gnu /
gcc /
fd8dd6c0384969170e594be34da278a072d5eb76 tree-optimization/107852 - missed optimization with PHIs
The following deals with the situation where we have
<bb 2> [local count: 1073741824]:
_5 = bytes.D.25336._M_impl.D.24643._M_start;
_6 = bytes.D.25336._M_impl.D.24643._M_finish;
pretmp_66 = bytes.D.25336._M_impl.D.24643._M_end_of_storage;
if (_5 != _6)
goto <bb 3>; [70.00%]
else
goto <bb 4>; [30.00%]
...
<bb 6> [local count: 329045359]:
_89 = operator new (4);
_43 = bytes.D.25336._M_impl.D.24643._M_start;
_Num_44 = _137 - _43;
if (_Num_44 != 0)
but fail to see that _137 is equal to _5 and thus eventually _Num_44
is zero if not operator new would possibly clobber the global
bytes variable.
The following resolves this in value-numbering by using the
predicated values for _5 == _6 recorded for the dominating
condition.
PR tree-optimization/107852
* tree-ssa-sccvn.cc (visit_phi): Use equivalences recorded
as predicated values to elide more redundant PHIs.
* gcc.dg/tree-ssa/ssa-fre-101.c: New testcase.
2 files changed
