https://llvm.org/bugs/show_bug.cgi?id=23395
Bug ID: 23395
Summary: raw_svector_ostream will heap allocate without need
Product: new-bugs
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected],
[email protected]
Classification: Unclassified
The code line
OS.reserve(OS.size() + 64);
called from raw_svector_ostream::write_impl called from
raw_svector_ostream::~raw_svector_ostream will heap allocate without need. The
issue is already documented:
raw_svector_ostream::~raw_svector_ostream() {
// FIXME: Prevent resizing during this flush().
flush();
}
And a solution is provided in raw_svector_ostream::init():
// Set up the initial external buffer. We make sure that the buffer has at
// least 128 bytes free; raw_ostream itself only requires 64, but we want to
// make sure that we don't grow the buffer unnecessarily on destruction (when
// the data is flushed). See the FIXME below.
OS.reserve(OS.size() + 128);
This solution may be worse than the problem. In total:
* If the SmallString was less than 128 bytes init() will always(!) heap
allocate.
* If it's more or equal to 128 bytes but upon destruction less than 64 bytes
are left unused it will heap allocate for no reason.
A careful programmer who did size the SmallString to match its use + small
reserve will find either of these heap allocations surprising, negating the
SmallString advantage and then paying memcpy cost.
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