diff --git a/llvm/include/llvm/Analysis/TargetLibraryInfo.h b/llvm/include/llvm/Analysis/TargetLibraryInfo.h
index e913b91..adc6363 100644
--- a/llvm/include/llvm/Analysis/TargetLibraryInfo.h
+++ b/llvm/include/llvm/Analysis/TargetLibraryInfo.h
@@ -192,6 +192,11 @@
   /// vector functions.
   void getWidestVF(StringRef ScalarF, ElementCount &FixedVF,
                    ElementCount &Scalable) const;
+
+  /// Returns true if call site / callee has cdecl-compatible calling
+  /// conventions.
+  static bool isCallingConvCCompatible(CallBase *CI);
+  static bool isCallingConvCCompatible(Function *Callee);
 };
 
 /// Provides information about what library functions are available for
diff --git a/llvm/lib/Analysis/TargetLibraryInfo.cpp b/llvm/lib/Analysis/TargetLibraryInfo.cpp
index 31cea8f..14dcaa6 100644
--- a/llvm/lib/Analysis/TargetLibraryInfo.cpp
+++ b/llvm/lib/Analysis/TargetLibraryInfo.cpp
@@ -65,6 +65,49 @@
   return TT.isOSFreeBSD() || TT.isOSSolaris();
 }
 
+static bool isCallingConvCCompatible(CallingConv::ID CC, Triple TT,
+                                     FunctionType *FuncTy) {
+  switch (CC) {
+  default:
+    return false;
+  case llvm::CallingConv::C:
+    return true;
+  case llvm::CallingConv::ARM_APCS:
+  case llvm::CallingConv::ARM_AAPCS:
+  case llvm::CallingConv::ARM_AAPCS_VFP: {
+
+    // The iOS ABI diverges from the standard in some cases, so for now don't
+    // try to simplify those calls.
+    if (TT.isiOS())
+      return false;
+
+    if (!FuncTy->getReturnType()->isPointerTy() &&
+        !FuncTy->getReturnType()->isIntegerTy() &&
+        !FuncTy->getReturnType()->isVoidTy())
+      return false;
+
+    for (auto *Param : FuncTy->params()) {
+      if (!Param->isPointerTy() && !Param->isIntegerTy())
+        return false;
+    }
+    return true;
+  }
+  }
+  return false;
+}
+
+bool TargetLibraryInfoImpl::isCallingConvCCompatible(CallBase *CI) {
+  return ::isCallingConvCCompatible(CI->getCallingConv(),
+                                    Triple(CI->getModule()->getTargetTriple()),
+                                    CI->getFunctionType());
+}
+
+bool TargetLibraryInfoImpl::isCallingConvCCompatible(Function *F) {
+  return ::isCallingConvCCompatible(F->getCallingConv(),
+                                    Triple(F->getParent()->getTargetTriple()),
+                                    F->getFunctionType());
+}
+
 /// Initialize the set of available library functions based on the specified
 /// target triple. This should be carefully written so that a missing target
 /// triple gets a sane set of defaults.
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 78f4511..615f10e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -2158,9 +2158,14 @@
       return &Call;
     }
 
-    // If the call and callee calling conventions don't match, this call must
-    // be unreachable, as the call is undefined.
-    if (CalleeF->getCallingConv() != Call.getCallingConv() &&
+    // If the call and callee calling conventions don't match, and neither one
+    // of the calling conventions is compatible with C calling convention
+    // this call must be unreachable, as the call is undefined.
+    if ((CalleeF->getCallingConv() != Call.getCallingConv() &&
+         !(CalleeF->getCallingConv() == llvm::CallingConv::C &&
+           TargetLibraryInfoImpl::isCallingConvCCompatible(&Call)) &&
+         !(Call.getCallingConv() == llvm::CallingConv::C &&
+           TargetLibraryInfoImpl::isCallingConvCCompatible(CalleeF))) &&
         // Only do this for calls to a function with a body.  A prototype may
         // not actually end up matching the implementation's calling conv for a
         // variety of reasons (e.g. it may be written in assembly).
diff --git a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
index c279d18..2e44051 100644
--- a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -56,38 +56,6 @@
          Func == LibFunc_llabs || Func == LibFunc_strlen;
 }
 
