[BPI] Consider deoptimize calls as "unreachable"

[sanjoy]

Summary

Calls to @llvm.experimental.deoptimize are expected to "never execute",
so optimize them as such.

[chandlerc]

LGTM

llvm/trunk/lib/Analysis/BranchProbabilityInfo.cpp

 
 /// \brief Calculate edge weights for successors lead to unreachable.
 ///
 /// Predict that a successor which leads necessarily to an
 /// unreachable-terminated block as extremely unlikely.
 bool BranchProbabilityInfo::calcUnreachableHeuristics(const BasicBlock *BB) {
   const TerminatorInst *TI = BB->getTerminator();
   if (TI->getNumSuccessors() == 0) {
-    if (isa<UnreachableInst>(TI))
+    if (isa<UnreachableInst>(TI) ||
+        // If this block is terminated by a call to
+        // @llvm.experimental.deoptimize then treat it like an unreachable since
+        // the @llvm.experimental.deoptimize call is expected to practically
+        // never execute.
+        BB->getTerminatingDeoptimizeCall())
       PostDominatedByUnreachable.insert(BB);
     return false;
   }
 
   SmallVector<unsigned, 4> UnreachableEdges;
   SmallVector<unsigned, 4> ReachableEdges;
 
   for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {

llvm/trunk/test/Analysis/BranchProbabilityInfo/deopt-intrinsic.ll

+; RUN: opt -analyze -branch-prob < %s | FileCheck %s
+
+declare i32 @llvm.experimental.deoptimize.i32(...)
+
+define i32 @test1(i32 %a, i32 %b) {
+; CHECK-LABEL: Printing analysis 'Branch Probability Analysis' for function 'test1':
+entry:
+  %cond = icmp eq i32 %a, 42
+  br i1 %cond, label %exit, label %deopt
+
+; CHECK:  edge entry -> exit probability is 0x7ffff800 / 0x80000000 = 100.00% [HOT edge]
+; CHECK:  edge entry -> deopt probability is 0x00000800 / 0x80000000 = 0.00%
+
+deopt:
+  %rval = call i32(...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+  ret i32 %rval
+
+exit:
+  ret i32 %b
+}