[llvm] r212816 - raw_svector_ostream: grow and reserve atomically

[Alp Toker]

On 11/07/2014 19:45, Chandler Carruth wrote:
>
> On Fri, Jul 11, 2014 at 7:02 AM, Alp Toker <alp at nuanti.com 
> <mailto:alp at nuanti.com>> wrote:
>
>     Author: alp
>     Date: Fri Jul 11 09:02:04 2014
>     New Revision: 212816
>
>     URL: http://llvm.org/viewvc/llvm-project?rev=212816&view=rev
>     Log:
>     raw_svector_ostream: grow and reserve atomically
>
>     Including the scratch buffer size in the initial reservation
>     eliminates the
>     subsequent malloc+move operation and offers a healthier constant
>     growth with
>     less memory wastage.
>
>
> What benchmarks did you run to measure the memory waste, and what were 
> the numbers? I think it is really important to provide these kinds of 
> details with potentially performance impacting changes like this.

I've been getting some good timings with a synthetic clang syntax-only 
diagnostics engine stress test (10 runs, Before: 0m3.317s After: 
0m3.094s). I can't be sure this is a win for every workload so I've 
boiled the commit down into a cleanup in r212837.

I'll post my thoughts shortly so we can discuss ideas to resolve that 
old FIXME:

raw_svector_ostream::~raw_svector_ostream() {
   // FIXME: Prevent resizing during this flush().
   flush();
}

My secondary interest here is to cook up a seekable raw_svector_ostream 
so we can run more of the compilation in memory / via API without file 
descriptors, where raw_fd_ostream is currently required. Beware, patches 
with even more new ostream subclasses are getting proposed shortly ;-)

Alp.

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