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

[Justin Holewinski]

On Mon, Jun 4, 2012 at 11:46 AM, Eli Friedman <eli.friedman at gmail.com>wrote:

> On Sun, Jun 3, 2012 at 7:12 PM, Justin Holewinski
> <justin.holewinski at gmail.com> wrote:
> > On Sun, Jun 3, 2012 at 4:15 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> >>
> >> On Sun, 03 Jun 2012 12:12:06 -0700
> >> Chris Lattner <clattner at apple.com> wrote:
> >>
> >> >
> >> > On Jun 2, 2012, at 11:01 AM, Mikael Lyngvig wrote:
> >> >
> >> > > If I may add my two cents:
> >> > >
> >> > > 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.
> >> >
> >> > LLVM should not be doing this.
> >> >
> >> > > 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:
> >> > >
> >> > > class ErrorHandler
> >> > > {
> >> > > public:
> >> > >         virtual void Report(ErrorKind kind, uint code, const
> >> > > Position &position, const unichar *text, ...) = 0; };
> >> > >
> >> > > 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.
> >> >
> >> > You're right, this would be better.  We have even already have it :)
> >> >
> >> >   llvm/Support/ErrorHandling.h
> >>
> >> This seems to only handle fatal errors. If that's correct, it will
> >> probably need to be extended to handle non-fatal errors, warnings,
> >> suggestions, notes, etc.
> >
> >
> > Okay, it looks like the combination of llvm/Support/ErrorHandling.h and
> > LLVMContext::emitError solves a part of my use case.  I found the
> > CrashRecoveryContext class, which seems to allow me to intercept the
> > report_fatal_error() calls by installing a custom handler that writes to
> a
> > custom stream and then calls abort() instead of the default exit().
> >  Alternatively, it looks like I can just spawn a thread for the LLVM work
> > and call pthread_exit() instead of abort() in the error handler, which
> does
> > not require messing with the process signal handlers.  Is there any
> reason
> > to prefer one over the other (thread vs. signal)?
> >
> > This leaves the following issues:
> >
> > Why is the error handling stuff in LLVMContext tied to inline asm?  The
> > error handler setter is even called setInlineAsmDiagnosticHandler.  Can
> we
> > make this more generic?  I glossed right over the emitError methods the
> last
> > time I read through this stuff because I saw "InlineAsm" everywhere and
> > figured it wasn't the right tool for the job! :)
>
> Inline asm is the only place where we have a way of mapping form LLVM
> IR to frontend source locations at the moment.
>

But is that strictly required?  Lacking the right annotations, we could
print IR information like function name, basic block name, global variable
name, etc.


>
> > As Hal points out, this only covers errors at the moment.  Is there any
> > resistance to changing emitError to emitDiagnostic and having a
> > SourceMgr::DiagKind argument?  This should cover the case where a pass
> wants
> > to emit a warning or info message that is not part to DEBUG.
> >
> > With LLVMContext::emitDiagnostic, this should cover my use case.
>
> I'm not exactly opposed, but what is your use case?
>

Generic warnings and notes, basically any case where compilation can
continue but the results may be sub-optimal.  For example, this could be
used by the vectorizer to provide a report on which loops were vectorized,
and why the others were not.  Backends could use this to report when a
certain feature is unavailable for the target and software emulation is
used.  Think of Intel/PGI C/C++/Fortran and their performance notes.
 Statistics output can give you some of this information, but not all.  Nor
can it put it into context if you happen to have debug information for
source level mapping.


>
> -Eli
>



-- 

Thanks,

Justin Holewinski
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