llvm dev - May 2012 - [LLVMdev] [PATCH] Allow per-thread re-direction of outs()/errs()

The attached patch add the ability to programmatically re-direct outs()/errs()
to an arbitrary raw_ostream instance, maintaining the raw_ostream instances in a
stack structure so clients can push/pop streams at will.  The stack is also
maintained in thread-local storage, so different threads can re-direct
individually.  This allows for two use cases:

1. Compilers can attach custom streams to outs()/errs() to intercept output from
LLVM without needing to play with STDOUT/STDERR.
2. Compilers can receive LLVM output from different threads individually,
instead of having all diagnostics dumped to a single stream.

Unit tests are included in the patch.


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On May 31, 2012, at 2:39 PM, Justin Holewinski wrote:
> The attached patch add the ability to programmatically re-direct
outs()/errs() to an arbitrary raw_ostream instance, maintaining the raw_ostream
instances in a stack structure so clients can push/pop streams at will.  The
stack is also maintained in thread-local storage, so different threads can
re-direct individually.  This allows for two use cases:
> 
> 1. Compilers can attach custom streams to outs()/errs() to intercept output
from LLVM without needing to play with STDOUT/STDERR.
> 2. Compilers can receive LLVM output from different threads individually,
instead of having all diagnostics dumped to a single stream.
This isn't the right approach.  Nothing in the library part of the compiler
should be hard coding a stream to write to.  What are you trying to accomplish?

-Chris

On Thu, May 31, 2012 at 8:06 PM, Chris Lattner <clattner at apple.com>
wrote:
>
> On May 31, 2012, at 2:39 PM, Justin Holewinski wrote:
>
> > The attached patch add the ability to programmatically re-direct
> outs()/errs() to an arbitrary raw_ostream instance, maintaining the
> raw_ostream instances in a stack structure so clients can push/pop streams
> at will.  The stack is also maintained in thread-local storage, so
> different threads can re-direct individually.  This allows for two use
> cases:
> >
> > 1. Compilers can attach custom streams to outs()/errs() to intercept
> output from LLVM without needing to play with STDOUT/STDERR.
> > 2. Compilers can receive LLVM output from different threads
> individually, instead of having all diagnostics dumped to a single stream.
>
> This isn't the right approach.  Nothing in the library part of the
> compiler should be hard coding a stream to write to.  What are you trying
> to accomplish?
>
There are a lot of places where warning/debug information is passed
directly to errs().  For example, take the Linker class.  You can tell it
to omit errors/warnings, but it would be preferable to let it emit the
errors/warnings to some compiler-controlled stream for message triaging.

For compilers that are (a) multi-threaded, or (b) invoked as part of an
embedded library, writing any information from the core libraries directly
to stderr is bad.  I want to be able to capture this output without messing
with the process' stdout/stderr streams.

This patch essentially prevents LLVM library code from having to hard-code
streams for error/warning/diagnostic output.  Library code can just use
outs()/errs() as they do now, but the compiler process/thread now has the
ability to re-direct this output as it sees fit. If no such functionality
is needed, the default behavior is to write to stdout/stderr as it does now.

>
> -Chris
>
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>


-- 

Thanks,

Justin Holewinski
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