[Bug middle-end/45058] New: GCC doesn't create functions with multiple entry points.

[svfuerst at gmail dot com]

A possible optimization in some cases is to construct a function with multiple
entry points.  This can save in both size and speed in certain cases.

#include <stdio.h>

/* Example function that does something complex enough not to be optimized away
*/
static foo(int x)
{
        int i;
        int j = x;
        for (i = 0; i < 1024; i++)
        {
                j += printf("%d%d%d\n", i);
        }

        return j;
}

/* Since foo is static and whose address is never taken, foo1 can have the same
address as foo. */
int foo1(int x)
{
        return foo(x);
}

/* This function could point to an "add $1, %rdi" instruction immediately
before foo. (Giving foo two entry points.) */ 
int foo2(int x)
{
        return foo(x + 1);
}

At the moment, GCC will at -O3 construct two cloned versions of foo for foo1
and foo2.  At -Os, it will jump to foo in foo1 and foo2.  With multiple entry
points, the code can be as fast as generated for -O3 but be half the size,
slightly smaller than the current -Os.


-- 
           Summary: GCC doesn't create functions with multiple entry points.
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: svfuerst at gmail dot com
 GCC build triplet: x86_64-linux
  GCC host triplet: x86_64-linux
GCC target triplet: x86_64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45058