Index: llvm/lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -24,6 +24,7 @@
 #include "llvm/Analysis/AssumptionCache.h"
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/EHPersonalities.h"
+#include "llvm/Analysis/GuardUtils.h"
 #include "llvm/Analysis/InstructionSimplify.h"
 #include "llvm/Analysis/MemorySSA.h"
 #include "llvm/Analysis/MemorySSAUpdater.h"
@@ -3212,6 +3213,46 @@
   return Changed;
 }
 
+
+/// If the previous block ended with a widenable branch, determine if reusing
+/// the target block is profitable and legal.  This will have the effect of
+/// "widening" PBI, but doesn't require us to reason about hosting safety.
+static bool tryWidenCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI) {
+  // TODO: This can be generalized in two important ways:
+  // 1) We can allow phi nodes in IfFalseBB and simply reuse all the input
+  //    values from the PBI edge.
+  // 2) We can sink side effecting instructions into BI's fallthrough
+  //    successor provided they doesn't contribute to computation of
+  //    BI's condition.
+  Value *CondWB, *WC;
+  BasicBlock *IfTrueBB, *IfFalseBB;
+  if (!parseWidenableBranch(PBI, CondWB, WC, IfTrueBB, IfFalseBB) ||
+      IfTrueBB != BI->getParent() || !BI->getParent()->getSinglePredecessor())
+    return false;
+  if (!IfFalseBB->phis().empty())
+    return false; // TODO
+  auto NoSideEffects = [](BasicBlock &BB) {
+    return !llvm::any_of(BB, [](const Instruction &I) {
+        return I.mayWriteToMemory() || I.mayHaveSideEffects();
+      });
+  };
+  if (BI->getSuccessor(1) != IfFalseBB && // no inf looping
+      BI->getSuccessor(1)->getTerminatingDeoptimizeCall() && // profitability
+      NoSideEffects(*BI->getParent())) {
+    BI->getSuccessor(1)->removePredecessor(BI->getParent());
+    BI->setSuccessor(1, IfFalseBB);
+    return true;
+  }
+  if (BI->getSuccessor(0) != IfFalseBB && // no inf looping
+      BI->getSuccessor(0)->getTerminatingDeoptimizeCall() && // profitability
+      NoSideEffects(*BI->getParent())) {
+    BI->getSuccessor(0)->removePredecessor(BI->getParent());
+    BI->setSuccessor(0, IfFalseBB);
+    return true;
+  }
+  return false;
+}
+
 /// If we have a conditional branch as a predecessor of another block,
 /// this function tries to simplify it.  We know
 /// that PBI and BI are both conditional branches, and BI is in one of the
@@ -3267,6 +3308,12 @@
     }
   }
 
+  // If the previous block ended with a widenable branch, determine if reusing
+  // the target block is profitable and legal.  This will have the effect of
+  // "widening" PBI, but doesn't require us to reason about hosting safety.
+  if (tryWidenCondBranchToCondBranch(PBI, BI))
+    return true;
+
   if (auto *CE = dyn_cast<ConstantExpr>(BI->getCondition()))
     if (CE->canTrap())
       return false;
Index: llvm/test/Transforms/SimplifyCFG/wc-widen-block.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/SimplifyCFG/wc-widen-block.ll
@@ -0,0 +1,316 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -passes=simplify-cfg -S < %s | FileCheck %s
+
+define i32 @basic(i1 %cond_0, i32* %p) {
+; CHECK-LABEL: @basic(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[WIDENABLE_COND:%.*]] = call i1 @llvm.experimental.widenable.condition()
+; CHECK-NEXT:    [[EXIPLICIT_GUARD_COND:%.*]] = and i1 [[COND_0:%.*]], [[WIDENABLE_COND]]
+; CHECK-NEXT:    br i1 [[EXIPLICIT_GUARD_COND]], label [[GUARDED:%.*]], label [[DEOPT:%.*]], !prof !0
+; CHECK:       deopt:
+; CHECK-NEXT:    [[DEOPTRET:%.*]] = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+; CHECK-NEXT:    ret i32 [[DEOPTRET]]
+; CHECK:       guarded:
+; CHECK-NEXT:    [[V:%.*]] = load i32, i32* [[P:%.*]]
