Index: include/clang/Basic/DiagnosticDriverKinds.td
===================================================================
--- include/clang/Basic/DiagnosticDriverKinds.td
+++ include/clang/Basic/DiagnosticDriverKinds.td
@@ -226,6 +226,8 @@
 def warn_drv_disabling_vptr_no_rtti_default : Warning<
   "implicitly disabling vptr sanitizer because rtti wasn't enabled">,
   InGroup<DiagGroup<"auto-disable-vptr-sanitizer">>;
+def warn_drv_object_size_disabled_O0 : Warning<
+  "the object size sanitizer has no effect at -O0, but is explicitly enabled: %0">;
 
 def note_drv_command_failed_diag_msg : Note<
   "diagnostic msg: %0">;
Index: lib/Driver/SanitizerArgs.cpp
===================================================================
--- lib/Driver/SanitizerArgs.cpp
+++ lib/Driver/SanitizerArgs.cpp
@@ -208,12 +208,28 @@
   SanitizerMask TrappingKinds = parseSanitizeTrapArgs(D, Args);
   SanitizerMask InvalidTrappingKinds = TrappingKinds & NotAllowedWithTrap;
 
+  // The object size sanitizer should not be enabled at -O0.
+  Arg *OptLevel = Args.getLastArg(options::OPT_O_Group);
+  bool RemoveObjectSizeAtO0 =
+      !OptLevel || OptLevel->getOption().matches(options::OPT_O0);
+
   for (ArgList::const_reverse_iterator I = Args.rbegin(), E = Args.rend();
        I != E; ++I) {
     const auto *Arg = *I;
     if (Arg->getOption().matches(options::OPT_fsanitize_EQ)) {
       Arg->claim();
-      SanitizerMask Add = parseArgValues(D, Arg, true);
+      SanitizerMask Add = parseArgValues(D, Arg, /*AllowGroups=*/true);
+
+      if (RemoveObjectSizeAtO0) {
+        AllRemove |= SanitizerKind::ObjectSize;
+
+        // The user explicitly enabled the object size sanitizer. Warn that
+        // that this does nothing at -O0.
+        if (Add & SanitizerKind::ObjectSize)
+          D.Diag(diag::warn_drv_object_size_disabled_O0)
+              << Arg->getAsString(Args);
+      }
+
       AllAddedKinds |= expandSanitizerGroups(Add);
 
