diff --git a/src/all_types.hpp b/src/all_types.hpp
index c386587a68..ea52be51cc 100644
--- a/src/all_types.hpp
+++ b/src/all_types.hpp
@@ -1045,6 +1045,13 @@ enum FnAnalState {
     FnAnalStateSkipped,
 };
 
+
+enum WantPure {
+    WantPureAuto,
+    WantPureFalse,
+    WantPureTrue,
+};
+
 struct FnTableEntry {
     LLVMValueRef fn_value;
     AstNode *proto_node;
@@ -1060,6 +1067,7 @@ struct FnTableEntry {
     bool is_extern;
     bool is_test;
     bool is_pure;
+    WantPure want_pure;
     bool safety_off;
     bool is_noinline;
     BlockContext *parent_block_context;
diff --git a/src/analyze.cpp b/src/analyze.cpp
index b7f325bde3..5eb1be4381 100644
--- a/src/analyze.cpp
+++ b/src/analyze.cpp
@@ -1060,7 +1060,7 @@ static void resolve_function_proto(CodeGen *g, AstNode *node, FnTableEntry *fn_t
                     bool ok = resolve_const_expr_bool(g, import, import->block_context,
                             &directive_node->data.directive.expr, &enable);
                     if (!enable || !ok) {
-                        fn_table_entry->is_pure = false;
+                        fn_table_entry->want_pure = WantPureFalse;
                     }
                     // TODO cause compile error if enable is true and impure fn
                 }
@@ -5153,7 +5153,7 @@ static TypeTableEntry *analyze_fn_call_ptr(CodeGen *g, ImportTableEntry *import,
 
     FnTableEntry *fn_table_entry = node->data.fn_call_expr.fn_entry;
     ConstExprValue *result_val = &get_resolved_expr(node)->const_val;
-    if (ok_invocation && fn_table_entry && fn_table_entry->is_pure) {
+    if (ok_invocation && fn_table_entry && fn_table_entry->is_pure && fn_table_entry->want_pure != WantPureFalse) {
         if (fn_table_entry->anal_state == FnAnalStateReady) {
             analyze_fn_body(g, fn_table_entry);
         }
@@ -5167,7 +5167,7 @@ static TypeTableEntry *analyze_fn_call_ptr(CodeGen *g, ImportTableEntry *import,
             }
         }
     }
-    if (!ok_invocation || !fn_table_entry || !fn_table_entry->is_pure) {
+    if (!ok_invocation || !fn_table_entry || !fn_table_entry->is_pure || fn_table_entry->want_pure == WantPureFalse) {
         // calling an impure fn is impure
         mark_impure_fn(context);
     }
diff --git a/src/codegen.cpp b/src/codegen.cpp
index dc66680ecf..d9a581b570 100644
--- a/src/codegen.cpp
+++ b/src/codegen.cpp
@@ -3885,6 +3885,7 @@ static void do_code_gen(CodeGen *g) {
 
         TypeTableEntry *fn_type = fn_table_entry->type_entry;
 
+        bool is_sret = false;
         if (!type_has_bits(fn_type->data.fn.fn_type_id.return_type)) {
             // nothing to do
         } else if (fn_type->data.fn.fn_type_id.return_type->id == TypeTableEntryIdPointer) {
@@ -3893,7 +3894,12 @@ static void do_code_gen(CodeGen *g) {
             LLVMValueRef first_arg = LLVMGetParam(fn_table_entry->fn_value, 0);
             LLVMAddAttribute(first_arg, LLVMStructRetAttribute);
             LLVMZigAddNonNullAttr(fn_table_entry->fn_value, 1);
+            is_sret = true;
         }
+        if (fn_table_entry->is_pure && !is_sret) {
+            LLVMAddFunctionAttr(fn_table_entry->fn_value, LLVMReadOnlyAttribute);
+        }
+
 
         // set parameter attributes
         for (int param_decl_i = 0; param_decl_i < fn_proto->params.length; param_decl_i += 1) {
@@ -3914,7 +3920,7 @@ static void do_code_gen(CodeGen *g) {
             if (param_type->id == TypeTableEntryIdPointer && param_is_noalias) {
                 LLVMAddAttribute(argument_val, LLVMNoAliasAttribute);
             }
-            if ((param_type->id == TypeTableEntryIdPointer && param_type->data.pointer.is_const) ||
+            if ((param_type->id == TypeTableEntryIdPointer && (param_type->data.pointer.is_const || fn_table_entry->is_pure)) ||
                 is_byval)
             {
                 LLVMAddAttribute(argument_val, LLVMReadOnlyAttribute);
diff --git a/test/run_tests.cpp b/test/run_tests.cpp
index c710c0325b..1d7e82e0b0 100644
--- a/test/run_tests.cpp
+++ b/test/run_tests.cpp
@@ -1410,8 +1410,10 @@ fn baz(a: i32) {}
     )SOURCE");
 
