Index: llvm/trunk/lib/Transforms/InstCombine/InstCombinePHI.cpp
===================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombinePHI.cpp
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombinePHI.cpp
@@ -15,8 +15,11 @@
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/Analysis/InstructionSimplify.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/PatternMatch.h"
 #include "llvm/Transforms/Utils/Local.h"
 using namespace llvm;
+using namespace llvm::PatternMatch;
 
 #define DEBUG_TYPE "instcombine"
 
@@ -641,6 +644,16 @@
   return true;
 }
 
+/// Return an existing non-zero constant if this phi node has, otherwise ruturn
+/// constant 1.
+static ConstantInt *GetAnyNonZeroConstInt(PHINode &PN) {
+  assert(isa<IntegerType>(PN.getType()) && "Expect only intger type phi");
+  for (Value *V : PN.operands())
+    if (auto *ConstVA = dyn_cast<ConstantInt>(V))
+      if (!ConstVA->isZeroValue())
+        return ConstVA;
+  return ConstantInt::get(cast<IntegerType>(PN.getType()), 1);
+}
 
 namespace {
 struct PHIUsageRecord {
@@ -919,6 +932,29 @@
         PHIUser->user_back() == &PN) {
       return replaceInstUsesWith(PN, UndefValue::get(PN.getType()));
     }
+    // When a PHI is used only to be compared with zero, it is safe to replace
+    // an incoming value proved as known nonzero with any non-zero constant.
+    // For example, in below code, the incoming value %v can be replaced with
+    // any non-zero constant based on the fact that the PHI is only used to be
+    // compared with zero and %v is a known non-zero value:
+    // %v = select %cond, 1, 2
+    // %p = phi [%v, BB] ...
+    //      icmp eq, %p, 0
+    auto *CmpInst = dyn_cast<ICmpInst>(PHIUser);
+    // FIXME: To be simple, handle only integer type for now.
+    if (CmpInst && isa<IntegerType>(PN.getType()) && CmpInst->isEquality() &&
+        match(CmpInst->getOperand(1), m_Zero())) {
+      ConstantInt *NonZeroConst = nullptr;
+      for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
+        Instruction *CtxI = PN.getIncomingBlock(i)->getTerminator();
+        Value *VA = PN.getIncomingValue(i);
+        if (isKnownNonZero(VA, DL, 0, AC, CtxI, DT)) {
+          if (!NonZeroConst)
+            NonZeroConst = GetAnyNonZeroConstInt(PN);
+          PN.setIncomingValue(i, NonZeroConst);
+        }
+      }
+    }
   }
 
   // We sometimes end up with phi cycles that non-obviously end up being the
Index: llvm/trunk/test/Transforms/InstCombine/phi.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/phi.ll
+++ llvm/trunk/test/Transforms/InstCombine/phi.ll
@@ -784,3 +784,98 @@
 
 declare void @dummy()
 
+; CHECK-LABEL: @phi_knownnonzero_eq
+; CHECK-LABEL: if.then:
+; CHECK-NOT: select
+; CHECK-LABEL: if.end:
+; CHECK: phi i32 [ 1, %if.then ]
+define i1 @phi_knownnonzero_eq(i32 %n, i32 %s, i32* nocapture readonly %P) {
+entry:
+  %tobool = icmp slt  i32 %n, %s
+  br i1 %tobool, label %if.end, label %if.then
+
+if.then:                                          ; preds = %entry
+  %0 = load i32, i32* %P
+  %cmp = icmp eq i32 %n, %0
+  %1 = select i1 %cmp, i32 1, i32 2
+  br label %if.end
+
+if.end:                                           ; preds = %entry, %if.then
+  %a.0 = phi i32 [ %1,  %if.then ], [ %n, %entry ]
+  %cmp1 = icmp eq i32 %a.0, 0
+  ret i1  %cmp1
+}
+
+; CHECK-LABEL: @phi_knownnonzero_ne
+; CHECK-LABEL: if.then:
+; CHECK-NOT: select
+; CHECK-LABEL: if.end:
+; CHECK: phi i32 [ 1, %if.then ]
+define i1 @phi_knownnonzero_ne(i32 %n, i32 %s, i32* nocapture readonly %P) {
+entry:
+  %tobool = icmp slt  i32 %n, %s
+  br i1 %tobool, label %if.end, label %if.then
+
+if.then:                                          ; preds = %entry
+  %0 = load i32, i32* %P
+  %cmp = icmp eq i32 %n, %0
+  %1 = select i1 %cmp, i32 1, i32 2
+  br label %if.end
+
+if.end:                                           ; preds = %entry, %if.then
+  %a.0 = phi i32 [ %1,  %if.then ], [ %n, %entry ]
+  %cmp1 = icmp ne i32 %a.0, 0
+  ret i1  %cmp1
+}
+
+; CHECK-LABEL: @phi_knownnonzero_eq_2
+; CHECK-LABEL: if.then:
+; CHECK-NOT: select
+; CHECK-LABEL: if.end:
+; CHECK: phi i32 [ 2, %if.else ]
+define i1 @phi_knownnonzero_eq_2(i32 %n, i32 %s, i32* nocapture readonly %P) {
+entry:
+  %tobool = icmp slt  i32 %n, %s
+  br i1 %tobool, label %if.then, label %if.end
+
+if.then:
+  %tobool2 = icmp slt  i32 %n, %s
+  br i1 %tobool2, label %if.else, label %if.end
+
+if.else:                                          ; preds = %entry
+  %0 = load i32, i32* %P
+  %cmp = icmp eq i32 %n, %0
+  %1 = select i1 %cmp, i32 1, i32 2
+  br label %if.end
+
+if.end:                                           ; preds = %entry, %if.then
+  %a.0 = phi i32 [ %1,  %if.else], [ %n, %entry ], [2, %if.then]
+  %cmp1 = icmp eq i32 %a.0, 0
+  ret i1  %cmp1
+}
+
+; CHECK-LABEL: @phi_knownnonzero_ne_2
+; CHECK-LABEL: if.then:
+; CHECK-NOT: select
+; CHECK-LABEL: if.end:
+; CHECK: phi i32 [ 2, %if.else ]
+define i1 @phi_knownnonzero_ne_2(i32 %n, i32 %s, i32* nocapture readonly %P) {
+entry:
+  %tobool = icmp slt  i32 %n, %s
+  br i1 %tobool, label %if.then, label %if.end
+
+if.then:
+  %tobool2 = icmp slt  i32 %n, %s
+  br i1 %tobool2, label %if.else, label %if.end
+
+if.else:                                          ; preds = %entry
+  %0 = load i32, i32* %P
+  %cmp = icmp eq i32 %n, %0
+  %1 = select i1 %cmp, i32 1, i32 2
+  br label %if.end
+
+if.end:                                           ; preds = %entry, %if.then
+  %a.0 = phi i32 [ %1,  %if.else], [ %n, %entry ], [2, %if.then]
+  %cmp1 = icmp ne i32 %a.0, 0
+  ret i1  %cmp1
+}