Re: Tr : [redundency elimination, code motion, commun expression elimination] GCC optimizations

[David Brown <david at westcontrol dot com>]

On 22/09/2011 21:59, charfi asma wrote:
> hello,
>
> If GCC could optimize this code
>
>
> if (x=1) { a=b; c=d; e=f; foo();  }
> if (x=2) { a=b; c=d; e=f; foo(); }
>
>
> to this one
>
> if (x=1) || (x=2) { a=b; c=d; e=f; foo(); }
>
> it will be implemented in which optimizations ? pre: partiel
> redundent elimination optimizaion or another one may be code motion?
>
> thank you very much
>
> Asma

gcc can't optimize code like this, because the two sequences have 
different effects.  In the first case, both sections are executed - 
first it sets "x" to 1, then does the {a=b;...foo();} code.  Then it 
sets "x" to 2 and does it again.  Depending on the locality and linkage 
of the variables, the compiler may be able to eliminate some of the 
assignments.  It will certainly be able to eliminate the conditional 
checks (unless "x" is volatile).  Your second sequence will only execute 
the code once (after the missing parenthesis are added).

If you had meant to write "if (x == 1)" instead of "if (x = 1)", then 
the question makes a little more sense.  But even then, it depends on 
whether foo() can access or change "x" (or more precisely, it depends on 
whether or not the compiler can be sure that foo() cannot read or change 
"x").