On 23/09/2011 10:46, Georg-Johann Lay wrote:
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
You are right - which is odd, since there is no reason why it could not
(unlike the original case where there is no "else"). Surely this would
count as a significant missed optimisation, especially for big switch()
code.