       // Avoid diagnosing any sanitizer which is disabled later.
Index: test/Driver/fsanitize-object-size.c
===================================================================
--- /dev/null
+++ test/Driver/fsanitize-object-size.c
@@ -0,0 +1,31 @@
+// Check that the object size check is disabled at -O0.
+//
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=object-size %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-NO-OSIZE
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=object-size %s -O0 -### 2>&1 | FileCheck %s --check-prefix=CHECK-NO-OSIZE
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=null,object-size %s -### 2>&1 | FileCheck %s --check-prefixes=CHECK-NO-OSIZE
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=undefined %s -### 2>&1 | FileCheck %s --check-prefixes=CHECK-NO-OSIZE-NO-WARNING
+
+// Check that the object size check is enabled at other optimization levels.
+//
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=undefined -O1 %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-HAS-OSIZE
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=object-size -O2 %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-HAS-OSIZE
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=object-size -O3 %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-HAS-OSIZE
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=object-size -O4 %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-HAS-OSIZE
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=object-size -Ofast %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-HAS-OSIZE
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=object-size -Os %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-HAS-OSIZE
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=object-size -Oz %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-HAS-OSIZE
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=object-size -Og %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-HAS-OSIZE
+
+// Use of trap mode shouldn't affect the object size check.
+//
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=undefined -fsanitize-trap=undefined -O1 %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-HAS-OSIZE
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=undefined-trap -O1 %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-HAS-OSIZE
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=undefined-trap -fsanitize-undefined-trap-on-error -O1 %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-HAS-OSIZE
+
+// CHECK-HAS-OSIZE-NOT: warning: the object size sanitizer
+// CHECK-HAS-OSIZE: -fsanitize={{[^ ]*}}object-size
+
+// CHECK-NO-OSIZE: warning: the object size sanitizer has no effect at -O0, but is explicitly enabled: -fsanitize={{[^ ]*}}object-size
+// CHECK-NO-OSIZE-NOT: -fsanitize={{[^ ]*}}object-size
+
+// CHECK-NO-OSIZE-NO-WARNING-NOT: -fsanitize={{[^ ]*}}object-size
Index: test/Driver/fsanitize.c
===================================================================
--- test/Driver/fsanitize.c
+++ test/Driver/fsanitize.c
@@ -3,18 +3,18 @@
 // RUN: %clang -target x86_64-linux-gnu -fsanitize=undefined -fsanitize-undefined-trap-on-error %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-UNDEFINED-TRAP
 // RUN: %clang -target x86_64-linux-gnu -fsanitize=undefined-trap -fsanitize-undefined-trap-on-error %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-UNDEFINED-TRAP
 // RUN: %clang -target x86_64-linux-gnu -fsanitize-undefined-trap-on-error -fsanitize=undefined-trap %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-UNDEFINED-TRAP
-// CHECK-UNDEFINED-TRAP: "-fsanitize={{((signed-integer-overflow|integer-divide-by-zero|float-divide-by-zero|shift-base|shift-exponent|unreachable|return|vla-bound|alignment|null|object-size|pointer-overflow|float-cast-overflow|array-bounds|enum|bool|returns-nonnull-attribute|nonnull-attribute|function),?){19}"}}
-// CHECK-UNDEFINED-TRAP: "-fsanitize-trap=alignment,array-bounds,bool,enum,float-cast-overflow,float-divide-by-zero,function,integer-divide-by-zero,nonnull-attribute,null,object-size,pointer-overflow,return,returns-nonnull-attribute,shift-base,shift-exponent,signed-integer-overflow,unreachable,vla-bound"
-// CHECK-UNDEFINED-TRAP2: "-fsanitize-trap=alignment,array-bounds,bool,enum,float-cast-overflow,float-divide-by-zero,function,integer-divide-by-zero,nonnull-attribute,null,object-size,pointer-overflow,return,returns-nonnull-attribute,shift-base,shift-exponent,unreachable,vla-bound"
+// CHECK-UNDEFINED-TRAP: "-fsanitize={{((signed-integer-overflow|integer-divide-by-zero|float-divide-by-zero|shift-base|shift-exponent|unreachable|return|vla-bound|alignment|null|pointer-overflow|float-cast-overflow|array-bounds|enum|bool|returns-nonnull-attribute|nonnull-attribute|function),?){18}"}}
+// CHECK-UNDEFINED-TRAP: "-fsanitize-trap=alignment,array-bounds,bool,enum,float-cast-overflow,float-divide-by-zero,function,integer-divide-by-zero,nonnull-attribute,null,pointer-overflow,return,returns-nonnull-attribute,shift-base,shift-exponent,signed-integer-overflow,unreachable,vla-bound"
+// CHECK-UNDEFINED-TRAP2: "-fsanitize-trap=alignment,array-bounds,bool,enum,float-cast-overflow,float-divide-by-zero,function,integer-divide-by-zero,nonnull-attribute,null,pointer-overflow,return,returns-nonnull-attribute,shift-base,shift-exponent,unreachable,vla-bound"
 
 // RUN: %clang -target x86_64-linux-gnu -fsanitize=undefined %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-UNDEFINED
-// CHECK-UNDEFINED: "-fsanitize={{((signed-integer-overflow|integer-divide-by-zero|float-divide-by-zero|function|shift-base|shift-exponent|unreachable|return|vla-bound|alignment|null|vptr|object-size|pointer-overflow|float-cast-overflow|array-bounds|enum|bool|returns-nonnull-attribute|nonnull-attribute),?){20}"}}
+// CHECK-UNDEFINED: "-fsanitize={{((signed-integer-overflow|integer-divide-by-zero|float-divide-by-zero|function|shift-base|shift-exponent|unreachable|return|vla-bound|alignment|null|vptr|pointer-overflow|float-cast-overflow|array-bounds|enum|bool|returns-nonnull-attribute|nonnull-attribute),?){19}"}}
 
