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
@@ -400,7 +400,7 @@
 def ObjC : Flag<["-"], "ObjC">, Flags<[DriverOption]>,
   HelpText<"Treat source input files as Objective-C inputs">;
 def O : Joined<["-"], "O">, Group<O_Group>, Flags<[CC1Option]>;
-def O_flag : Flag<["-"], "O">, Flags<[CC1Option]>, Alias<O>, AliasArgs<["2"]>;
+def O_flag : Flag<["-"], "O">, Flags<[CC1Option]>, Alias<O>, AliasArgs<["1"]>;
 def Ofast : Joined<["-"], "Ofast">, Group<O_Group>, Flags<[CC1Option]>;
 def P : Flag<["-"], "P">, Flags<[CC1Option]>, Group<Preprocessor_Group>,
   HelpText<"Disable linemarker output in -E mode">;
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -137,7 +137,7 @@
     assert(A->getOption().matches(options::OPT_O));
 
     StringRef S(A->getValue());
-    if (S == "s" || S == "z" || S.empty())
+    if (S == "s" || S == "z")
       return llvm::CodeGenOpt::Default;
 
     if (S == "g")
diff --git a/clang/test/CodeGen/builtins-systemz-zvector-constrained.c b/clang/test/CodeGen/builtins-systemz-zvector-constrained.c
--- a/clang/test/CodeGen/builtins-systemz-zvector-constrained.c
+++ b/clang/test/CodeGen/builtins-systemz-zvector-constrained.c
@@ -1,10 +1,10 @@
 // REQUIRES: systemz-registered-target
 // RUN: %clang_cc1 -target-cpu z13 -triple s390x-linux-gnu \
-// RUN: -O -fzvector -flax-vector-conversions=none \
+// RUN: -O2 -fzvector -flax-vector-conversions=none \
 // RUN: -ffp-exception-behavior=strict \
 // RUN: -Wall -Wno-unused -Werror -emit-llvm %s -o - | FileCheck %s
 // RUN: %clang_cc1 -target-cpu z13 -triple s390x-linux-gnu \
-// RUN: -O -fzvector -flax-vector-conversions=none \
+// RUN: -O2 -fzvector -flax-vector-conversions=none \
 // RUN: -ffp-exception-behavior=strict \
 // RUN: -Wall -Wno-unused -Werror -S %s -o - | FileCheck %s --check-prefix=CHECK-ASM
 
diff --git a/clang/test/CodeGen/builtins-systemz-zvector.c b/clang/test/CodeGen/builtins-systemz-zvector.c
--- a/clang/test/CodeGen/builtins-systemz-zvector.c
+++ b/clang/test/CodeGen/builtins-systemz-zvector.c
@@ -1,9 +1,9 @@
 // REQUIRES: systemz-registered-target
 // RUN: %clang_cc1 -target-cpu z13 -triple s390x-linux-gnu \
-// RUN: -O -fzvector -flax-vector-conversions=none \
+// RUN: -O2 -fzvector -flax-vector-conversions=none \
 // RUN: -Wall -Wno-unused -Werror -emit-llvm %s -o - | FileCheck %s
 // RUN: %clang_cc1 -target-cpu z13 -triple s390x-linux-gnu \
-// RUN: -O -fzvector -flax-vector-conversions=none \
+// RUN: -O2 -fzvector -flax-vector-conversions=none \
 // RUN: -Wall -Wno-unused -Werror -S %s -o - | FileCheck %s --check-prefix=CHECK-ASM
 