+; CHECK-NEXT:    [[COND_1:%.*]] = icmp eq i32 [[V]], 0
+; CHECK-NEXT:    br i1 [[COND_1]], label [[RETURN:%.*]], label [[DEOPT]], !prof !0
+; CHECK:       return:
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  %widenable_cond = call i1 @llvm.experimental.widenable.condition()
+  %exiplicit_guard_cond = and i1 %cond_0, %widenable_cond
+  br i1 %exiplicit_guard_cond, label %guarded, label %deopt, !prof !0
+
+deopt:
+  %deoptret = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+  ret i32 %deoptret
+
+guarded:
+  %v = load i32, i32* %p
+  %cond_1 = icmp eq i32 %v, 0
+  br i1 %cond_1, label %return, label %deopt2, !prof !0
+
+deopt2:
+  %deoptret2 = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+  ret i32 %deoptret2
+
+return:
+  ret i32 0
+}
+
+define i32 @mergeable(i1 %cond_0, i1 %cond_1) {
+; CHECK-LABEL: @mergeable(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[WIDENABLE_COND:%.*]] = call i1 @llvm.experimental.widenable.condition()
+; CHECK-NEXT:    [[EXIPLICIT_GUARD_COND:%.*]] = and i1 [[COND_0:%.*]], [[WIDENABLE_COND]]
+; CHECK-NEXT:    [[EXIPLICIT_GUARD_COND_NOT:%.*]] = xor i1 [[EXIPLICIT_GUARD_COND]], true
+; CHECK-NEXT:    [[COND_1_NOT:%.*]] = xor i1 [[COND_1:%.*]], true
+; CHECK-NEXT:    [[BRMERGE:%.*]] = or i1 [[EXIPLICIT_GUARD_COND_NOT]], [[COND_1_NOT]]
+; CHECK-NEXT:    br i1 [[BRMERGE]], label [[DEOPT:%.*]], label [[RETURN:%.*]], !prof !1
+; CHECK:       deopt:
+; CHECK-NEXT:    [[DEOPTRET:%.*]] = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+; CHECK-NEXT:    ret i32 [[DEOPTRET]]
+; CHECK:       return:
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  %widenable_cond = call i1 @llvm.experimental.widenable.condition()
+  %exiplicit_guard_cond = and i1 %cond_0, %widenable_cond
+  br i1 %exiplicit_guard_cond, label %guarded, label %deopt, !prof !0
+
+deopt:
+  %deoptret = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+  ret i32 %deoptret
+
+guarded:
+  br i1 %cond_1, label %return, label %deopt2, !prof !0
+
+deopt2:
+  %deoptret2 = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+  ret i32 %deoptret2
+
+return:
+  ret i32 0
+}
+
+define i32 @basic_swapped_branch(i1 %cond_0, i32* %p) {
+; CHECK-LABEL: @basic_swapped_branch(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[WIDENABLE_COND:%.*]] = call i1 @llvm.experimental.widenable.condition()
+; CHECK-NEXT:    [[EXIPLICIT_GUARD_COND:%.*]] = and i1 [[COND_0:%.*]], [[WIDENABLE_COND]]
+; CHECK-NEXT:    br i1 [[EXIPLICIT_GUARD_COND]], label [[GUARDED:%.*]], label [[DEOPT:%.*]], !prof !0
+; CHECK:       deopt:
+; CHECK-NEXT:    [[DEOPTRET:%.*]] = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+; CHECK-NEXT:    ret i32 [[DEOPTRET]]
+; CHECK:       guarded:
+; CHECK-NEXT:    [[V:%.*]] = load i32, i32* [[P:%.*]]
+; CHECK-NEXT:    [[COND_1:%.*]] = icmp eq i32 [[V]], 0
+; CHECK-NEXT:    br i1 [[COND_1]], label [[DEOPT]], label [[RETURN:%.*]], !prof !0
+; CHECK:       return:
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  %widenable_cond = call i1 @llvm.experimental.widenable.condition()
+  %exiplicit_guard_cond = and i1 %cond_0, %widenable_cond
+  br i1 %exiplicit_guard_cond, label %guarded, label %deopt, !prof !0
+
+deopt:
+  %deoptret = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+  ret i32 %deoptret
+
+guarded:
+  %v = load i32, i32* %p
+  %cond_1 = icmp eq i32 %v, 0
+  br i1 %cond_1, label %deopt2, label %return, !prof !0
+
+deopt2:
+  %deoptret2 = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+  ret i32 %deoptret2
+
+return:
+  ret i32 0
+}
+
+define i32 @todo_sink_side_effect(i1 %cond_0, i1 %cond_1) {
