[Bug rtl-optimization/38544] New: missed opportunity to use adc

[Bug rtl-optimization/38544] New: missed opportunity to use adc

[rrh at google dot com]

The optimizer at -O3 for x86_64 finds opportunities to generate the "adc" (add
with carry) instruction to avoid a conditional branch.  However, the dst of the
adc can only be in a register.  The optimizer misses a chance to use "adc" with
the destination in memory, and will instead use a conditional branch in a
straightforward manner.  The following program shows that foo1 uses adc, but
foo0 does not.

Although I haven't tested this, I'm sure that the other 3 possibilities in this
family (involving sbb and/or different literal values to add/subtract) will
also not find the opportunity to do a memory update with adc.

extern void consumep(int *);
extern void consume(int);
void foo0(unsigned a, unsigned b, int *victim)
{
  if (a > b) { (*victim)++; }
  consumep(victim);
}
void foo1(unsigned a, unsigned b, int victim)
{
  if (a > b) { victim++; }
  consume(victim);
}


-- 
           Summary: missed opportunity to use adc
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rrh at google dot com
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38544

[Bug rtl-optimization/38544] missed opportunity to use adc

[pinskia at gcc dot gnu dot org]

-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
           Keywords|                            |missed-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38544

[Bug target/38544] missed opportunity to use adc

[ubizjak at gmail dot com]

------- Comment #1 from ubizjak at gmail dot com  2008-12-17 08:56 -------
There are various scary comments in ifcvt.c, noce_process_if_block() regarding
memory operands, like:

  /* Only operate on register destinations, and even then avoid extending
     the lifetime of hard registers on small register class machines.  */

and 

  /* Don't operate on sources that may trap or are volatile.  */

and

      /* Avoid store speculation: given "if (...) x = a" where x is a
         MEM, we only want to do the store if x is always set
         somewhere in the function.  This avoids cases like
           if (pthread_mutex_trylock(mutex))
             ++global_variable;
         where we only want global_variable to be changed if the mutex
         is held.  FIXME: This should ideally be expressed directly in
         RTL somehow.  */

I don't think it is always safe to simplify global memory operands.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38544

[Bug rtl-optimization/38544] missed opportunity to use adc

[ubizjak at gmail dot com]

------- Comment #2 from ubizjak at gmail dot com  2008-12-17 08:58 -------
Setting Component to Generic RTL optimization.


-- 

ubizjak at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|target                      |rtl-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38544