http://llvm.org/bugs/show_bug.cgi?id=8504
Summary: Unused calls to operator new/delete not optimized away
Product: clang
Version: trunk
Platform: PC
OS/Version: All
Status: NEW
Severity: enhancement
Priority: P
Component: C++
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected], [email protected]
for this silly testcase:
void t() {
int *x = new int;
delete x;
}
clang generates at -O3:
define void @_Z1tv() ssp {
entry:
%call = tail call noalias i8* @_Znwm(i64 4)
%isnull = icmp eq i8* %call, null
br i1 %isnull, label %delete.end, label %delete.notnull
delete.notnull: ; preds = %entry
tail call void @_ZdlPv(i8* %call) nounwind
ret void
delete.end: ; preds = %entry
ret void
}
One issue is that we unconditionally emit the isnull check even if it's
unneeded. We could teach llvm to optimize away null checks before operator
delete (or free), which I'm not entirely sure is what the user expects, or just
avoid emitting the check when we don't need it.
Another problem is that LLVM knows nothing about operator new/delete so it
cannot remove the calls if the allocated memory isn't used at all.
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