 // RUN: %clang -target x86_64-apple-darwin10 -fsanitize=undefined %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-UNDEFINED-DARWIN
-// CHECK-UNDEFINED-DARWIN: "-fsanitize={{((signed-integer-overflow|integer-divide-by-zero|float-divide-by-zero|shift-base|shift-exponent|unreachable|return|vla-bound|alignment|null|object-size|pointer-overflow|float-cast-overflow|array-bounds|enum|bool|returns-nonnull-attribute|nonnull-attribute),?){18}"}}
+// CHECK-UNDEFINED-DARWIN: "-fsanitize={{((signed-integer-overflow|integer-divide-by-zero|float-divide-by-zero|shift-base|shift-exponent|unreachable|return|vla-bound|alignment|null|pointer-overflow|float-cast-overflow|array-bounds|enum|bool|returns-nonnull-attribute|nonnull-attribute),?){17}"}}
 
 // RUN: %clang -target i386-unknown-openbsd -fsanitize=undefined %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-UNDEFINED-OPENBSD
-// CHECK-UNDEFINED-OPENBSD: "-fsanitize={{((signed-integer-overflow|integer-divide-by-zero|float-divide-by-zero|shift-base|shift-exponent|unreachable|return|vla-bound|alignment|null|object-size|pointer-overflow|float-cast-overflow|array-bounds|enum|bool|returns-nonnull-attribute|nonnull-attribute),?){18}"}}
+// CHECK-UNDEFINED-OPENBSD: "-fsanitize={{((signed-integer-overflow|integer-divide-by-zero|float-divide-by-zero|shift-base|shift-exponent|unreachable|return|vla-bound|alignment|null|pointer-overflow|float-cast-overflow|array-bounds|enum|bool|returns-nonnull-attribute|nonnull-attribute),?){17}"}}
 
 // RUN: %clang -target i386-pc-win32 -fsanitize=undefined %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-UNDEFINED-WIN --check-prefix=CHECK-UNDEFINED-WIN32
 // RUN: %clang -target i386-pc-win32 -fsanitize=undefined -x c++ %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-UNDEFINED-WIN --check-prefix=CHECK-UNDEFINED-WIN32 --check-prefix=CHECK-UNDEFINED-WIN-CXX
@@ -23,7 +23,7 @@
 // CHECK-UNDEFINED-WIN32: "--dependent-lib={{[^"]*}}ubsan_standalone-i386.lib"
 // CHECK-UNDEFINED-WIN64: "--dependent-lib={{[^"]*}}ubsan_standalone-x86_64.lib"
 // CHECK-UNDEFINED-WIN-CXX: "--dependent-lib={{[^"]*}}ubsan_standalone_cxx{{[^"]*}}.lib"
-// CHECK-UNDEFINED-WIN-SAME: "-fsanitize={{((signed-integer-overflow|integer-divide-by-zero|float-divide-by-zero|shift-base|shift-exponent|unreachable|return|vla-bound|alignment|null|object-size|pointer-overflow|float-cast-overflow|array-bounds|enum|bool|returns-nonnull-attribute|nonnull-attribute),?){18}"}}
+// CHECK-UNDEFINED-WIN-SAME: "-fsanitize={{((signed-integer-overflow|integer-divide-by-zero|float-divide-by-zero|shift-base|shift-exponent|unreachable|return|vla-bound|alignment|null|pointer-overflow|float-cast-overflow|array-bounds|enum|bool|returns-nonnull-attribute|nonnull-attribute),?){17}"}}
 
