diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -447,8 +447,11 @@
 
 .. _release-notes-ubsan:
 
-Undefined Behavior Sanitizer (UBSan)
-------------------------------------
+Sanitizers
+----------
+- ``-fsanitize-memory-param-retval`` is turned on by default. With
+  ``-fsanitize=memory``, passing uninitialized variables to functions and
+  returning uninitialized variables from functions is more aggressively reported.
 
 Core Analysis Improvements
 ==========================
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -1767,7 +1767,7 @@
 defm sanitize_memory_param_retval
     : BoolFOption<"sanitize-memory-param-retval",
         CodeGenOpts<"SanitizeMemoryParamRetval">,
-        DefaultFalse,
+        DefaultTrue,
         PosFlag<SetTrue, [CC1Option], "Enable">, NegFlag<SetFalse, [], "Disable">,
         BothFlags<[], " detection of uninitialized parameters and return values">>;
 //// Note: This flag was introduced when it was necessary to distinguish between
diff --git a/clang/include/clang/Driver/SanitizerArgs.h b/clang/include/clang/Driver/SanitizerArgs.h
--- a/clang/include/clang/Driver/SanitizerArgs.h
+++ b/clang/include/clang/Driver/SanitizerArgs.h
@@ -34,7 +34,7 @@
   int BinaryMetadataFeatures = 0;
   int MsanTrackOrigins = 0;
   bool MsanUseAfterDtor = true;
-  bool MsanParamRetval = false;
+  bool MsanParamRetval = true;
   bool CfiCrossDso = false;
   bool CfiICallGeneralizePointers = false;
   bool CfiCanonicalJumpTables = false;
diff --git a/clang/lib/Driver/SanitizerArgs.cpp b/clang/lib/Driver/SanitizerArgs.cpp
--- a/clang/lib/Driver/SanitizerArgs.cpp
+++ b/clang/lib/Driver/SanitizerArgs.cpp
@@ -1184,8 +1184,8 @@
   if (MsanUseAfterDtor)
     CmdArgs.push_back("-fsanitize-memory-use-after-dtor");
 
-  if (MsanParamRetval)
-    CmdArgs.push_back("-fsanitize-memory-param-retval");
+  if (!MsanParamRetval)
+    CmdArgs.push_back("-fno-sanitize-memory-param-retval");
 
   // FIXME: Pass these parameters as function attributes, not as -llvm flags.
   if (!TsanMemoryAccess) {
diff --git a/clang/test/CodeGen/kmsan-param-retval.c b/clang/test/CodeGen/kmsan-param-retval.c
--- a/clang/test/CodeGen/kmsan-param-retval.c
+++ b/clang/test/CodeGen/kmsan-param-retval.c
@@ -1,12 +1,12 @@
 // RUN: %clang_cc1 -triple x86_64-linux-gnu -S -emit-llvm -O2 -fsanitize=kernel-memory -no-enable-noundef-analysis -o - %s | \
 // RUN:     FileCheck %s --check-prefix=CLEAN
-// RUN: %clang_cc1 -triple x86_64-linux-gnu -S -emit-llvm -O2 -fsanitize=kernel-memory -o - %s | \
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -S -emit-llvm -O2 -fsanitize=kernel-memory -fno-sanitize-memory-param-retval -o - %s | \
 // RUN:     FileCheck %s --check-prefixes=NOUNDEF,NOUNDEF_ONLY
 // RUN: %clang_cc1 -triple x86_64-linux-gnu -S -emit-llvm -O2 -fsanitize=kernel-memory -mllvm -msan-eager-checks -o - %s | \
 // RUN:     FileCheck %s --check-prefixes=NOUNDEF,EAGER
 // RUN: %clang_cc1 -triple x86_64-linux-gnu -S -emit-llvm -O2 -fsanitize=kernel-memory -no-enable-noundef-analysis -fsanitize-memory-param-retval -o - %s | \
 // RUN:     FileCheck %s --check-prefixes=CLEAN
-// RUN: %clang_cc1 -triple x86_64-linux-gnu -S -emit-llvm -O2 -fsanitize=kernel-memory -fsanitize-memory-param-retval -o - %s | \
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -S -emit-llvm -O2 -fsanitize=kernel-memory -o - %s | \
 // RUN:     FileCheck %s --check-prefixes=NOUNDEF,EAGER
 
