[NFC] Add detector function for explicitly expressed guards

[mkazantsev]

Summary

This patch adds a function to detect guards expressed in explicit form:

  %wc = call i1 @llvm.experimental.widenable.condition()
  %guard_cond = and i1, %some_cond, %wc
  br i1 %guard_cond, label %guarded, label %deopt

deopt:
  <maybe some non-side-effecting instructions>
  deoptimize()

This form can be used as alternative to implicit control flow guard
representation expressed by experimental_guard intrinsic.

[apilipenko]

lib/Analysis/GuardUtils.cpp

30

Why do we need this restriction?

[reames]

LGTM w/one required change.

include/llvm/Analysis/GuardUtils.h

23

This comment applies to your whole set of reviews.

I would suggest avoiding the term "explicit". Despite glancing at your patch titles a few times over the last week, I still get confused each time. Is explicit the guard? Or the widenable condition? Possible alternate wording:
isWidenableBranchToDeopt?
isGuardAsWidenableBranch?

[mkazantsev]

lib/Analysis/GuardUtils.cpp

30

The simplest example is that there is a throwing call before the deopt, we may never actually go to deopt, and this is not what we want here.

include/llvm/Analysis/GuardUtils.h

 
 namespace llvm {
 
 class User;
 
 /// Returns true iff \p U has semantics of a guard.
 bool isGuard(const User *U);
 
+/// Returns true iff \p U is a branch that has semantics of a guard.
+bool isExplicitGuard(const User *U);
+
 } // llvm
 
 #endif // LLVM_ANALYSIS_GUARDUTILS_H
 

lib/Analysis/GuardUtils.cpp

 #include "llvm/IR/PatternMatch.h"
 
 using namespace llvm;
 
 bool llvm::isGuard(const User *U) {
   using namespace llvm::PatternMatch;
   return match(U, m_Intrinsic<Intrinsic::experimental_guard>());
 }
+
+bool llvm::isExplicitGuard(const User *U) {
+  using namespace llvm::PatternMatch;
+  const BranchInst *BI = dyn_cast<BranchInst>(U);
+
+  // We are looking for the following pattern:
+  //   br i1 %cond & widenable_condition(), label %guarded, label %deopt
+  // deopt:
+  //   <non-side-effecting instructions>
+  //   deoptimize()
+  if (!BI || !BI->isConditional())
+    return false;
+
+  if (!match(BI->getCondition(),
+             m_And(m_Value(),
+                   m_Intrinsic<Intrinsic::experimental_widenable_condition>())))
+    return false;
+
+  const BasicBlock *DeoptBlock = BI->getSuccessor(1);
+  for (auto &Insn : *DeoptBlock) {
+    if (match(&Insn, m_Intrinsic<Intrinsic::experimental_deoptimize>()))
+      return true;
+    if (Insn.mayHaveSideEffects())
+      return false;
+  }
+  return false;
+}