------- Comment #17 from changpeng dot fang at amd dot com 2010-03-17 00:18
-------
(In reply to comment #8)
> And
>
> int foo (int b, int j)
> {
> if (b)
> {
> int i;
> for (i = 0; i<1000; ++i)
> ;
> j = b;
> }
> return j;
> }
>
With "j=b", "b" is not folded as a phi argument:
<bb 5>:
# i_2 = PHI <0(3), i_6(4)>
if (i_2 <= 999)
goto <bb 4>;
else
goto <bb 6>;
<bb 6>:
j_7 = b_3(D);
<bb 7>:
# j_1 = PHI <j_4(D)(2), j_7(6)>
However, if "j=0", it is:
<bb 6>:
j_7 = 0;
<bb 7>:
# j_1 = PHI <j_4(D)(2), 0(6)>
j_8 = j_1;
return j_8;
Then copy propagation will remove "j_7 = 0" (and thus <bb 6>) because it has no
user.
So, one possible solution is "do not remove trival dead code" in
copy_propagation pass. Any dce pass will remove such code.
Of course, if we follow Steven's suggestion not use constants as phi arguments,
"j_7=0" will not be removed by constant propagation, and we are all fine.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42906