+; CHECK-LABEL: @todo_sink_side_effect(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[WIDENABLE_COND:%.*]] = call i1 @llvm.experimental.widenable.condition()
+; CHECK-NEXT:    [[EXIPLICIT_GUARD_COND:%.*]] = and i1 [[COND_0:%.*]], [[WIDENABLE_COND]]
+; CHECK-NEXT:    br i1 [[EXIPLICIT_GUARD_COND]], label [[GUARDED:%.*]], label [[DEOPT:%.*]], !prof !0
+; CHECK:       deopt:
+; CHECK-NEXT:    [[DEOPTRET:%.*]] = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+; CHECK-NEXT:    ret i32 [[DEOPTRET]]
+; CHECK:       guarded:
+; CHECK-NEXT:    call void @unknown()
+; CHECK-NEXT:    br i1 [[COND_1:%.*]], label [[RETURN:%.*]], label [[DEOPT2:%.*]], !prof !0
+; CHECK:       deopt2:
+; CHECK-NEXT:    [[DEOPTRET2:%.*]] = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+; CHECK-NEXT:    ret i32 [[DEOPTRET2]]
+; CHECK:       return:
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  %widenable_cond = call i1 @llvm.experimental.widenable.condition()
+  %exiplicit_guard_cond = and i1 %cond_0, %widenable_cond
+  br i1 %exiplicit_guard_cond, label %guarded, label %deopt, !prof !0
+
+deopt:
+  %deoptret = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+  ret i32 %deoptret
+
+guarded:
+  call void @unknown()
+  br i1 %cond_1, label %return, label %deopt2, !prof !0
+
+deopt2:
+  %deoptret2 = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+  ret i32 %deoptret2
+
+return:
+  ret i32 0
+}
+
+define i32 @neg_unsinkable_side_effect(i1 %cond_0) {
+; CHECK-LABEL: @neg_unsinkable_side_effect(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[WIDENABLE_COND:%.*]] = call i1 @llvm.experimental.widenable.condition()
+; CHECK-NEXT:    [[EXIPLICIT_GUARD_COND:%.*]] = and i1 [[COND_0:%.*]], [[WIDENABLE_COND]]
+; CHECK-NEXT:    br i1 [[EXIPLICIT_GUARD_COND]], label [[GUARDED:%.*]], label [[DEOPT:%.*]], !prof !0
+; CHECK:       deopt:
+; CHECK-NEXT:    [[DEOPTRET:%.*]] = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+; CHECK-NEXT:    ret i32 [[DEOPTRET]]
+; CHECK:       guarded:
+; CHECK-NEXT:    [[V:%.*]] = call i32 @unknown_i32()
+; CHECK-NEXT:    [[COND_1:%.*]] = icmp eq i32 [[V]], 0
+; CHECK-NEXT:    br i1 [[COND_1]], label [[RETURN:%.*]], label [[DEOPT2:%.*]], !prof !0
+; CHECK:       deopt2:
+; CHECK-NEXT:    [[DEOPTRET2:%.*]] = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+; CHECK-NEXT:    ret i32 [[DEOPTRET2]]
+; CHECK:       return:
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  %widenable_cond = call i1 @llvm.experimental.widenable.condition()
+  %exiplicit_guard_cond = and i1 %cond_0, %widenable_cond
+  br i1 %exiplicit_guard_cond, label %guarded, label %deopt, !prof !0
+
+deopt:
+  %deoptret = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+  ret i32 %deoptret
+
+guarded:
+  %v = call i32 @unknown_i32()
+  %cond_1 = icmp eq i32 %v, 0
+  br i1 %cond_1, label %return, label %deopt2, !prof !0
+
+deopt2:
+  %deoptret2 = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+  ret i32 %deoptret2
+
+return:
+  ret i32 0
+}
+
+
+define i32 @neg_inf_loop(i1 %cond_0, i1 %cond_1) {
+; CHECK-LABEL: @neg_inf_loop(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[WIDENABLE_COND:%.*]] = call i1 @llvm.experimental.widenable.condition()
+; CHECK-NEXT:    [[EXIPLICIT_GUARD_COND:%.*]] = and i1 [[COND_0:%.*]], [[WIDENABLE_COND]]
+; CHECK-NEXT:    br i1 [[EXIPLICIT_GUARD_COND]], label [[GUARDED:%.*]], label [[DEOPT:%.*]], !prof !0
+; CHECK:       deopt:
+; CHECK-NEXT:    [[DEOPTRET:%.*]] = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+; CHECK-NEXT:    ret i32 [[DEOPTRET]]
+; CHECK:       guarded:
+; CHECK-NEXT:    call void @unknown()
+; CHECK-NEXT:    br i1 [[COND_1:%.*]], label [[RETURN:%.*]], label [[DEOPT]], !prof !0
+; CHECK:       return:
