If I may add my two cents: <div><br></div><div>I am planning to use LLVM as the backend for a compiler I am working on. And I wholeheartedly agree with Justin that it is a problem, if LLVM is allowed to freely write to stdout and stderr as it is a component which can be used in all sorts of code, be it a GUI IDE, a CLI driver, or whatever.<div>
<br></div><div>Also, I have a number of times wondered about the somewhat unusual use of error strings in LLVM (that you pass in a string, which can be assigned a descriptive error message). Better would be, IMHO, to provide an interface along the lines of this:</div>
<div><br></div><div><div>class ErrorHandler</div><div>{</div><div>public:</div><div> virtual void Report(ErrorKind kind, uint code, const Position &position, const unichar *text, ...) = 0;</div><div>};</div><div>
<br></div><div>And then let the client, i.e. the frontend, derive from it and handle all the output in whichever way it wants to. The above example is probably too complex for LLVM, but that's what I use in my compiler. ErrorKind is ErrorKind_Fatal, ErrorKind_Error, etc. Position is simply a filename, a line number, and a character position. unichar is either char or wchar_t, depending on the build mode and target environment.</div>
<div><br></div><div>I hook it up so that I can buffer all errors encountered for later user.</div><div><br></div><div><br></div><div>Cheers,</div><div>Mikael</div>-- Love Thy Frog! </div><div><br><div class="gmail_quote">
2012/6/2 Justin Holewinski <span dir="ltr"><<a href="mailto:justin.holewinski@gmail.com" target="_blank">justin.holewinski@gmail.com</a>></span><br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div class="gmail_quote"><div class="im">On Sat, Jun 2, 2012 at 8:56 AM, Chris Lattner <span dir="ltr"><<a href="mailto:clattner@apple.com" target="_blank">clattner@apple.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div style="word-wrap:break-word"><div><div><div>On Jun 1, 2012, at 11:13 PM, Justin Holewinski wrote:</div><blockquote type="cite"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
<div style="word-wrap:break-word"><div><div><blockquote type="cite"><span style="border-collapse:separate;font-family:Helvetica;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:-webkit-auto;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;font-size:medium"><div>
In a way, they are. What if I want a debug trace in a multi-threaded context? Right now, all threads would just spew to the same stream and the result would be unreadable. If you allow threads to setup their own outs() and errs() streams (transparently to the rest of LLVM), you can intercept these as you see fit, perhaps dumping each to a separate file. I acknowledge that you could also enforce single-threaded execution for debug runs, but what if you are in the context of a library with no valid stdout/stderr buffers?</div>
<div><br></div></span></blockquote><br></div></div><div>This doesn't make any sense. There is very little sense in trying to use DEBUG (which gets compiled out of optimized builds) in a multithreaded context. Polluting the purity of outs() and errs() because of this sort of use case doesn't make any sense to me.</div>
<div><br></div><div>outs() and errs() are useful independent of how LLVM happens to use them. The extensions that you are proposing doesn't make any sense give nthe design of raw_ostream.</div></div></blockquote><div>
<br></div><div>Is the problem more that outs() and errs() should always point to valid stdout/stderr streams, regardless of how the hosting compiler is implemented?</div></div></blockquote><div><br></div></div><div>No. outs() and errs() are equivalent to stdout and stderr. Your request to change how they work to be thread local is like saying that we should change how printf works because LLVM is misusing printf somewhere.</div>
</div></div></blockquote><div><br></div></div><div>Fair enough. I've always been under the impression that outs() and errs() were simply convenience wrappers around *some* output mechanism, not strictly just stdout/stderr.</div>
<div class="im">
<div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><div><br><blockquote type="cite"><div class="gmail_quote">
<div>If so, how do you feel about introducing a new global stream, logs()? This could replace all uses of outs()/errs() in the LLVM libraries, and default to errs(). Existing command-line tools could just use this default, but compilers that are hosted inside of libraries or other non-command line scenarios can attach a custom stream to logs() that captures all LLVM output, including DEBUG and any legitimate pass output. Command-line tools can continue to use outs()/errs() just as they do now. Nothing would really change in the way LLVM operates by default</div>
</div></blockquote><br></div></div><div>In fact, before we had raw_ostream, we had a dbgs() sort of stream, which was intended to be used in debug statements. I'm not strongly opposed to this (it would be much better than violating the sanctity of outs() :), but I'm still unclear why you care so much about DEBUG output. If we had a dbgs() again, it should *only* be used from within DEBUG macros (for example, the linker shouldn't use it). Would that be enough to achieve your goal?</div>
</div></blockquote><div><br></div></div><div>It's not so much DEBUG output that I care specifically about, that's just an example of a place where the LLVM libraries assume an stderr stream exists and is freely writable. What I really want is a way to capture *all* LLVM library output that would normally go to stderr, similar to how Clang has its Diagnostic functionality that allows a driver to collect all output messages.</div>
<div><br></div><div>There are legitimate cases where passes may output warnings or even errors that the compiler should be able to direct to the user. errs() is fine for this if you know you're working with a command-line tool, but that's not always true.</div>
<div><br></div><div>It sounds like dbgs() is not really the solution here, since I want to capture more than just DEBUG output. If the module linker wants to output a warning, I want to capture that. If an optimization pass wants to emit a performance warning, I want to capture that too. Neither of these cases are DEBUG-only.</div>
<div><br></div><div>I see two possible solutions:</div><div><ol><li>For every pass (or class) that may emit warnings/errors, provide it a raw_ostream instance. This seems to be how some of the utility classes work now, but this puts a burden on the compiler to know which passes actually need a raw_ostream parameter. If you want to add a warning output to any pass, you need to change its interface (create***Pass and constructor) to add the raw_ostream parameter.</li>
<li>Provide a generic mechanism through which passes and any other utility classes can write warning/error information. This gives more control to the compiler without introducing any interface changes.</li></ol><div>Option (1) is do-able, but option (2) seems more general. The specifics of the implementation that I envision are:</div>
</div><div><ul><li>logs() is added as another stream to raw_ostream.h</li><li>By default, logs() forwards to errs() [no change in the way LLVM works in the default case]</li><li>Any pass or utility class that wants to emit warnings/errors should write to logs() instead of errs()</li>
<li>DEBUG code should use logs() instead of errs() [again, this will default to errs()]</li><li>An API is provided that allows a client to re-direct logs() to any arbitrary raw_ostream, on a per-thread level</li></ul><div>
You could also envision something along the lines of Clang's Diagnostics functionality. You could thread a diagnostics interface through the pass manager, and passes could output any messages as diagnostics. These diagnostics would be collected and the compiler could query for them after the pass manager completes.</div>
</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><span><font color="#888888"><div><br></div><div>-Chris</div><span class="HOEnZb"><font color="#888888">
</font></span></font></span></div></blockquote></div><span class="HOEnZb"><font color="#888888"><br><br clear="all"><div><br></div>-- <br><br><div>Thanks,</div><div><br></div><div>Justin Holewinski</div><br>
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