Index: llvm/lib/Transforms/Utils/CloneFunction.cpp
===================================================================
--- llvm/lib/Transforms/Utils/CloneFunction.cpp
+++ llvm/lib/Transforms/Utils/CloneFunction.cpp
@@ -112,6 +112,13 @@
                  ModuleLevelChanges ? RF_None : RF_NoModuleLevelChanges,
                  TypeMapper, Materializer));
 
+  if (OldFunc->hasPrefixData()) {
+    NewFunc->setPrefixData(
+        MapValue(OldFunc->getPrefixData(), VMap,
+                 ModuleLevelChanges ? RF_None : RF_NoModuleLevelChanges,
+                 TypeMapper, Materializer));
+  }
+
   SmallVector<AttributeSet, 4> NewArgAttrs(NewFunc->arg_size());
   AttributeList OldAttrs = OldFunc->getAttributes();
 
Index: llvm/test/tools/llvm-reduce/reduce-prefix-data.ll
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-reduce/reduce-prefix-data.ll
@@ -0,0 +1,47 @@
+; RUN: llvm-reduce --abort-on-invalid-reduction --delta-passes=function-data --test FileCheck --test-arg --check-prefix=CHECK --test-arg %s --test-arg --input-file %s -o %t
+; RUN: FileCheck -check-prefixes=CHECK,RESULT %s < %t
+
+@gv = linkonce_odr global i32 1
+
+; CHECK: define void @drop_prefix_i32_const()
+define void @drop_prefix_i32_const() prefix i32 1 {
+  ret void
+}
+
+; CHECK: define void @keep_prefix_i32_const() prefix i32 1 {
+define void @keep_prefix_i32_const() prefix i32 1 {
+  ret void
+}
+
+; CHECK: define void @drop_prefix_ptr_global()
+define void @drop_prefix_ptr_global() prefix ptr @gv {
+  ret void
+}
+
+; CHECK: define void @keep_prefix_ptr_global() prefix ptr @gv {
+define void @keep_prefix_ptr_global() prefix ptr @gv {
+  ret void
+}
+
+; Make sure there's no invalid reduction if the prefix data is really
+; accessed
+; CHECK: define i32 @drop_uses_prefix_i32_const()
+define i32 @drop_uses_prefix_i32_const() prefix i32 1 {
+  %gep = getelementptr inbounds i32, ptr @drop_uses_prefix_i32_const, i32 -1
+  %load = load i32, ptr %gep
+  ret i32 %load
+}
+
+; CHECK: define ptr @drop_uses_prefix_gv(
+define ptr @drop_uses_prefix_gv() prefix ptr @gv {
+  %gep = getelementptr inbounds i32, ptr @drop_uses_prefix_gv, i32 -1
+  %load = load ptr, ptr %gep
+  ret ptr %load
+}
+
+
+; RESULT: declare void @declaration_prefix_i32_const(){{$}}
+declare void @declaration_prefix_i32_const() prefix i32 2
+
+; RESULT: void @declaration_prefix_ptr_gv(){{$}}
+declare void @declaration_prefix_ptr_gv() prefix ptr @gv
Index: llvm/tools/llvm-reduce/deltas/ReduceFunctionBodies.cpp
===================================================================
--- llvm/tools/llvm-reduce/deltas/ReduceFunctionBodies.cpp
+++ llvm/tools/llvm-reduce/deltas/ReduceFunctionBodies.cpp
@@ -38,9 +38,6 @@
 
 static void reduceFunctionData(Oracle &O, Module &M) {
   for (Function &F : M) {
-    if (F.isDeclaration())
-      continue;
-
     if (F.hasPersonalityFn()) {
       if (none_of(F,
                   [](const BasicBlock &BB) {
@@ -51,7 +48,10 @@
       }
     }
 
-    // TODO: Handle prefix data and prologue data
+    if (F.hasPrefixData() && !O.shouldKeep())
+      F.setPrefixData(nullptr);
+
+    // TODO: Handle prologue data
   }
 }