[DomTree] Add API forceVerifyDomTree

[anemet]

Summary

This API bypasses the -verify-dom-info flag to always check DT
consistency. This is useful to stress-test a single pass without
slowing down other passes.

Looks like the LoopVectorizer was trying to do something similar when it
calls verifyDomTree directly. That's a no-op unless -verify-dom-info is
used. But if -verify-dom-info is passed the verification would be run
anyway. We should probably also fix this in LV.

[reames]

LGTM. I would mildly prefer a boolean parameter to verifyDomTree with a default value, but I'm okay with either version.

You might also consider making the force version declaration under a #ifndef NDEBUG to ensure it doesn't accidentally get left on in a release build.

[anemet]

In D8818#153879, @reames wrote:

LGTM. I would mildly prefer a boolean parameter to verifyDomTree with a default value, but I'm okay with either version.

Thanks. I'll make that change -- fewer functions to remember.

[anemet]

Philip,

Rather than introducing another API flag besides the -verify-dom-info
command-line flag, I've decided to clean this up.

The new commit log should explain the situation:

[DomTree] verifyDomTree to unconditionally perform DT verification

I folded the check for the flag -verify-dom-info into the only caller
where I think it is supposed to be checked: verifyAnalysis.  (The idea
of the flag is to enable this expensive verification in
verifyPreservedAnalysis.)

I'm assuming that when manually scheduling the verification pass with
-passes=verify<domtree>, we do want to perform the verification, so I
don't check for the flag there.

Please let me know if you're OK with this.

Thanks,
Adam

[reames]

LGTM.

[anemet]

It's r236575, thanks Philip.

include/llvm/IR/Dominators.h

public:
   /// \brief Provide an overload for a Use.
   bool isReachableFromEntry(const Use &U) const;
 
   /// \brief Verify the correctness of the domtree by re-computing it.
   ///
   /// This should only be used for debugging as it aborts the program if the
   /// verification fails.
   void verifyDomTree() const;
+
+  /// \brief Same as verifyDomTree but it performs the verification even without
+  /// -verify-dom-info.
+  ///
+  /// Useful to call from a pass to stress-test DT updating.
+  void forceVerifyDomTree() const;
 };
 
 //===-------------------------------------
 // DominatorTree GraphTraits specializations so the DominatorTree can be
 // iterable by generic graph iterators.
 
 template <> struct GraphTraits<DomTreeNode*> {
   typedef DomTreeNode NodeType;

lib/IR/Dominators.cpp

bool DominatorTree::isReachableFromEntry(const Use &U) const {
 
   // Everything else uses their operands in their own block.
   return isReachableFromEntry(I->getParent());
 }
 
 void DominatorTree::verifyDomTree() const {
   if (!VerifyDomInfo)
     return;
+  forceVerifyDomTree();
+}
 
+void DominatorTree::forceVerifyDomTree() const {
   Function &F = *getRoot()->getParent();
 
   DominatorTree OtherDT;
   OtherDT.recalculate(F);
   if (compare(OtherDT)) {
     errs() << "DominatorTree is not up to date!\nComputed:\n";
     print(errs());
     errs() << "\nActual:\n";