------- Comment #10 from rguenth at gcc dot gnu dot org 2007-11-01 15:26
-------
That is, the following testcase:
int f(void);
void acceptloop_th(int *t, int options) {
if (f()) options |= 0x1 << 0;
if (f()) options |= 0x1 << 1;
if (f()) options |= 0x1 << 2;
if (f()) options |= 0x1 << 3;
if (f()) options |= 0x1 << 4;
if (f()) options |= 0x1 << 5;
if (f()) options |= 0x1 << 6;
if (f()) options |= 0x1 << 7;
if (f()) options |= 0x1 << 8;
if (f()) options |= 0x1 << 9;
if (f()) options |= 0x1 << 10;
if (f()) options |= 0x1 << 11;
if (f()) options |= 0x1 << 12;
if (f()) options |= 0x1 << 13;
if (f()) options |= 0x1 << 14;
if (f()) options |= 0x1 << 15;
if (f()) options |= 0x1 << 16;
if (f()) options |= 0x1 << 17;
if (f()) options |= 0x1 << 18;
if (f()) options |= 0x1 << 19;
if (f()) options |= 0x1 << 20;
if (f()) options |= 0x1 << 21;
if (f()) options |= 0x1 << 22;
if (f()) options |= 0x1 << 23;
if (f()) options |= 0x1 << 24;
if (f()) options |= 0x1 << 25;
if (f()) options |= 0x1 << 26;
if (f()) *t = options;
}
it's interesting that this one is not different from the zero-initialized
options case just because PHIOPT which runs before PRE changes
<bb 2>:
D.1179_5 = f ();
if (D.1179_5 != 0)
goto <bb 3>;
else
goto <bb 4>;
<bb 3>:
<bb 4>:
# options_1 = PHI <0(2), 1(3)>
to
<bb 2>:
D.1179_5 = f ();
options_1 = D.1179_5 != 0;
which then causes PHI translation to not be able to figure out the constants
(but create value expressions). With PHIOPT disabled we even perform PRE
on this testcase ;) So, take the following modified testcase as well:
int f(void);
void acceptloop_th(int *t) {
int options = 0;
if (f()) options |= 0x1 << 1;
if (f()) options |= 0x1 << 2;
if (f()) options |= 0x1 << 3;
if (f()) options |= 0x1 << 4;
if (f()) options |= 0x1 << 5;
if (f()) options |= 0x1 << 6;
if (f()) options |= 0x1 << 7;
if (f()) options |= 0x1 << 8;
if (f()) options |= 0x1 << 9;
if (f()) options |= 0x1 << 10;
if (f()) options |= 0x1 << 11;
if (f()) options |= 0x1 << 12;
if (f()) options |= 0x1 << 13;
if (f()) options |= 0x1 << 14;
if (f()) options |= 0x1 << 15;
if (f()) options |= 0x1 << 16;
if (f()) options |= 0x1 << 17;
if (f()) options |= 0x1 << 18;
if (f()) options |= 0x1 << 19;
if (f()) options |= 0x1 << 20;
if (f()) options |= 0x1 << 21;
if (f()) options |= 0x1 << 22;
if (f()) options |= 0x1 << 23;
if (f()) options |= 0x1 << 24;
if (f()) options |= 0x1 << 25;
if (f()) options |= 0x1 << 26;
if (f()) *t = options;
}
which causes an exponential number of PHI nodes to be inserted. Like for
example (shortened testcase):
<bb 4>:
# prephitmp.9_30 = PHI <16(13), 18(3)>
# prephitmp.9_29 = PHI <24(13), 26(3)>
# prephitmp.9_28 = PHI <8(13), 10(3)>
# prephitmp.9_27 = PHI <20(13), 22(3)>
# prephitmp.9_26 = PHI <28(13), 30(3)>
# prephitmp.9_25 = PHI <12(13), 14(3)>
# prephitmp.9_24 = PHI <4(13), 6(3)>
# options_1 = PHI <0(13), 2(3)>
# SMT.4_18 = VDEF <SMT.4_17>
D.1180_8 = f ();
if (D.1180_8 != 0)
goto <bb 5>;
else
goto <bb 14>;
where all the computed constants materialize! :)
How interesting.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32540
