russo@M.CS.UIUC.EDU (Vince Russo) (01/04/89)
The following code fragment is an example of something output from the C++
translator when inlining a function.
When compiled on the sun3 with gcc-1.31 it generates the code below it.
The three local variables are assigned to in the comma expression, but then
immediately go out of scope.
My question is, why does it generate any code at all. Shouldnt the
assignments be dead coded since the variables immediately go out of
scope? Then no space for the variables even needs allocated since they
are never used. The next result of the comma expression is just 0.
Therefore as long as any parts of it dont have side effect, the whole
expression is dead.
Is there some subtle reason to leave in the dead assignments or is this
just an optimization gcc misses?
The only compiler flags I used where "-O -S". Ive tried others but nothing
deadcodes the assignments.
--Vince Russo
russo@cs.uiuc.edu
-------------------------------------------------------------------------
union debugArg {
long l__8debugArg ;
char * p__8debugArg ;
};
char
main ()
{
{ /* in scope */
union debugArg __0_V1 ;
union debugArg __0_V2 ;
union debugArg __0_V3 ;
( ( ((& __0_V1 )-> l__8debugArg = ((long )0 )),
(((& __0_V1 )))) ,
( ( ((& __0_V2 )-> l__8debugArg = ((long )1 )),
(((& __0_V2 )))) ,
( ( ((& __0_V3 )-> p__8debugArg = ((char *)"aa")),
(((& __0_V3 )))) ,
( ( 0 ) ) ) )
);
} /* out of scope */
}
----------------------------------------------------------------------------
#NO_APP
.text
LC0:
.ascii "aa\0"
.even
.globl _main
_main:
link a6,#-12 <<- Stack space is wasted storing them
clrl a6@(-4) <<- Why is this here (dead assignment).
moveq #1,d0 <<- Why is this here (ditto)
movel d0,a6@(-8) <<- Why is this here (ditto)
movel #LC0,a6@(-12) <<- Why is this here (ditto)
unlk a6
rts
---------------------------------------------------------------------------