 #include <vecintrin.h>
diff --git a/clang/test/CodeGen/builtins-systemz-zvector2-constrained.c b/clang/test/CodeGen/builtins-systemz-zvector2-constrained.c
--- a/clang/test/CodeGen/builtins-systemz-zvector2-constrained.c
+++ b/clang/test/CodeGen/builtins-systemz-zvector2-constrained.c
@@ -1,10 +1,10 @@
 // REQUIRES: systemz-registered-target
 // RUN: %clang_cc1 -target-cpu z14 -triple s390x-linux-gnu \
-// RUN: -O -fzvector -flax-vector-conversions=none \
+// RUN: -O2 -fzvector -flax-vector-conversions=none \
 // RUN: -ffp-exception-behavior=strict \
 // RUN: -Wall -Wno-unused -Werror -emit-llvm %s -o - | FileCheck %s
 // RUN: %clang_cc1 -target-cpu z14 -triple s390x-linux-gnu \
-// RUN: -O -fzvector -flax-vector-conversions=none \
+// RUN: -O2 -fzvector -flax-vector-conversions=none \
 // RUN: -ffp-exception-behavior=strict \
 // RUN: -Wall -Wno-unused -Werror -S %s -o - | FileCheck %s --check-prefix=CHECK-ASM
 
diff --git a/clang/test/CodeGen/builtins-systemz-zvector2.c b/clang/test/CodeGen/builtins-systemz-zvector2.c
--- a/clang/test/CodeGen/builtins-systemz-zvector2.c
+++ b/clang/test/CodeGen/builtins-systemz-zvector2.c
@@ -1,9 +1,9 @@
 // REQUIRES: systemz-registered-target
 // RUN: %clang_cc1 -target-cpu z14 -triple s390x-linux-gnu \
-// RUN: -O -fzvector -flax-vector-conversions=none \
+// RUN: -O2 -fzvector -flax-vector-conversions=none \
 // RUN: -Wall -Wno-unused -Werror -emit-llvm %s -o - | FileCheck %s
 // RUN: %clang_cc1 -target-cpu z14 -triple s390x-linux-gnu \
-// RUN: -O -fzvector -flax-vector-conversions=none \
+// RUN: -O2 -fzvector -flax-vector-conversions=none \
 // RUN: -Wall -Wno-unused -Werror -S %s -o - | FileCheck %s --check-prefix=CHECK-ASM
 
 #include <vecintrin.h>
diff --git a/clang/test/CodeGen/builtins-systemz-zvector3-constrained.c b/clang/test/CodeGen/builtins-systemz-zvector3-constrained.c
--- a/clang/test/CodeGen/builtins-systemz-zvector3-constrained.c
+++ b/clang/test/CodeGen/builtins-systemz-zvector3-constrained.c
@@ -1,10 +1,10 @@
 // REQUIRES: systemz-registered-target
 // RUN: %clang_cc1 -target-cpu z15 -triple s390x-linux-gnu \
-// RUN: -O -fzvector -flax-vector-conversions=none \
+// RUN: -O2 -fzvector -flax-vector-conversions=none \
 // RUN: -ffp-exception-behavior=strict \
 // RUN: -Wall -Wno-unused -Werror -emit-llvm %s -o - | FileCheck %s
 // RUN: %clang_cc1 -target-cpu z15 -triple s390x-linux-gnu \
-// RUN: -O -fzvector -flax-vector-conversions=none \
+// RUN: -O2 -fzvector -flax-vector-conversions=none \
 // RUN: -ffp-exception-behavior=strict \
 // RUN: -Wall -Wno-unused -Werror -S %s -o - | FileCheck %s --check-prefix=CHECK-ASM
 
diff --git a/clang/test/CodeGen/builtins-systemz-zvector3.c b/clang/test/CodeGen/builtins-systemz-zvector3.c
--- a/clang/test/CodeGen/builtins-systemz-zvector3.c
+++ b/clang/test/CodeGen/builtins-systemz-zvector3.c
@@ -1,9 +1,9 @@
 // REQUIRES: systemz-registered-target
 // RUN: %clang_cc1 -target-cpu z15 -triple s390x-linux-gnu \
-// RUN: -O -fzvector -flax-vector-conversions=none \
+// RUN: -O2 -fzvector -flax-vector-conversions=none \
 // RUN: -Wall -Wno-unused -Werror -emit-llvm %s -o - | FileCheck %s
 // RUN: %clang_cc1 -target-cpu z15 -triple s390x-linux-gnu \
-// RUN: -O -fzvector -flax-vector-conversions=none \
+// RUN: -O2 -fzvector -flax-vector-conversions=none \
 // RUN: -Wall -Wno-unused -Werror -S %s -o - | FileCheck %s --check-prefix=CHECK-ASM
 