     add_debug_safety_case("integer addition overflow", R"SOURCE(
+error Whatever;
 pub fn main(args: [][]u8) -> %void {
-    add(65530, 10);
+    const x = add(65530, 10);
+    if (x == 0) return error.Whatever;
 }
 #static_eval_enable(false)
 fn add(a: u16, b: u16) -> u16 {
@@ -1420,8 +1422,10 @@ fn add(a: u16, b: u16) -> u16 {
     )SOURCE");
 
     add_debug_safety_case("integer subtraction overflow", R"SOURCE(
+error Whatever;
 pub fn main(args: [][]u8) -> %void {
-    sub(10, 20);
+    const x = sub(10, 20);
+    if (x == 0) return error.Whatever;
 }
 #static_eval_enable(false)
 fn sub(a: u16, b: u16) -> u16 {
@@ -1430,8 +1434,10 @@ fn sub(a: u16, b: u16) -> u16 {
     )SOURCE");
 
     add_debug_safety_case("integer multiplication overflow", R"SOURCE(
+error Whatever;
 pub fn main(args: [][]u8) -> %void {
-    mul(300, 6000);
+    const x = mul(300, 6000);
+    if (x == 0) return error.Whatever;
 }
 #static_eval_enable(false)
 fn mul(a: u16, b: u16) -> u16 {
@@ -1440,8 +1446,10 @@ fn mul(a: u16, b: u16) -> u16 {
     )SOURCE");
 
     add_debug_safety_case("integer negation overflow", R"SOURCE(
+error Whatever;
 pub fn main(args: [][]u8) -> %void {
-    neg(-32768);
+    const x = neg(-32768);
+    if (x == 0) return error.Whatever;
 }
 #static_eval_enable(false)
 fn neg(a: i16) -> i16 {
@@ -1450,8 +1458,10 @@ fn neg(a: i16) -> i16 {
     )SOURCE");
 
     add_debug_safety_case("signed shift left overflow", R"SOURCE(
+error Whatever;
 pub fn main(args: [][]u8) -> %void {
-    shl(-16385, 1);
+    const x = shl(-16385, 1);
+    if (x == 0) return error.Whatever;
 }
 #static_eval_enable(false)
 fn shl(a: i16, b: i16) -> i16 {
@@ -1460,8 +1470,10 @@ fn shl(a: i16, b: i16) -> i16 {
     )SOURCE");
 
     add_debug_safety_case("unsigned shift left overflow", R"SOURCE(
+error Whatever;
 pub fn main(args: [][]u8) -> %void {
-    shl(0b0010111111111111, 3);
+    const x = shl(0b0010111111111111, 3);
+    if (x == 0) return error.Whatever;
 }
 #static_eval_enable(false)
 fn shl(a: u16, b: u16) -> u16 {
@@ -1470,8 +1482,9 @@ fn shl(a: u16, b: u16) -> u16 {
     )SOURCE");
 
     add_debug_safety_case("integer division by zero", R"SOURCE(
+error Whatever;
 pub fn main(args: [][]u8) -> %void {
-    div0(999, 0);
+    const x = div0(999, 0);
 }
 #static_eval_enable(false)
 fn div0(a: i32, b: i32) -> i32 {
@@ -1480,8 +1493,10 @@ fn div0(a: i32, b: i32) -> i32 {
     )SOURCE");
 
     add_debug_safety_case("exact division failure", R"SOURCE(
+error Whatever;
 pub fn main(args: [][]u8) -> %void {
-    div_exact(10, 3);
+    const x = div_exact(10, 3);
+    if (x == 0) return error.Whatever;
 }
 #static_eval_enable(false)
 fn div_exact(a: i32, b: i32) -> i32 {
@@ -1490,8 +1505,10 @@ fn div_exact(a: i32, b: i32) -> i32 {
     )SOURCE");
 
     add_debug_safety_case("cast []u8 to bigger slice of wrong size", R"SOURCE(
+error Whatever;
 pub fn main(args: [][]u8) -> %void {
-    widen_slice([]u8{1, 2, 3, 4, 5});
+    const x = widen_slice([]u8{1, 2, 3, 4, 5});
+    if (x.len == 0) return error.Whatever;
 }
 #static_eval_enable(false)
 fn widen_slice(slice: []u8) -> []i32 {
@@ -1500,8 +1517,10 @@ fn widen_slice(slice: []u8) -> []i32 {
     )SOURCE");
 
     add_debug_safety_case("value does not fit in shortening cast", R"SOURCE(
+error Whatever;
 pub fn main(args: [][]u8) -> %void {
-    shorten_cast(200);
+    const x = shorten_cast(200);
+    if (x == 0) return error.Whatever;
 }
 #static_eval_enable(false)
 fn shorten_cast(x: i32) -> i8 {
@@ -1510,8 +1529,10 @@ fn shorten_cast(x: i32) -> i8 {
     )SOURCE");
 
     add_debug_safety_case("signed integer not fitting in cast to unsigned integer", R"SOURCE(
+error Whatever;
 pub fn main(args: [][]u8) -> %void {
-    unsigned_cast(-10);
+    const x = unsigned_cast(-10);
+    if (x == 0) return error.Whatever;
 }
 #static_eval_enable(false)
 fn unsigned_cast(x: i32) -> u32 {