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  %widenable_cond = call i1 @llvm.experimental.widenable.condition()
+  %exiplicit_guard_cond = and i1 %cond_0, %widenable_cond
+  br i1 %exiplicit_guard_cond, label %guarded, label %deopt, !prof !0
+
+deopt:
+  %deoptret = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+  ret i32 %deoptret
+
+guarded:
+  call void @unknown()
+  br i1 %cond_1, label %return, label %deopt, !prof !0
+
+return:
+  ret i32 0
+}
+
+
+define i32 @todo_phi(i1 %cond_0, i1 %cond_1) {
+; CHECK-LABEL: @todo_phi(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[WIDENABLE_COND:%.*]] = call i1 @llvm.experimental.widenable.condition()
+; CHECK-NEXT:    [[EXIPLICIT_GUARD_COND:%.*]] = and i1 [[COND_0:%.*]], [[WIDENABLE_COND]]
+; CHECK-NEXT:    br i1 [[EXIPLICIT_GUARD_COND]], label [[GUARDED:%.*]], label [[DEOPT:%.*]], !prof !0
+; CHECK:       deopt:
+; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[DEOPTRET:%.*]] = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"(i32 [[PHI]]) ]
+; CHECK-NEXT:    ret i32 [[DEOPTRET]]
+; CHECK:       guarded:
+; CHECK-NEXT:    br i1 [[COND_1:%.*]], label [[RETURN:%.*]], label [[DEOPT2:%.*]], !prof !0
+; CHECK:       deopt2:
+; CHECK-NEXT:    [[DEOPTRET2:%.*]] = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+; CHECK-NEXT:    ret i32 [[DEOPTRET2]]
+; CHECK:       return:
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  %widenable_cond = call i1 @llvm.experimental.widenable.condition()
+  %exiplicit_guard_cond = and i1 %cond_0, %widenable_cond
+  br i1 %exiplicit_guard_cond, label %guarded, label %deopt, !prof !0
+
+deopt:
+  %phi = phi i32 [0, %entry]
+  %deoptret = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"(i32 %phi) ]
+  ret i32 %deoptret
+
+guarded:
+  br i1 %cond_1, label %return, label %deopt2, !prof !0
+
+deopt2:
+  %deoptret2 = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+  ret i32 %deoptret2
+
+return:
+  ret i32 0
+}
+
+
+define i32 @neg_loop(i1 %cond_0, i1 %cond_1) {
+; CHECK-LABEL: @neg_loop(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[GUARDED:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[WIDENABLE_COND:%.*]] = call i1 @llvm.experimental.widenable.condition()
+; CHECK-NEXT:    [[EXIPLICIT_GUARD_COND:%.*]] = and i1 [[COND_0:%.*]], [[WIDENABLE_COND]]
+; CHECK-NEXT:    br i1 [[EXIPLICIT_GUARD_COND]], label [[GUARDED]], label [[DEOPT:%.*]], !prof !0
+; CHECK:       deopt:
+; CHECK-NEXT:    [[DEOPTRET:%.*]] = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+; CHECK-NEXT:    ret i32 [[DEOPTRET]]
+; CHECK:       guarded:
+; CHECK-NEXT:    call void @unknown()
+; CHECK-NEXT:    br i1 [[COND_1:%.*]], label [[LOOP:%.*]], label [[DEOPT2:%.*]], !prof !0
+; CHECK:       deopt2:
+; CHECK-NEXT:    [[DEOPTRET2:%.*]] = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+; CHECK-NEXT:    ret i32 [[DEOPTRET2]]
+;
+entry:
+  br label %guarded
+
+loop:
+  %widenable_cond = call i1 @llvm.experimental.widenable.condition()
+  %exiplicit_guard_cond = and i1 %cond_0, %widenable_cond
+  br i1 %exiplicit_guard_cond, label %guarded, label %deopt, !prof !0
+
+deopt:
+  %deoptret = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+  ret i32 %deoptret
+
+guarded:
+  call void @unknown()
+  br i1 %cond_1, label %loop, label %deopt2, !prof !0
+
+deopt2:
+  %deoptret2 = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"() ]
+  ret i32 %deoptret2
+}
+
+
+declare void @unknown()
+declare i32 @unknown_i32()
+
+declare i1 @llvm.experimental.widenable.condition()
+declare i32 @llvm.experimental.deoptimize.i32(...)
+
+!0 = !{!"branch_weights", i32 1048576, i32 1}
+!1 = !{i32 1, i32 -2147483648}