 #include <vecintrin.h>
diff --git a/clang/test/CodeGen/fma-builtins-constrained.c b/clang/test/CodeGen/fma-builtins-constrained.c
--- a/clang/test/CodeGen/fma-builtins-constrained.c
+++ b/clang/test/CodeGen/fma-builtins-constrained.c
@@ -1,8 +1,8 @@
 // REQUIRES: x86-registered-target
-// RUN: %clang_cc1 -ffreestanding %s -triple=x86_64-unknown-linux-gnu -target-feature +fma -O -emit-llvm -o - | FileCheck --check-prefix=COMMON --check-prefix=COMMONIR --check-prefix=UNCONSTRAINED %s
-// RUN: %clang_cc1 -ffreestanding %s -triple=x86_64-unknown-linux-gnu -target-feature +fma -ffp-exception-behavior=strict -O -emit-llvm -o - | FileCheck --check-prefix=COMMON --check-prefix=COMMONIR --check-prefix=CONSTRAINED %s
-// RUN: %clang_cc1 -ffreestanding %s -triple=x86_64-unknown-linux-gnu -target-feature +fma -O -S -o - | FileCheck --check-prefix=COMMON --check-prefix=CHECK-ASM --check-prefix=CHECK-ASM-UNCONSTRAINED %s
-// RUN: %clang_cc1 -ffreestanding %s -triple=x86_64-unknown-linux-gnu -target-feature +fma -O -ffp-exception-behavior=strict -S -o - | FileCheck --check-prefix=COMMON --check-prefix=CHECK-ASM --check-prefix=CHECK-ASM-CONSTRAINED %s
+// RUN: %clang_cc1 -ffreestanding %s -triple=x86_64-unknown-linux-gnu -target-feature +fma -O2 -emit-llvm -o - | FileCheck --check-prefix=COMMON --check-prefix=COMMONIR --check-prefix=UNCONSTRAINED %s
+// RUN: %clang_cc1 -ffreestanding %s -triple=x86_64-unknown-linux-gnu -target-feature +fma -ffp-exception-behavior=strict -O2 -emit-llvm -o - | FileCheck --check-prefix=COMMON --check-prefix=COMMONIR --check-prefix=CONSTRAINED %s
+// RUN: %clang_cc1 -ffreestanding %s -triple=x86_64-unknown-linux-gnu -target-feature +fma -O2 -S -o - | FileCheck --check-prefix=COMMON --check-prefix=CHECK-ASM --check-prefix=CHECK-ASM-UNCONSTRAINED %s
+// RUN: %clang_cc1 -ffreestanding %s -triple=x86_64-unknown-linux-gnu -target-feature +fma -O2 -ffp-exception-behavior=strict -S -o - | FileCheck --check-prefix=COMMON --check-prefix=CHECK-ASM --check-prefix=CHECK-ASM-CONSTRAINED %s
 
 #include <immintrin.h>
 
diff --git a/clang/test/Driver/O.c b/clang/test/Driver/O.c
--- a/clang/test/Driver/O.c
+++ b/clang/test/Driver/O.c
@@ -1,7 +1,7 @@
 // Test that we parse and translate the -O option correctly.
 
