Fix Crashes when 'AttributeList::get'ing with an ArrayRef<AttributeList> where all pImpl are null

[thewilsonator]

Summary

As the Title says, this causes crashes if you go

AttributeList::get(getContext(), { AttributeList(),AttributeList() });

It is sensitive to I/O, I have no idea why it is I/O sensitive but it is, at least it disappears when logging is turned on in LDC (the LLVM D Compiler) without fail.
Sometimes the last function in the stacktrace is

_vsnprintf + 586
llvm::AttributeList::get(llvm::LLVMContext&, llvm::ArrayRef<llvm::AttributeList>) + 798

Anyway this appears to fix this and will speed up merges of null AttributeList.

Nicholas

[javed.absar]

Hi Nicholas:

This looks like a manifestation of a problem that is possibly elsewhere (quick look makes me think in the getImpl when it is supplied an empty AttrSet). But I will have a better look later and get back. By the way, the context is missing (probably your diff to create the patch didnt have -U9999).
--Javed

[thewilsonator]

Thanks.

I just pasted the diff from my terminal and it accepted it with whatever the default of git diff is, I don't know what U9999 is. *shrug*
This is my first contribution to LLVM so I don't know the way things are supposed to work.

The AttributeList::get that takes an ArrayRef<AttributeList> checks for Attrs.empty() and for Attrs.size() == 1 as fast paths.
the getImpl asserts that the ArrayRef<AttributeSet> is not empty but I must have had asserts disabled.

Nicholas

[javed.absar]

No problem. Please have a look at http://llvm.org/docs/Phabricator.html (sec. Requesting a review...). Basically, 'git diff -U9999 your-branch' will create a patch with more context.

[rnk]

Thanks for the diagnosis and patch! I think I didn't find this problem when testing LLVM and clang because they test each AttributeList for emptiness before adding it to the list of lists to merge. I'll land this soon.

[thewilsonator]

Thanks!

lib/IR/Attributes.cpp

   for (AttributeList List : Attrs)
     MaxSize = std::max(MaxSize, List.getNumAttrSets());
 
+  if (MaxSize == 0)
+    return AttributeList();
+
   SmallVector<AttributeSet, 8> NewAttrSets(MaxSize);
   for (unsigned I = 0; I < MaxSize; ++I) {
     AttrBuilder CurBuilder;