[Bug c/40748] New: simple switch/case, if/else and arithmetics result in different code

[Bug c/40748] New: simple switch/case, if/else and arithmetics result in different code

[zsojka at seznam dot cz]

Tested r149624, 4.4.0 and 4.3.3

# ./gcc -v
Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: ../configure --enable-languages=c,c++
--prefix=/mnt/svn/gcc-trunk/build/
Thread model: posix
gcc version 4.5.0 20090714 (experimental) (GCC)

4.4.0 and 4.5 have better behaviour in the case of "switch"

The following code:
-------------------------------
unsigned f1(unsigned i)
{
        switch (i) {
                case 0: return 0;
                case 1: return 1;
                case 2: return 2;
                case 3: return 3;
                default: return 4;
        }
}

unsigned f2(unsigned i)
{
        if (i == 0) return 0;
        if (i == 1) return 1;
        if (i == 2) return 2;
        if (i == 3) return 3;
        return 4;
}

unsigned f3(unsigned i)
{
        return i < 4 ? i : 4;
}
-------------------------------

f1(), f2() and f3() do the same, but the resulting code differs a lot.


-- 
           Summary: simple switch/case, if/else and arithmetics result in
                    different code
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: zsojka at seznam dot cz
  GCC host triplet: x86_64-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40748

[Bug c/40748] simple switch/case, if/else and arithmetics result in different code

[zsojka at seznam dot cz]

------- Comment #1 from zsojka at seznam dot cz  2009-07-14 17:11 -------
Created an attachment (id=18195)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=18195&action=view)
testcase

Even at -O3, f1() and f2() don't have the same code as f3().


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40748

[Bug middle-end/40748] simple switch/case, if/else and arithmetics result in different code

[rguenth at gcc dot gnu dot org]

------- Comment #2 from rguenth at gcc dot gnu dot org  2009-07-15 09:34 -------
switch-conversion could try to handle this.  Generally perfect hashing during
switch expansion is another thing.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
          Component|c                           |middle-end
     Ever Confirmed|0                           |1
           Keywords|                            |missed-optimization
   Last reconfirmed|0000-00-00 00:00:00         |2009-07-15 09:34:55
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40748