diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -3395,6 +3395,10 @@
     if (CallerPAL.hasParamAttr(i, Attribute::SwiftError))
       return false;
 
+    if (!CallerPAL.hasParamAttr(i, Attribute::ByVal) &&
+        Callee->getAttributes().hasParamAttr(i, Attribute::ByVal))
+      return false; // Cannot transform to byval.
+
     // If the parameter is passed as a byval argument, then we have to have a
     // sized type and the sized type has to have the same size as the old type.
     if (ParamTy != ActTy && CallerPAL.hasParamAttr(i, Attribute::ByVal)) {
diff --git a/llvm/test/Transforms/InstCombine/cast-to-byval.ll b/llvm/test/Transforms/InstCombine/cast-to-byval.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/cast-to-byval.ll
@@ -0,0 +1,14 @@
+; Check that function calls involving conversion to byval aren't transformed.
+; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+
+define i32 @bar(i32 %0) {
+  %2 = tail call i32 @foo(i32 %0)
+; CHECK: %2 = tail call i32 @foo(i32 %0)
+  ret i32 %2
+}
+
+%Foo = type { i32 }
+define i32 @foo (ptr byval(%Foo) %foo) {
+  %1 = load i32, ptr %foo, align 4
+  ret i32 %1
+}