diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index bf7c91bf36306..a38c990b9ea83 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -4106,6 +4106,12 @@ bool InstCombinerImpl::transformConstExprCastCall(CallBase &Call) {
   assert(!isa<CallBrInst>(Call) &&
          "CallBr's don't have a single point after a def to insert at");
 
+  // Don't perform the transform for declarations, which may not be fully
+  // accurate. For example, void @foo() is commonly used as a placeholder for
+  // unknown prototypes.
+  if (Callee->isDeclaration())
+    return false;
+
   // If this is a call to a thunk function, don't remove the cast. Thunks are
   // used to transparently forward all incoming parameters and outgoing return
   // values, so it's important to leave the cast in place.
@@ -4142,9 +4148,6 @@ bool InstCombinerImpl::transformConstExprCastCall(CallBase &Call) {
       return false; // TODO: Handle multiple return values.
 
     if (!CastInst::isBitOrNoopPointerCastable(NewRetTy, OldRetTy, DL)) {
-      if (Callee->isDeclaration())
-        return false;   // Cannot transform this return value.
-
       if (!Caller->use_empty())
         return false;   // Cannot transform this return value.
     }
@@ -4212,25 +4215,6 @@ bool InstCombinerImpl::transformConstExprCastCall(CallBase &Call) {
       return false; // Cannot transform to or from byval.
   }
 
-  if (Callee->isDeclaration()) {
-    // Do not delete arguments unless we have a function body.
-    if (FT->getNumParams() < NumActualArgs && !FT->isVarArg())
-      return false;
-
-    // If the callee is just a declaration, don't change the varargsness of the
-    // call.  We don't want to introduce a varargs call where one doesn't
-    // already exist.
-    if (FT->isVarArg() != Call.getFunctionType()->isVarArg())
-      return false;
-
-    // If both the callee and the cast type are varargs, we still have to make
-    // sure the number of fixed parameters are the same or we have the same
-    // ABI issues as if we introduce a varargs call.
-    if (FT->isVarArg() && Call.getFunctionType()->isVarArg() &&
-        FT->getNumParams() != Call.getFunctionType()->getNumParams())
-      return false;
-  }
-
   if (FT->getNumParams() < NumActualArgs && FT->isVarArg() &&
       !CallerPAL.isEmpty()) {
     // In this case we have more arguments than the new function type, but we
diff --git a/llvm/test/Transforms/InstCombine/apint-call-cast-target.ll b/llvm/test/Transforms/InstCombine/apint-call-cast-target.ll
index d491762133549..6cb195e3d5092 100644
--- a/llvm/test/Transforms/InstCombine/apint-call-cast-target.ll
+++ b/llvm/test/Transforms/InstCombine/apint-call-cast-target.ll
@@ -4,8 +4,19 @@
 target datalayout = "e-p:32:32"
 target triple = "i686-pc-linux-gnu"
 
-declare i32 @main2()
-declare ptr @ctime2(ptr)
+define i32 @main2() {
+; CHECK-LABEL: @main2(
+; CHECK-NEXT:    ret i32 0
+;
+  ret i32 0
+}
+
+define ptr @ctime2(ptr %p) {
+; CHECK-LABEL: @ctime2(
+; CHECK-NEXT:    ret ptr [[P:%.*]]
+;
+  ret ptr %p
+}
 
 define ptr @ctime(ptr) {
 ; CHECK-LABEL: @ctime(
diff --git a/llvm/test/Transforms/InstCombine/call-cast-target.ll b/llvm/test/Transforms/InstCombine/call-cast-target.ll
index b9f57dbbd1c30..e5bb929ce92f0 100644
--- a/llvm/test/Transforms/InstCombine/call-cast-target.ll
+++ b/llvm/test/Transforms/InstCombine/call-cast-target.ll
@@ -16,7 +16,13 @@ entry:
   ret i32 %tmp
 }
 
-declare ptr @ctime(ptr)
+define ptr @ctime(ptr %p) {
+; CHECK-LABEL: define ptr @ctime(
+; CHECK-SAME: ptr [[P:%.*]]) {
+; CHECK-NEXT:    ret ptr [[P]]
+;
+  ret ptr %p
+}
 
 define internal { i8 } @foo(ptr) {
 ; CHECK-LABEL: define internal { i8 } @foo(
@@ -39,7 +45,13 @@ entry:
   ret void
 }
 
-declare i32 @fn1(i32)
+define i32 @fn1(i32 %x) {
+; CHECK-LABEL: define i32 @fn1(
+; CHECK-SAME: i32 [[X:%.*]]) {
+; CHECK-NEXT:    ret i32 [[X]]
+;
+  ret i32 %x
+}
 
 define i32 @test1(ptr %a) {
 ; CHECK-LABEL: define i32 @test1(
@@ -116,7 +128,13 @@ define i1 @test5() {
   ret i1 %6
 }
 