 void foo();
diff --git a/clang/test/CodeGen/msan-param-retval.c b/clang/test/CodeGen/msan-param-retval.c
--- a/clang/test/CodeGen/msan-param-retval.c
+++ b/clang/test/CodeGen/msan-param-retval.c
@@ -1,12 +1,12 @@
 // RUN: %clang_cc1 -triple x86_64-linux-gnu -S -emit-llvm -fsanitize=memory -no-enable-noundef-analysis -o - %s | \
 // RUN:     FileCheck %s --check-prefix=CLEAN
-// RUN: %clang_cc1 -triple x86_64-linux-gnu -S -emit-llvm -fsanitize=memory -o - %s | \
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -S -emit-llvm -fsanitize=memory -fno-sanitize-memory-param-retval -o - %s | \
 // RUN:     FileCheck %s --check-prefixes=NOUNDEF,NOUNDEF_ONLY
 // RUN: %clang_cc1 -triple x86_64-linux-gnu -S -emit-llvm -fsanitize=memory -mllvm -msan-eager-checks -o - %s | \
 // RUN:     FileCheck %s --check-prefixes=NOUNDEF,EAGER
 // RUN: %clang_cc1 -triple x86_64-linux-gnu -S -emit-llvm -fsanitize=memory -no-enable-noundef-analysis -fsanitize-memory-param-retval -o - %s | \
 // RUN:     FileCheck %s --check-prefixes=CLEAN
-// RUN: %clang_cc1 -triple x86_64-linux-gnu -S -emit-llvm -fsanitize=memory -fsanitize-memory-param-retval -o - %s | \
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -S -emit-llvm -fsanitize=memory -o - %s | \
 // RUN:     FileCheck %s --check-prefixes=NOUNDEF,EAGER
 
 void bar(int x) {
diff --git a/clang/test/Driver/fsanitize-memory-param-retval.c b/clang/test/Driver/fsanitize-memory-param-retval.c
--- a/clang/test/Driver/fsanitize-memory-param-retval.c
+++ b/clang/test/Driver/fsanitize-memory-param-retval.c
@@ -1,14 +1,14 @@
-// RUN: %clang -target i386-gnu-linux %s -fsanitize=memory -fsanitize-memory-param-retval -c -### 2>&1 | FileCheck %s
-// RUN: %clang -target x86_64-linux-gnu %s -fsanitize=memory -fsanitize-memory-param-retval -c -### 2>&1 | FileCheck %s
-// RUN: %clang -target aarch64-linux-gnu %s -fsanitize=memory -fsanitize-memory-param-retval -c -### 2>&1 | FileCheck %s
-// RUN: %clang -target riscv32-linux-gnu %s -fsanitize=memory -fsanitize-memory-param-retval -c -### 2>&1 | FileCheck %s
-// RUN: %clang -target riscv64-linux-gnu %s -fsanitize=memory -fsanitize-memory-param-retval -c -### 2>&1 | FileCheck %s
-// RUN: %clang -target x86_64-linux-gnu %s -fsanitize=kernel-memory -fsanitize-memory-param-retval -c -### 2>&1 | FileCheck %s
+// RUN: %clang -target i386-gnu-linux %s -fsanitize=memory -fno-sanitize-memory-param-retval -c -### 2>&1 | FileCheck %s
+// RUN: %clang -target x86_64-linux-gnu %s -fsanitize=memory -fno-sanitize-memory-param-retval -c -### 2>&1 | FileCheck %s
+// RUN: %clang -target aarch64-linux-gnu %s -fsanitize=memory -fno-sanitize-memory-param-retval -c -### 2>&1 | FileCheck %s
+// RUN: %clang -target riscv32-linux-gnu %s -fsanitize=memory -fno-sanitize-memory-param-retval -c -### 2>&1 | FileCheck %s
+// RUN: %clang -target riscv64-linux-gnu %s -fsanitize=memory -fno-sanitize-memory-param-retval -c -### 2>&1 | FileCheck %s
+// RUN: %clang -target x86_64-linux-gnu %s -fsanitize=kernel-memory -fno-sanitize-memory-param-retval -c -### 2>&1 | FileCheck %s
 
-// CHECK: "-fsanitize-memory-param-retval"
+// CHECK: "-fno-sanitize-memory-param-retval"
 
-// RUN: %clang -target aarch64-linux-gnu -fsyntax-only %s -fsanitize=memory -fsanitize-memory-param-retval -c -### 2>&1 | FileCheck --check-prefix=11 %s
-// 11: "-fsanitize-memory-param-retval"
+// RUN: %clang -target aarch64-linux-gnu -fsyntax-only %s -fsanitize=memory -fno-sanitize-memory-param-retval -c -### 2>&1 | FileCheck --check-prefix=11 %s
+// 11: "-fno-sanitize-memory-param-retval"
 
-// RUN: not %clang -target x86_64-linux-gnu -fsyntax-only %s -fsanitize=memory -fsanitize-memory-param-retval=1 2>&1 | FileCheck --check-prefix=EXCESS %s
-// EXCESS: error: unknown argument: '-fsanitize-memory-param-retval=
+// RUN: not %clang -target x86_64-linux-gnu -fsyntax-only %s -fsanitize=memory -fno-sanitize-memory-param-retval=1 2>&1 | FileCheck --check-prefix=EXCESS %s
+// EXCESS: error: unknown argument: '-fno-sanitize-memory-param-retval=