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

[Georg-Johann Lay]

David Brown schrieb:
> 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").

It's the same for

  if (x == 1)      { a=b; c=d; e=f; foo(); }
  else if (x == 2) { a=b; c=d; e=f; foo(); }

GCC don't factor out the common part

Johann