Use llvm_unreachable instead of errs+abort in MCStreamer.cpp (in EmitRawTextImpl). NFC.

[kristina]

Summary

NFC change to handle it more consistently in line with other errors, though I'd prefer a quick review since I'm not sure if there's a reason it's was written like that.

(Tidying up parts of MCStreamer code before a bigger differential)

[rnk]

I don't think this should be llvm_unreachable, I think it should be report_fatal_error. LLVM has some some support for out of tree backends, and they could plausibly call this when using a release build of LLVM as a library. If all the callers were in-tree, then calling this would represent an LLVM-internal logic error (forgetting to check hasRawTextSupport()), and unreachable or assert would be appropriate.

[kristina]

Revised to be a fatal error after discussion and added comment for any future maintainers. Also made empty braced blocks consistent.

[dblaikie]

In D56245#1345316, @rnk wrote:

I don't think this should be llvm_unreachable, I think it should be report_fatal_error. LLVM has some some support for out of tree backends, and they could plausibly call this when using a release build of LLVM as a library. If all the callers were in-tree, then calling this would represent an LLVM-internal logic error (forgetting to check hasRawTextSupport()), and unreachable or assert would be appropriate.

Not a strong disagreement, but I'm not sure I follow this entirely - even for external users, there are preconditions for certain API calls that must be met & can still be considered assertion-level failures (ie: the calling code is buggy if it makes this call, not "the calling code can expect a specific failure behavior & rely on this if it wants to") & sounds like this could be one of those.

llvm/trunk/lib/MC/MCStreamer.cpp

void MCStreamer::EmitWinCFIEndProlog(SMLoc Loc) {
   if (!CurFrame)
     return;
 
   MCSymbol *Label = EmitCFILabel();
 
   CurFrame->PrologEnd = Label;
 }
 
-void MCStreamer::EmitCOFFSafeSEH(MCSymbol const *Symbol) {
-}
+void MCStreamer::EmitCOFFSafeSEH(MCSymbol const *Symbol) {}
 
 void MCStreamer::EmitCOFFSymbolIndex(MCSymbol const *Symbol) {}
 
-void MCStreamer::EmitCOFFSectionIndex(MCSymbol const *Symbol) {
-}
+void MCStreamer::EmitCOFFSectionIndex(MCSymbol const *Symbol) {}
 
 void MCStreamer::EmitCOFFSecRel32(MCSymbol const *Symbol, uint64_t Offset) {}
 
 void MCStreamer::EmitCOFFImgRel32(MCSymbol const *Symbol, int64_t Offset) {}
 
 /// EmitRawText - If this file is backed by an assembly streamer, this dumps
 /// the specified string in the output .s file.  This capability is
 /// indicated by the hasRawTextSupport() predicate.
 void MCStreamer::EmitRawTextImpl(StringRef String) {
-  errs() << "EmitRawText called on an MCStreamer that doesn't support it, "
-  " something must not be fully mc'ized\n";
-  abort();
+  // This is not llvm_unreachable for the sake of out of tree backend
+  // developers who may not have assembly streamers and should serve as a
+  // reminder to not accidentally call EmitRawText in the absence of such.
+  report_fatal_error("EmitRawText called on an MCStreamer that doesn't support "
+                     "it (target backend is likely missing an AsmStreamer "
+                     "implementation)");
 }
 
 void MCStreamer::EmitRawText(const Twine &T) {
   SmallString<128> Str;
   EmitRawTextImpl(T.toStringRef(Str));
 }
 
 void MCStreamer::EmitWindowsUnwindTables() {