 // RUN: %clang -target i386-pc-win32 -fsanitize-coverage=bb %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-COVERAGE-WIN32
 // CHECK-COVERAGE-WIN32: "--dependent-lib={{[^"]*}}ubsan_standalone-i386.lib"
@@ -43,7 +43,7 @@
 // CHECK-FNO-SANITIZE-ALL: "-fsanitize=thread"
 
 // RUN: %clang -target x86_64-linux-gnu -fsanitize=thread,undefined -fno-sanitize=thread -fno-sanitize=float-cast-overflow,vptr,bool,enum %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-PARTIAL-UNDEFINED
-// CHECK-PARTIAL-UNDEFINED: "-fsanitize={{((signed-integer-overflow|integer-divide-by-zero|float-divide-by-zero|function|shift-base|shift-exponent|unreachable|return|vla-bound|alignment|null|object-size|pointer-overflow|array-bounds|returns-nonnull-attribute|nonnull-attribute),?){16}"}}
+// CHECK-PARTIAL-UNDEFINED: "-fsanitize={{((signed-integer-overflow|integer-divide-by-zero|float-divide-by-zero|function|shift-base|shift-exponent|unreachable|return|vla-bound|alignment|null|pointer-overflow|array-bounds|returns-nonnull-attribute|nonnull-attribute),?){15}"}}
 
 // RUN: %clang -target x86_64-linux-gnu -fsanitize=shift -fno-sanitize=shift-base %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-FSANITIZE-SHIFT-PARTIAL
 // CHECK-FSANITIZE-SHIFT-PARTIAL: "-fsanitize=shift-exponent"
@@ -217,11 +217,11 @@
 // RUN: %clang -target x86_64-linux-gnu %s -fsanitize=undefined -fno-sanitize-recover=undefined -### 2>&1 | FileCheck %s --check-prefix=CHECK-NO-RECOVER-UBSAN
 // RUN: %clang -target x86_64-linux-gnu %s -fsanitize=undefined -fno-sanitize-recover=all -fsanitize-recover=thread -### 2>&1 | FileCheck %s --check-prefix=CHECK-NO-RECOVER-UBSAN
 // RUN: %clang -target x86_64-linux-gnu %s -fsanitize=undefined -fsanitize-recover=all -fno-sanitize-recover=undefined -### 2>&1 | FileCheck %s --check-prefix=CHECK-NO-RECOVER-UBSAN
-// CHECK-RECOVER-UBSAN: "-fsanitize-recover={{((signed-integer-overflow|integer-divide-by-zero|float-divide-by-zero|function|shift-base|shift-exponent|vla-bound|alignment|null|vptr|object-size|pointer-overflow|float-cast-overflow|array-bounds|enum|bool|returns-nonnull-attribute|nonnull-attribute),?){18}"}}
+// CHECK-RECOVER-UBSAN: "-fsanitize-recover={{((signed-integer-overflow|integer-divide-by-zero|float-divide-by-zero|function|shift-base|shift-exponent|vla-bound|alignment|null|vptr|pointer-overflow|float-cast-overflow|array-bounds|enum|bool|returns-nonnull-attribute|nonnull-attribute),?){17}"}}
 // CHECK-NO-RECOVER-UBSAN-NOT: sanitize-recover
 
 // RUN: %clang -target x86_64-linux-gnu %s -fsanitize=undefined -fno-sanitize-recover=all -fsanitize-recover=object-size,shift-base -### 2>&1 | FileCheck %s --check-prefix=CHECK-PARTIAL-RECOVER
-// CHECK-PARTIAL-RECOVER: "-fsanitize-recover={{((object-size|shift-base),?){2}"}}
+// CHECK-PARTIAL-RECOVER: "-fsanitize-recover={{((shift-base),?){1}"}}
 
 // RUN: %clang -target x86_64-linux-gnu %s -fsanitize=address -fsanitize-recover=all -### 2>&1 | FileCheck %s --check-prefix=CHECK-RECOVER-ASAN
 // CHECK-RECOVER-ASAN: "-fsanitize-recover=address"