diff --git a/reference/shaders-hlsl/frag/partial-write-preserve.frag b/reference/shaders-hlsl/frag/partial-write-preserve.frag
new file mode 100644
index 00000000..abbd8769
--- /dev/null
+++ b/reference/shaders-hlsl/frag/partial-write-preserve.frag
@@ -0,0 +1,78 @@
+struct B
+{
+    float a;
+    float b;
+};
+
+struct _UBO
+{
+    int some_value;
+};
+
+cbuffer UBO : register(c0)
+{
+    _UBO _42;
+};
+
+void partial_inout(inout float4 x)
+{
+    x.x = 10.0f;
+}
+
+void complete_inout(out float4 x)
+{
+    x = float4(50.0f, 50.0f, 50.0f, 50.0f);
+}
+
+void branchy_inout(inout float4 v)
+{
+    v.y = 20.0f;
+    if (_42.some_value == 20)
+    {
+        v = float4(50.0f, 50.0f, 50.0f, 50.0f);
+    }
+}
+
+void branchy_inout_2(out float4 v)
+{
+    if (_42.some_value == 20)
+    {
+        v = float4(50.0f, 50.0f, 50.0f, 50.0f);
+    }
+    else
+    {
+        v = float4(70.0f, 70.0f, 70.0f, 70.0f);
+    }
+    v.y = 20.0f;
+}
+
+void partial_inout(inout B b)
+{
+    b.b = 40.0f;
+}
+
+void frag_main()
+{
+    float4 a = float4(10.0f, 10.0f, 10.0f, 10.0f);
+    float4 param = a;
+    partial_inout(param);
+    a = param;
+    float4 param_1;
+    complete_inout(param_1);
+    a = param_1;
+    float4 param_2 = a;
+    branchy_inout(param_2);
+    a = param_2;
+    float4 param_3;
+    branchy_inout_2(param_3);
+    a = param_3;
+    B b = { 10.0f, 20.0f };
+    B param_4 = b;
+    partial_inout(param_4);
+    b = param_4;
+}
+
+void main()
+{
+    frag_main();
+}
diff --git a/reference/shaders/comp/inout-struct.invalid.comp b/reference/shaders/comp/inout-struct.invalid.comp
index eec37a19..640e25bb 100644
--- a/reference/shaders/comp/inout-struct.invalid.comp
+++ b/reference/shaders/comp/inout-struct.invalid.comp
@@ -24,7 +24,7 @@ layout(binding = 2, std430) readonly buffer SSBO3
     Foo foos[];
 } foobar;
 
-void baz(out Foo foo)
+void baz(inout Foo foo)
 {
     uint ident = gl_GlobalInvocationID.x;
     foo.a = indata.data[(4u * ident) + 0u];
diff --git a/reference/shaders/frag/flush_params.frag b/reference/shaders/frag/flush_params.frag
index ee99395a..b4b36ff9 100644
--- a/reference/shaders/frag/flush_params.frag
+++ b/reference/shaders/frag/flush_params.frag
@@ -9,7 +9,7 @@ struct Structy
 
 layout(location = 0) out vec4 FragColor;
 
