diff --git a/Test/baseResults/spv.subgroupSizeARB.frag.out b/Test/baseResults/spv.subgroupSizeARB.frag.out
new file mode 100644
index 00000000..7ce78183
--- /dev/null
+++ b/Test/baseResults/spv.subgroupSizeARB.frag.out
@@ -0,0 +1,34 @@
+spv.subgroupSizeARB.frag
+// Module Version 10000
+// Generated by (magic number): 8000a
+// Id's are bound by 12
+
+                              Capability Shader
+                              Capability SubgroupBallotKHR
+                              Extension  "SPV_KHR_shader_ballot"
+               1:             ExtInstImport  "GLSL.std.450"
+                              MemoryModel Logical GLSL450
+                              EntryPoint Fragment 4  "main" 8 10
+                              ExecutionMode 4 OriginUpperLeft
+                              Source GLSL 450
+                              SourceExtension  "GL_ARB_shader_ballot"
+                              SourceExtension  "GL_KHR_shader_subgroup_basic"
+                              Name 4  "main"
+                              Name 8  "result"
+                              Name 10  "gl_SubGroupSizeARB"
+                              Decorate 8(result) Location 0
+                              Decorate 10(gl_SubGroupSizeARB) Flat
+                              Decorate 10(gl_SubGroupSizeARB) BuiltIn SubgroupSize
+               2:             TypeVoid
+               3:             TypeFunction 2
+               6:             TypeInt 32 0
+               7:             TypePointer Output 6(int)
+       8(result):      7(ptr) Variable Output
+               9:             TypePointer Input 6(int)
+10(gl_SubGroupSizeARB):      9(ptr) Variable Input
+         4(main):           2 Function None 3
+               5:             Label
+              11:      6(int) Load 10(gl_SubGroupSizeARB)
+                              Store 8(result) 11
+                              Return
+                              FunctionEnd
diff --git a/Test/spv.subgroupSizeARB.frag b/Test/spv.subgroupSizeARB.frag
new file mode 100644
index 00000000..45a1701d
--- /dev/null
+++ b/Test/spv.subgroupSizeARB.frag
@@ -0,0 +1,10 @@
+#version 450
+#extension GL_ARB_shader_ballot : enable
+#extension GL_KHR_shader_subgroup_basic : enable
+
+layout(location = 0) out uint result;
+
+void main (void)
+{
+  result = gl_SubGroupSizeARB;
+}
diff --git a/glslang/MachineIndependent/Initialize.cpp b/glslang/MachineIndependent/Initialize.cpp
index 35681a5f..31b448ad 100644
--- a/glslang/MachineIndependent/Initialize.cpp
+++ b/glslang/MachineIndependent/Initialize.cpp
@@ -7653,6 +7653,23 @@ static void SpecialQualifier(const char* name, TStorageQualifier qualifier, TBui
     symQualifier.builtIn = builtIn;
 }
 
+//
+// Modify the symbol's flat decoration.
+//
+// Safe to call even if name is not present.
+//
+// Originally written to transform gl_SubGroupSizeARB from uniform to fragment input in Vulkan.
+//
+static void ModifyFlatDecoration(const char* name, bool flat, TSymbolTable& symbolTable)
+{
+    TSymbol* symbol = symbolTable.find(name);
+    if (symbol == nullptr)
+        return;
+
+    TQualifier& symQualifier = symbol->getWritableType().getQualifier();
+    symQualifier.flat = flat;
+}
+
 //
 // To tag built-in variables with their TBuiltInVariable enum.  Use this when the
 // normal declaration text already gets the qualifier right, and all that's needed
@@ -8036,9 +8053,12 @@ void TBuiltIns::identifyBuiltIns(int version, EProfile profile, const SpvVersion
             BuiltInVariable("gl_SubGroupLeMaskARB",     EbvSubGroupLeMask,     symbolTable);
             BuiltInVariable("gl_SubGroupLtMaskARB",     EbvSubGroupLtMask,     symbolTable);
 
-            if (spvVersion.vulkan > 0)
+            if (spvVersion.vulkan > 0) {
                 // Treat "gl_SubGroupSizeARB" as shader input instead of uniform for Vulkan
                 SpecialQualifier("gl_SubGroupSizeARB", EvqVaryingIn, EbvSubGroupSize, symbolTable);
+                if (language == EShLangFragment)
+                    ModifyFlatDecoration("gl_SubGroupSizeARB", true, symbolTable);
+            }
             else
                 BuiltInVariable("gl_SubGroupSizeARB", EbvSubGroupSize, symbolTable);
         }
@@ -8470,9 +8490,12 @@ void TBuiltIns::identifyBuiltIns(int version, EProfile profile, const SpvVersion
             BuiltInVariable("gl_SubGroupLeMaskARB",     EbvSubGroupLeMask,     symbolTable);
             BuiltInVariable("gl_SubGroupLtMaskARB",     EbvSubGroupLtMask,     symbolTable);
 
