https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67438
Mikhail Maltsev <miyuki at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |miyuki at gcc dot gnu.org
--- Comment #4 from Mikhail Maltsev <miyuki at gcc dot gnu.org> ---
I looked at gimple dumps. The only difference looks like this. In the "good"
revision after forwprop1:
<bb 3>:
_13 = *in_2;
a_14 = ~_13;
_17 = MEM[(char *)in_2 + 1B];
b_18 = ~_17;
in_20 = &MEM[(void *)in_2 + 3B];
_21 = MEM[(char *)in_2 + 2B];
c_22 = ~_21;
if (a_14 < b_18)
goto <bb 4>;
else
goto <bb 5>;
In the "bad" revision this basic block is simplified:
<bb 3>:
_13 = *in_2;
a_14 = ~_13;
_17 = MEM[(char *)in_2 + 1B];
b_18 = ~_17;
in_20 = &MEM[(void *)in_2 + 3B];
_21 = MEM[(char *)in_2 + 2B];
c_22 = ~_21;
if (_13 > _17)
goto <bb 4>;
else
goto <bb 5>;
Next BB's are:
<bb 4>: d_23 = MIN_EXPR <a_14, c_22>;
<bb 5>: d_24 = MIN_EXPR <b_18, c_22>;
<bb 6>: # d_4 = PHI <d_23(4), d_24(5)>
The condition of "if" is not altered throughout all other passes (it gets
if-converted and vectorized).
Another small difference: VRP adds assertions in bb 4 (a_12 lt_expr b_14, b_14
gt_expr a_12) and bb5 (a_12 ge_expr b_14, b_14 le_expr a_12). For some reason
this does not happen in the "bad" revision.
As I understand, the problem is that if we do not fold the condition, values
_13 and _17 are killed after we calculate a_14 = ~_13 and b_18 = ~_17. But if
we do fold, they are still live (because they are used in the condition), thus,
register pressure increases.