 // RUN: %clang -O -### %s 2>&1 | FileCheck -check-prefix=CHECK-O %s
-// CHECK-O: -O2
+// CHECK-O: -O1
 
 // RUN: %clang -O0 -### %s 2>&1 | FileCheck -check-prefix=CHECK-O0 %s
 // CHECK-O0: -O0
diff --git a/clang/test/Driver/clang_f_opts.c b/clang/test/Driver/clang_f_opts.c
--- a/clang/test/Driver/clang_f_opts.c
+++ b/clang/test/Driver/clang_f_opts.c
@@ -135,7 +135,7 @@
 // RUN: %clang -### -S -fno-tree-vectorize -fvectorize %s 2>&1 | FileCheck -check-prefix=CHECK-VECTORIZE %s
 // RUN: %clang -### -S -fno-tree-vectorize %s 2>&1 | FileCheck -check-prefix=CHECK-NO-VECTORIZE %s
 // RUN: %clang -### -S -ftree-vectorize -fno-vectorize %s 2>&1 | FileCheck -check-prefix=CHECK-NO-VECTORIZE %s
-// RUN: %clang -### -S -O %s 2>&1 | FileCheck -check-prefix=CHECK-VECTORIZE %s
+// RUN: %clang -### -S -O %s 2>&1 | FileCheck -check-prefix=CHECK-NO-VECTORIZE %s
 // RUN: %clang -### -S -O2 %s 2>&1 | FileCheck -check-prefix=CHECK-VECTORIZE %s
 // RUN: %clang -### -S -Os %s 2>&1 | FileCheck -check-prefix=CHECK-VECTORIZE %s
 // RUN: %clang -### -S -O3 %s 2>&1 | FileCheck -check-prefix=CHECK-VECTORIZE %s
@@ -157,7 +157,7 @@
 // RUN: %clang -### -S -fno-tree-slp-vectorize -fslp-vectorize %s 2>&1 | FileCheck -check-prefix=CHECK-SLP-VECTORIZE %s
 // RUN: %clang -### -S -fno-tree-slp-vectorize %s 2>&1 | FileCheck -check-prefix=CHECK-NO-SLP-VECTORIZE %s
 // RUN: %clang -### -S -ftree-slp-vectorize -fno-slp-vectorize %s 2>&1 | FileCheck -check-prefix=CHECK-NO-SLP-VECTORIZE %s
-// RUN: %clang -### -S -O %s 2>&1 | FileCheck -check-prefix=CHECK-SLP-VECTORIZE %s
+// RUN: %clang -### -S -O %s 2>&1 | FileCheck -check-prefix=CHECK-NO-SLP-VECTORIZE %s
 // RUN: %clang -### -S -O2 %s 2>&1 | FileCheck -check-prefix=CHECK-SLP-VECTORIZE %s
 // RUN: %clang -### -S -Os %s 2>&1 | FileCheck -check-prefix=CHECK-SLP-VECTORIZE %s
 // RUN: %clang -### -S -Oz %s 2>&1 | FileCheck -check-prefix=CHECK-SLP-VECTORIZE %s
diff --git a/clang/test/Driver/lto.c b/clang/test/Driver/lto.c
--- a/clang/test/Driver/lto.c
+++ b/clang/test/Driver/lto.c
@@ -49,7 +49,7 @@
 // NO-LLVMGOLD-NOT: "-plugin" "{{.*}}{{[/\\]}}LLVMgold.{{dll|dylib|so}}"
 
 // RUN: %clang -target x86_64-unknown-linux-gnu --sysroot %S/Inputs/basic_cross_linux_tree %s \
-// RUN:   -fuse-ld=lld -flto -O -### 2>&1 | FileCheck --check-prefix=O2 %s
+// RUN:   -fuse-ld=lld -flto -O -### 2>&1 | FileCheck --check-prefix=O1 %s
 // RUN: %clang -target x86_64-unknown-linux-gnu --sysroot %S/Inputs/basic_cross_linux_tree %s \
 // RUN:   -fuse-ld=lld -flto -O1 -### 2>&1 | FileCheck --check-prefix=O1 %s
 // RUN: %clang -target x86_64-unknown-linux-gnu --sysroot %S/Inputs/basic_cross_linux_tree %s \