https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71347
--- Comment #2 from amker at gcc dot gnu.org ---
Thanks for reporting this.
The dump after IVOPT now is:
<bb 3>:
# prephitmp_21 = PHI <pretmp_20(4), powmult_2(2)>
# prephitmp_23 = PHI <pretmp_22(4), powmult_2(2)>
# ivtmp.17_16 = PHI <ivtmp.17_9(4), ivtmp.17_24(2)>
_6 = prephitmp_21 * prephitmp_23;
_4 = (void *) ivtmp.17_16;
MEM[base: _4, offset: 0B] = _6;
ivtmp.17_9 = ivtmp.17_16 + 8;
if (ivtmp.17_9 != _26)
goto <bb 4>;
else
goto <bb 5>;
<bb 4>:
_5 = (void *) ivtmp.17_9;
pretmp_20 = MEM[base: _5, offset: 4294967288B];
pretmp_22 = X[1];
goto <bb 3>;
That patch skips computing cost for sub iv_uses in a group:
Group 0:
Type: ADDRESS
Use 0.0:
At stmt: X[i_18] = _6;
At pos: X[i_18]
IV struct:
Type: double *
Base: (double *) (&X + 16)
Step: 8
Object: (void *) &X
Biv: N
Overflowness wrto loop niter: Overflow
Use 0.1:
At stmt: pretmp_20 = X[_15];
At pos: X[_15]
IV struct:
Type: double *
Base: (double *) (&X + 16)
Step: 8
Object: (void *) &X
Biv: N
Overflowness wrto loop niter: Overflow
Though use 0.0/0.1 have same {base, step}, but there are in different program
point, so if iv_cand is increased before use 0.0, the first use is transformed
into: MEM[var_before, 0], and use 0.1 will be transformed into: MEM[var_after,
-8]. Now the two memory reference have different expressions, though access to
the same object. Afterwards, DOM failed to CSE them.
It is hard to decide which sub iv_use we should compute the cost, and which one
we can skip computing the cost. Maybe I should revert this part of code which
was introduced to save a small amount of compilation time.
