx dot net> 2012-11-23 14:07:34 UTC ---
Here is another test case:


struct S {
  unsigned char * p;
  unsigned char * limit;
};

S g(S s);

inline __attribute__((always_inline)) S f(S s, unsigned int n);

S f(S s, unsigned int n)
{
  if (__builtin_expect(s.p + n < s.limit, 1)) {
    s.p += n;
    return s;
  }
  return g(s);
}

extern S sext;

int main()
{
  S s = sext;
  s = f(s, 4);
  s = f(s, 8);
  s = f(s, 2);
  return (int)(unsigned long)s.p;
}


leading to
        movq    %rax, (%rsp)
        movq    %rdx, 8(%rsp)
in the code paths calling g(s), although "s" is kept in registers throughout.