-void foo2(out Structy f)
+void foo2(inout Structy f)
 {
     f.c = vec4(10.0);
 }
diff --git a/reference/shaders/frag/partial-write-preserve.frag b/reference/shaders/frag/partial-write-preserve.frag
new file mode 100644
index 00000000..cf8a83cf
--- /dev/null
+++ b/reference/shaders/frag/partial-write-preserve.frag
@@ -0,0 +1,109 @@
+#version 310 es
+precision mediump float;
+precision highp int;
+
+struct B
+{
+    float a;
+    float b;
+};
+
+layout(binding = 0, std140) uniform UBO
+{
+    mediump int some_value;
+} _51;
+
+void partial_inout(inout vec4 x)
+{
+    x.x = 10.0;
+}
+
+void complete_inout(out vec4 x)
+{
+    x = vec4(50.0);
+}
+
+void branchy_inout(inout vec4 v)
+{
+    v.y = 20.0;
+    if (_51.some_value == 20)
+    {
+        v = vec4(50.0);
+    }
+}
+
+void branchy_inout_2(out vec4 v)
+{
+    if (_51.some_value == 20)
+    {
+        v = vec4(50.0);
+    }
+    else
+    {
+        v = vec4(70.0);
+    }
+    v.y = 20.0;
+}
+
+void partial_inout(inout B b)
+{
+    b.b = 40.0;
+}
+
+void complete_inout(out B b)
+{
+    b = B(100.0, 200.0);
+}
+
+void branchy_inout(inout B b)
+{
+    b.b = 20.0;
+    if (_51.some_value == 20)
+    {
+        b = B(10.0, 40.0);
+    }
+}
+
+void branchy_inout_2(out B b)
+{
+    if (_51.some_value == 20)
+    {
+        b = B(10.0, 40.0);
+    }
+    else
+    {
+        b = B(70.0, 70.0);
+    }
+    b.b = 20.0;
+}
+
+void main()
+{
+    vec4 a = vec4(10.0);
+    highp vec4 param = a;
+    partial_inout(param);
+    a = param;
+    highp vec4 param_1;
+    complete_inout(param_1);
+    a = param_1;
+    highp vec4 param_2 = a;
+    branchy_inout(param_2);
+    a = param_2;
+    highp vec4 param_3;
+    branchy_inout_2(param_3);
+    a = param_3;
+    B b = B(10.0, 20.0);
+    B param_4 = b;
+    partial_inout(param_4);
+    b = param_4;
+    B param_5;
+    complete_inout(param_5);
+    b = param_5;
+    B param_6 = b;
+    branchy_inout(param_6);
+    b = param_6;
+    B param_7;
+    branchy_inout_2(param_7);
+    b = param_7;
+}
+
diff --git a/shaders-hlsl/frag/partial-write-preserve.frag b/shaders-hlsl/frag/partial-write-preserve.frag
new file mode 100644
index 00000000..f30270b9
--- /dev/null
+++ b/shaders-hlsl/frag/partial-write-preserve.frag
@@ -0,0 +1,64 @@
+#version 310 es
+precision mediump float;
+
+layout(std140, binding = 0) uniform UBO
+{
+	int some_value;
+};
+
+struct B
+{
+	float a;
+	float b;
+};
+
+void partial_inout(inout vec4 x)
+{
+	x.x = 10.0;
+}
+
+void partial_inout(inout B b)
+{
+	b.b = 40.0;
+}
+
+// Make a complete write, but only conditionally ...
+void branchy_inout(inout vec4 v)
+{
+	v.y = 20.0;
+	if (some_value == 20)
+	{
+		v = vec4(50.0);
+	}
+}
+
+void branchy_inout_2(out vec4 v)
+{
+	if (some_value == 20)
+	{
+		v = vec4(50.0);
+	}
+	else
+	{
+		v = vec4(70.0);
+	}
+	v.y = 20.0;
+}
+
+void complete_inout(out vec4 x)
+{
+	x = vec4(50.0);
+}
+
+void main()
+{
+	vec4 a = vec4(10.0);
+	partial_inout(a);
+	complete_inout(a);
+	branchy_inout(a);
+	branchy_inout_2(a);
+
+	B b = B(10.0, 20.0);
+	partial_inout(b);
+}
+
diff --git a/shaders/frag/partial-write-preserve.frag b/shaders/frag/partial-write-preserve.frag
new file mode 100644
index 00000000..227df950
--- /dev/null
+++ b/shaders/frag/partial-write-preserve.frag
@@ -0,0 +1,95 @@
+#version 310 es
+precision mediump float;
+
+layout(std140, binding = 0) uniform UBO
+{
+	int some_value;
+};
+
+struct B
+{
+	float a;
+	float b;
+};
+
+void partial_inout(inout vec4 x)
+{
+	x.x = 10.0;
+}
+
+void partial_inout(inout B b)
+{
+	b.b = 40.0;
+}
+
+// Make a complete write, but only conditionally ...
+void branchy_inout(inout vec4 v)
+{
+	v.y = 20.0;
+	if (some_value == 20)
+	{
+		v = vec4(50.0);
+	}
+}
+
+void branchy_inout(inout B b)
+{
+	b.b = 20.0;
+	if (some_value == 20)
+	{
+		b = B(10.0, 40.0);
+	}
+}
+
+void branchy_inout_2(out vec4 v)
+{
+	if (some_value == 20)
+	{
+		v = vec4(50.0);
+	}
+	else
+	{
+		v = vec4(70.0);
+	}
+	v.y = 20.0;
+}
+
+void branchy_inout_2(out B b)
+{
+	if (some_value == 20)
+	{
+		b = B(10.0, 40.0);
+	}
+	else
+	{
+		b = B(70.0, 70.0);
+	}
+	b.b = 20.0;
+}
+
+
+void complete_inout(out vec4 x)
+{
+	x = vec4(50.0);
+}
+
+void complete_inout(out B b)
+{
+	b = B(100.0, 200.0);
+}
+
+void main()
+{
+	vec4 a = vec4(10.0);
+	partial_inout(a);
+	complete_inout(a);
+	branchy_inout(a);
+	branchy_inout_2(a);
+
+	B b = B(10.0, 20.0);
+	partial_inout(b);
+	complete_inout(b);
+	branchy_inout(b);
+	branchy_inout_2(b);
+}
+
diff --git a/spirv_cross.cpp b/spirv_cross.cpp
index 4fcdebcf..6c6b86c5 100644
--- a/spirv_cross.cpp
+++ b/spirv_cross.cpp
@@ -2923,7 +2923,8 @@ static bool exists_unaccessed_path_to_return(const CFG &cfg, uint32_t block, con
 }
 
 void Compiler::analyze_parameter_preservation(
-    SPIRFunction &entry, const CFG &cfg, const unordered_map<uint32_t, unordered_set<uint32_t>> &variable_to_blocks)
+    SPIRFunction &entry, const CFG &cfg, const unordered_map<uint32_t, unordered_set<uint32_t>> &variable_to_blocks,
+    const unordered_map<uint32_t, unordered_set<uint32_t>> &complete_write_blocks)
 {
 	for (auto &arg : entry.arguments)
 	{
@@ -2958,7 +2959,16 @@ void Compiler::analyze_parameter_preservation(
 			continue;
 		}
 