-static bool isCallingConvCCompatible(CallInst *CI) {
-  switch(CI->getCallingConv()) {
-  default:
-    return false;
-  case llvm::CallingConv::C:
-    return true;
-  case llvm::CallingConv::ARM_APCS:
-  case llvm::CallingConv::ARM_AAPCS:
-  case llvm::CallingConv::ARM_AAPCS_VFP: {
-
-    // The iOS ABI diverges from the standard in some cases, so for now don't
-    // try to simplify those calls.
-    if (Triple(CI->getModule()->getTargetTriple()).isiOS())
-      return false;
-
-    auto *FuncTy = CI->getFunctionType();
-
-    if (!FuncTy->getReturnType()->isPointerTy() &&
-        !FuncTy->getReturnType()->isIntegerTy() &&
-        !FuncTy->getReturnType()->isVoidTy())
-      return false;
-
-    for (auto Param : FuncTy->params()) {
-      if (!Param->isPointerTy() && !Param->isIntegerTy())
-        return false;
-    }
-    return true;
-  }
-  }
-  return false;
-}
-
 /// Return true if it is only used in equality comparisons with With.
 static bool isOnlyUsedInEqualityComparison(Value *V, Value *With) {
   for (User *U : V->users()) {
@@ -2862,9 +2830,10 @@
   // Check for string/memory library functions.
   if (TLI->getLibFunc(*Callee, Func) && TLI->has(Func)) {
     // Make sure we never change the calling convention.
-    assert((ignoreCallingConv(Func) ||
-            isCallingConvCCompatible(CI)) &&
-      "Optimizing string/memory libcall would change the calling convention");
+    assert(
+        (ignoreCallingConv(Func) ||
+         TargetLibraryInfoImpl::isCallingConvCCompatible(CI)) &&
+        "Optimizing string/memory libcall would change the calling convention");
     switch (Func) {
     case LibFunc_strcat:
       return optimizeStrCat(CI, Builder);
@@ -3048,7 +3017,7 @@
 
   LibFunc Func;
   Function *Callee = CI->getCalledFunction();
-  bool isCallingConvC = isCallingConvCCompatible(CI);
+  bool IsCallingConvC = TargetLibraryInfoImpl::isCallingConvCCompatible(CI);
 
   SmallVector<OperandBundleDef, 2> OpBundles;
   CI->getOperandBundlesAsDefs(OpBundles);
@@ -3066,7 +3035,7 @@
 
   // First, check for intrinsics.
   if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(CI)) {
-    if (!isCallingConvC)
+    if (!IsCallingConvC)
       return nullptr;
     // The FP intrinsics have corresponding constrained versions so we don't
     // need to check for the StrictFP attribute here.
@@ -3119,7 +3088,7 @@
   // Then check for known library functions.
   if (TLI->getLibFunc(*Callee, Func) && TLI->has(Func)) {
     // We never change the calling convention.
-    if (!ignoreCallingConv(Func) && !isCallingConvC)
+    if (!ignoreCallingConv(Func) && !IsCallingConvC)
       return nullptr;
     if (Value *V = optimizeStringMemoryLibCall(CI, Builder))
       return V;
@@ -3503,7 +3472,7 @@
 
   LibFunc Func;
   Function *Callee = CI->getCalledFunction();
-  bool isCallingConvC = isCallingConvCCompatible(CI);
+  bool IsCallingConvC = TargetLibraryInfoImpl::isCallingConvCCompatible(CI);
 
   SmallVector<OperandBundleDef, 2> OpBundles;
   CI->getOperandBundlesAsDefs(OpBundles);
@@ -3517,7 +3486,7 @@
     return nullptr;
 
   // We never change the calling convention.
-  if (!ignoreCallingConv(Func) && !isCallingConvC)
+  if (!ignoreCallingConv(Func) && !IsCallingConvC)
     return nullptr;
 
   switch (Func) {
diff --git a/llvm/test/Transforms/InstCombine/call-callconv-mismatch.ll b/llvm/test/Transforms/InstCombine/call-callconv-mismatch.ll
new file mode 100644
index 0000000..de1110d
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/call-callconv-mismatch.ll
@@ -0,0 +1,22 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instcombine -S | FileCheck %s
+; Verify that a cdecl-compatible calling convention does not trigger emitting
+; unreachable idom `store i1 true, i1* undef`.
+
+define arm_aapcs_vfpcc i8 @bar(i8* %0) {
+; CHECK-LABEL: @bar(
+; CHECK-NEXT:    [[TMP2:%.*]] = load i8, i8* [[TMP0:%.*]], align 1
+; CHECK-NEXT:    ret i8 [[TMP2]]
+;
+  %2 = load i8, i8* %0, align 1
+  ret i8 %2
+}
+
+define dso_local arm_aapcs_vfpcc i8 @foo(i8* %0) {
+; CHECK-LABEL: @foo(
+; CHECK-NEXT:    [[TMP2:%.*]] = call i8 @bar(i8* [[TMP0:%.*]])
+; CHECK-NEXT:    ret i8 [[TMP2]]
+;
+  %2 = call i8 @bar(i8* %0)
+  ret i8 %2
+}