[patch][rfc] Add pwrite to raw_stream

[Duncan P. N. Exon Smith]

> On 2015-Apr-08, at 16:59, Justin Bogner <mail at justinbogner.com> wrote:
> 
> Rafael Espíndola <rafael.espindola at gmail.com> writes:
>> I noticed that object writer are somewhat convoluted because they have
>> to write to streams with to seek or pwrite support. For that to work
>> they have to do a pass over the data computing the size and only then
>> write it out.
>> 
>> That is a bit silly, as most of the time (and the ones we care about
>> performance) we are writing to an actual file with seek support.
>> 
>> When that is not the case, we can accumulate the data and write it on the end.
>> 
>> For accumulating the data, raw_svector_ostream works just fine, except
>> that it doesn't support seeking. Adding that is doable, but
>> complicates a performance critical structure.
>> 
>> Fortunately, adding pwrite to raw_svector_ostream is trivial.
>> 
>> The attached patch changes ELFObjectWriter.cpp to use pwrite to update
>> the one forward reference that is left at that point. Similar cleanups
>> should be possible for the other object writers.
>> 
>> The patch also adds a virtual pwrite to raw_stream and a helper class
>> for creating a raw_svector_ostream when a given FD does'n support
>> seeking (like a pipe).
>> 
>> It passes check-llvm with the exception of
>> CodeGen/R600/cf-stack-bug.ll which uses "2>&1" and depends on the
>> order the data from stdout and stderr is mixed.
>> 
>> The test could be fixed, but another issue is that clang is structured
>> to pass a plain raw_ostream around until the last moment when it is
>> wrapped in a formatted_raw_ostream. That makes it harder to detect if
>> a stream supports seeking or not.
>> 
>> So, if you guys are OK with this direction, I think the proper way to
>> split/commit this is
> 
> I like this direction. I've needed this sort of functionality before (in
> InstrProfWriter, I think) and worked around it at the expense of
> readability.
> 
>> * Implement classof for raw_ostream.
>> * Stop using formatted_raw_ostream when producing binaries.
>> * This patch, but SeekWrapper can then take a plain raw_stream and use
>> dyn_cast to see if it a fd stream. It would also not add pwrite to
>> formatted_raw_ostream.
> 
> SGTM.

SGTM too!