-		// If there is a path through the CFG where no block writes to the variable, the variable will be in an undefined state
+		// We have accessed a variable, but there was no complete writes to that variable.
+		// We deduce that we must preserve the argument.
+		itr = complete_write_blocks.find(arg.id);
+		if (itr == end(complete_write_blocks))
+		{
+			arg.read_count++;
+			continue;
+		}
+
+		// If there is a path through the CFG where no block completely writes to the variable, the variable will be in an undefined state
 		// when the function returns. We therefore need to implicitly preserve the variable in case there are writers in the function.
 		// Major case here is if a function is
 		// void foo(int &var) { if (cond) var = 10; }
@@ -3036,6 +3046,10 @@ void Compiler::analyze_variable_scope(SPIRFunction &entry)
 				auto *var = compiler.maybe_get_backing_variable(ptr);
 				if (var && var->storage == StorageClassFunction)
 					accessed_variables_to_block[var->self].insert(current_block->self);
+
+				// If we store through an access chain, we have a partial write.
+				if (var && var->self == ptr && var->storage == StorageClassFunction)
+					complete_write_variables_to_block[var->self].insert(current_block->self);
 				break;
 			}
 
@@ -3063,6 +3077,10 @@ void Compiler::analyze_variable_scope(SPIRFunction &entry)
 				if (var && var->storage == StorageClassFunction)
 					accessed_variables_to_block[var->self].insert(current_block->self);
 
+				// If we store through an access chain, we have a partial write.
+				if (var->self == lhs)
+					complete_write_variables_to_block[var->self].insert(current_block->self);
+
 				var = compiler.maybe_get_backing_variable(rhs);
 				if (var && var->storage == StorageClassFunction)
 					accessed_variables_to_block[var->self].insert(current_block->self);
@@ -3103,6 +3121,10 @@ void Compiler::analyze_variable_scope(SPIRFunction &entry)
 					auto *var = compiler.maybe_get_backing_variable(args[i]);
 					if (var && var->storage == StorageClassFunction)
 						accessed_variables_to_block[var->self].insert(current_block->self);
+
+					// Cannot easily prove if argument we pass to a function is completely written.
+					// Usually, functions write to a dummy variable,
+					// which is then copied to in full to the real argument.
 				}
 				break;
 			}
@@ -3128,6 +3150,7 @@ void Compiler::analyze_variable_scope(SPIRFunction &entry)
 
 		Compiler &compiler;
 		std::unordered_map<uint32_t, std::unordered_set<uint32_t>> accessed_variables_to_block;
+		std::unordered_map<uint32_t, std::unordered_set<uint32_t>> complete_write_variables_to_block;
 		const SPIRBlock *current_block = nullptr;
 	} handler(*this);
 
@@ -3139,7 +3162,8 @@ void Compiler::analyze_variable_scope(SPIRFunction &entry)
 	CFG cfg(*this, entry);
 
 	// Analyze if there are parameters which need to be implicitly preserved with an "in" qualifier.
-	analyze_parameter_preservation(entry, cfg, handler.accessed_variables_to_block);
+	analyze_parameter_preservation(entry, cfg, handler.accessed_variables_to_block,
+	                               handler.complete_write_variables_to_block);
 
 	unordered_map<uint32_t, uint32_t> potential_loop_variables;
 
diff --git a/spirv_cross.hpp b/spirv_cross.hpp
index 071075e6..346ef9bc 100644
--- a/spirv_cross.hpp
+++ b/spirv_cross.hpp
@@ -644,7 +644,8 @@ protected:
 
 	void analyze_parameter_preservation(
 	    SPIRFunction &entry, const CFG &cfg,
-	    const std::unordered_map<uint32_t, std::unordered_set<uint32_t>> &variable_to_blocks);
+	    const std::unordered_map<uint32_t, std::unordered_set<uint32_t>> &variable_to_blocks,
+	    const std::unordered_map<uint32_t, std::unordered_set<uint32_t>> &complete_write_blocks);
 
 	// If a variable ID or parameter ID is found in this set, a sampler is actually a shadow/comparison sampler.
 	// SPIR-V does not support this distinction, so we must keep track of this information outside the type system.