-declare void @bundles_callee(i32)
+define void @bundles_callee(i32) {
+; CHECK-LABEL: define void @bundles_callee(
+; CHECK-SAME: i32 [[TMP0:%.*]]) {
+; CHECK-NEXT:    ret void
+;
+  ret void
+}
 
 define void @bundles() {
 ; CHECK-LABEL: define void @bundles() {
diff --git a/llvm/test/Transforms/InstCombine/call.ll b/llvm/test/Transforms/InstCombine/call.ll
index 52de13ba06b35..333b3c499aba0 100644
--- a/llvm/test/Transforms/InstCombine/call.ll
+++ b/llvm/test/Transforms/InstCombine/call.ll
@@ -110,7 +110,7 @@ declare i32 @test6a(i32)
 
 define i32 @test6() {
 ; CHECK-LABEL: define i32 @test6() {
-; CHECK-NEXT:    [[X:%.*]] = call i32 @test6a(i32 0)
+; CHECK-NEXT:    [[X:%.*]] = call i32 @test6a()
 ; CHECK-NEXT:    ret i32 [[X]]
 ;
   %X = call i32 @test6a( )
@@ -141,12 +141,12 @@ declare void @test8a()
 define ptr @test8() personality ptr @__gxx_personality_v0 {
 ; CHECK-LABEL: define ptr @test8() personality ptr @__gxx_personality_v0 {
 ; CHECK-NEXT:    invoke void @test8a()
-; CHECK-NEXT:    to label [[INVOKE_CONT:%.*]] unwind label [[TRY_HANDLER:%.*]]
+; CHECK-NEXT:            to label [[INVOKE_CONT:%.*]] unwind label [[TRY_HANDLER:%.*]]
 ; CHECK:       invoke.cont:
 ; CHECK-NEXT:    unreachable
 ; CHECK:       try.handler:
 ; CHECK-NEXT:    [[EXN:%.*]] = landingpad { ptr, i32 }
-; CHECK-NEXT:    cleanup
+; CHECK-NEXT:            cleanup
 ; CHECK-NEXT:    ret ptr null
 ;
 ; Don't turn this into "unreachable": the callee and caller don't agree in
diff --git a/llvm/test/Transforms/InstCombine/opaque-ptr.ll b/llvm/test/Transforms/InstCombine/opaque-ptr.ll
index df85547f56d74..a52a07389baee 100644
--- a/llvm/test/Transforms/InstCombine/opaque-ptr.ll
+++ b/llvm/test/Transforms/InstCombine/opaque-ptr.ll
@@ -743,8 +743,7 @@ declare void @call_byval(i64, ptr byval(i64))
 
 define void @call_cast_ptr_to_int(ptr %p) {
 ; CHECK-LABEL: @call_cast_ptr_to_int(
-; CHECK-NEXT:    [[TMP1:%.*]] = ptrtoint ptr [[P:%.*]] to i64
-; CHECK-NEXT:    call void @call_i64(i64 [[TMP1]])
+; CHECK-NEXT:    call void @call_i64(ptr [[P:%.*]])
 ; CHECK-NEXT:    ret void
 ;
   call void @call_i64(ptr %p)
@@ -753,8 +752,7 @@ define void @call_cast_ptr_to_int(ptr %p) {
 
 define void @call_cast_byval(ptr %p, ptr %p2) {
 ; CHECK-LABEL: @call_cast_byval(
-; CHECK-NEXT:    [[TMP1:%.*]] = ptrtoint ptr [[P:%.*]] to i64
-; CHECK-NEXT:    call void @call_byval(i64 [[TMP1]], ptr byval(double) [[P2:%.*]])
+; CHECK-NEXT:    call void @call_byval(ptr [[P:%.*]], ptr byval(double) [[P2:%.*]])
 ; CHECK-NEXT:    ret void
 ;
   call void @call_byval(ptr %p, ptr byval(double) %p2)
@@ -765,8 +763,8 @@ declare float @fmodf(float, float)
 
 define i32 @const_fold_call_with_func_type_mismatch() {
 ; CHECK-LABEL: @const_fold_call_with_func_type_mismatch(
-; CHECK-NEXT:    [[V:%.*]] = call float @fmodf(float 0x40091EB860000000, float 2.000000e+00)
-; CHECK-NEXT:    ret i32 1066527622
+; CHECK-NEXT:    [[V:%.*]] = call i32 @fmodf(float 0x40091EB860000000, float 2.000000e+00)
+; CHECK-NEXT:    ret i32 [[V]]
 ;
   %v = call i32 @fmodf(float 0x40091EB860000000, float 2.000000e+00)
   ret i32 %v