Don't extract code containing an indirect jump from a callbr

[void]

Summary

The callbr instruction's indirect destinations are expected to be within
the current function. Extracting them will cause chaos as the callbr
will then want to branch to a label in another function, which isn't
allowed.

[vsk]

Could you add a test?

llvm/lib/Transforms/IPO/HotColdSplitting.cpp

135

nit: const BasicBlock &PredBB : predecessors(BB)?

[void]

Use range-based for loop.

[void]

In D70929#1766125, @vsk wrote:

Could you add a test?

Strange. There was a test, but somehow it wasn't included. I'll re-add it.

llvm/lib/Transforms/IPO/HotColdSplitting.cpp

if (blockEndsInUnreachable(BB)) {
     return true;
   }
 
   return false;
 }
 
 /// Check whether it's safe to outline \p BB.
 static bool mayExtractBlock(const BasicBlock &BB) {
+  // Don't extract the indirect destination of a "callbr" instruction as even
+  // though it's ice cold it's not safe to extract it, for reasons similar to
+  // extracting an EH block.
+  for (const auto *pred : predecessors(&BB))
+    if (isa<CallBrInst>(pred->getTerminator()))
+      return false;
+
   // EH pads are unsafe to outline because doing so breaks EH type tables. It
   // follows that invoke instructions cannot be extracted, because CodeExtractor
   // requires unwind destinations to be within the extraction region.
   //
   // Resumes that are not reachable from a cleanup landing pad are considered to
   // be unreachable. It’s not safe to split them out either.
   auto Term = BB.getTerminator();
   return !BB.hasAddressTaken() && !BB.isEHPad() && !isa<InvokeInst>(Term) &&
-         !isa<ResumeInst>(Term);
+         !isa<ResumeInst>(Term) && !isa<CallBrInst>(Term);
 }
 
 /// Mark \p F cold. Based on this assumption, also optimize it for minimum size.
 /// If \p UpdateEntryCount is true (set when this is a new split function and
 /// module has profile data), set entry count to 0 to ensure treated as cold.
 /// Return true if the function is changed.
 static bool markFunctionCold(Function &F, bool UpdateEntryCount = false) {
   assert(!F.hasOptNone() && "Can't mark this cold");