-            if (spvVersion.vulkan > 0)
+            if (spvVersion.vulkan > 0) {
                 // Treat "gl_SubGroupSizeARB" as shader input instead of uniform for Vulkan
                 SpecialQualifier("gl_SubGroupSizeARB", EvqVaryingIn, EbvSubGroupSize, symbolTable);
+                if (language == EShLangFragment)
+                    ModifyFlatDecoration("gl_SubGroupSizeARB", true, symbolTable);
+            }
             else
                 BuiltInVariable("gl_SubGroupSizeARB", EbvSubGroupSize, symbolTable);
         }
@@ -8687,9 +8710,12 @@ void TBuiltIns::identifyBuiltIns(int version, EProfile profile, const SpvVersion
             BuiltInVariable("gl_SubGroupLeMaskARB",     EbvSubGroupLeMask,     symbolTable);
             BuiltInVariable("gl_SubGroupLtMaskARB",     EbvSubGroupLtMask,     symbolTable);
 
-            if (spvVersion.vulkan > 0)
+            if (spvVersion.vulkan > 0) {
                 // Treat "gl_SubGroupSizeARB" as shader input instead of uniform for Vulkan
                 SpecialQualifier("gl_SubGroupSizeARB", EvqVaryingIn, EbvSubGroupSize, symbolTable);
+                if (language == EShLangFragment)
+                    ModifyFlatDecoration("gl_SubGroupSizeARB", true, symbolTable);
+            }
             else
                 BuiltInVariable("gl_SubGroupSizeARB", EbvSubGroupSize, symbolTable);
         }
@@ -8877,9 +8903,12 @@ void TBuiltIns::identifyBuiltIns(int version, EProfile profile, const SpvVersion
             BuiltInVariable("gl_SubGroupLeMaskARB",     EbvSubGroupLeMask,     symbolTable);
             BuiltInVariable("gl_SubGroupLtMaskARB",     EbvSubGroupLtMask,     symbolTable);
 
-            if (spvVersion.vulkan > 0)
+            if (spvVersion.vulkan > 0) {
                 // Treat "gl_SubGroupSizeARB" as shader input instead of uniform for Vulkan
                 SpecialQualifier("gl_SubGroupSizeARB", EvqVaryingIn, EbvSubGroupSize, symbolTable);
+                if (language == EShLangFragment)
+                    ModifyFlatDecoration("gl_SubGroupSizeARB", true, symbolTable);
+            }
             else
                 BuiltInVariable("gl_SubGroupSizeARB", EbvSubGroupSize, symbolTable);
 
@@ -9075,9 +9104,12 @@ void TBuiltIns::identifyBuiltIns(int version, EProfile profile, const SpvVersion
             BuiltInVariable("gl_SubGroupLeMaskARB",     EbvSubGroupLeMask,     symbolTable);
             BuiltInVariable("gl_SubGroupLtMaskARB",     EbvSubGroupLtMask,     symbolTable);
 
-            if (spvVersion.vulkan > 0)
+            if (spvVersion.vulkan > 0) {
                 // Treat "gl_SubGroupSizeARB" as shader input instead of uniform for Vulkan
                 SpecialQualifier("gl_SubGroupSizeARB", EvqVaryingIn, EbvSubGroupSize, symbolTable);
+                if (language == EShLangFragment)
+                    ModifyFlatDecoration("gl_SubGroupSizeARB", true, symbolTable);
+            }
             else
                 BuiltInVariable("gl_SubGroupSizeARB", EbvSubGroupSize, symbolTable);
         }
@@ -9202,9 +9234,12 @@ void TBuiltIns::identifyBuiltIns(int version, EProfile profile, const SpvVersion
             BuiltInVariable("gl_SubGroupLeMaskARB",     EbvSubGroupLeMask,     symbolTable);
             BuiltInVariable("gl_SubGroupLtMaskARB",     EbvSubGroupLtMask,     symbolTable);
 
-            if (spvVersion.vulkan > 0)
+            if (spvVersion.vulkan > 0) {
                 // Treat "gl_SubGroupSizeARB" as shader input instead of uniform for Vulkan
                 SpecialQualifier("gl_SubGroupSizeARB", EvqVaryingIn, EbvSubGroupSize, symbolTable);
+                if (language == EShLangFragment)
+                    ModifyFlatDecoration("gl_SubGroupSizeARB", true, symbolTable);
+            }
             else
                 BuiltInVariable("gl_SubGroupSizeARB", EbvSubGroupSize, symbolTable);
         }
diff --git a/gtests/Spv.FromFile.cpp b/gtests/Spv.FromFile.cpp
index ed0f5cad..93e364d9 100644
--- a/gtests/Spv.FromFile.cpp
+++ b/gtests/Spv.FromFile.cpp
@@ -480,6 +480,7 @@ INSTANTIATE_TEST_SUITE_P(
         "spv.storageBuffer.vert",
         "spv.terminate.frag",
         "spv.subgroupUniformControlFlow.vert",
+        "spv.subgroupSizeARB.frag",
         "spv.precise.tese",
         "spv.precise.tesc",
         "spv.viewportindex.tese",