[PATCH] Add support for generating MIPS legacy NaN

[Petar Jovanovic]

================
Comment at: lib/Basic/Targets.cpp:5679
@@ +5678,3 @@
+  bool isNan2008() const override {
+    return IsNan2008 || isNaN2008Default();
+  }
----------------
dsanders wrote:
> petarj wrote:
> > vradosavljevic wrote:
> > > dsanders wrote:
> > > > This isn't quite right. It should just be IsNan2008 since IsNan2008 is initialized with the result of isNaN2008Default().
> > > When you generating code for mips32r6/mips64r6 with option -mnan=legacy, IsNan2008 will be set to false.
> > > In function handleTargetFeatures there are lines that change that.
> > >       else if (*it == "-nan2008")
> > >         IsNan2008 = false;
> > > Because of that, I use the result of isNaN2008Default().
> > We should not allow that case. Clang should (at least) issue a warning if "-mnan=legacy" is passed for the revisions that do not support the NaN legacy encoding.
> Agreed. I'd prefer an error* since the user is requesting an impossible combination of options but it should at least warn instead of quietly ignoring the user.
> 
> *Ideally with a suggestion for fixing it, e.g. "error: IEEE754(2008) compliant NaN's are mandatory on Mips32r6. Did you mean -mnan=2008?"
I would also prefer an error, but gcc seems to produce a warning only.
Furthermore, we should do similar for "-mnan=2008" and earlier mips revisions. But this all can be part of another patch, not this one necessary.

http://reviews.llvm.org/D7882

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