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 +1,22 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; Check that function calls involving conversion to byval aren't transformed.
+; RUN: opt < %s -passes=instcombine -mtriple=x86_64-unknown-unknown -S | FileCheck %s
+
+%Foo = type { i64 }
+define i64 @foo (ptr byval(%Foo) %foo) {
+; CHECK-LABEL: @foo(
+; CHECK-NEXT:    [[TMP1:%.*]] = load i64, ptr [[FOO:%.*]], align 4
+; CHECK-NEXT:    ret i64 [[TMP1]]
+;
+  %1 = load i64, ptr %foo, align 4
+  ret i64 %1
+}
+
+define i64 @bar(i64 %0) {
+; CHECK-LABEL: @bar(
+; CHECK-NEXT:    [[TMP2:%.*]] = tail call i64 @foo(i64 [[TMP0:%.*]])
+; CHECK-NEXT:    ret i64 [[TMP2]]
+;
+  %2 = tail call i64 @foo(i64 %0)
+  ret i64 %2
+}