error return trace pointer prefixes other params

instead of being last. This increases the chances that it can
remain in the same register between calls.

2 files changed, 32 insertions, 20 deletions

diff --git a/src/analyze.cpp b/src/analyze.cpp
index b52cb019d3..f7c0f832a2 100644
--- a/src/analyze.cpp
+++ b/src/analyze.cpp
@@ -925,7 +925,7 @@ TypeTableEntry *get_fn_type(CodeGen *g, FnTypeId *fn_type_id) {
     if (!skip_debug_info) {
         bool first_arg_return = calling_convention_does_first_arg_return(fn_type_id->cc) &&
             handle_is_ptr(fn_type_id->return_type);
-        bool last_arg_error_return_trace = fn_type_id->return_type->id == TypeTableEntryIdErrorUnion || 
+        bool prefix_arg_error_return_trace = fn_type_id->return_type->id == TypeTableEntryIdErrorUnion || 
             fn_type_id->return_type->id == TypeTableEntryIdPureError;
         // +1 for maybe making the first argument the return value
         // +1 for maybe last argument the error return trace
@@ -951,6 +951,14 @@ TypeTableEntry *get_fn_type(CodeGen *g, FnTypeId *fn_type_id) {
         }
         fn_type->data.fn.gen_return_type = gen_return_type;
 
+        if (prefix_arg_error_return_trace) {
+            TypeTableEntry *gen_type = get_ptr_to_stack_trace_type(g);
+            gen_param_types[gen_param_index] = gen_type->type_ref;
+            gen_param_index += 1;
+            // after the gen_param_index += 1 because 0 is the return type
+            param_di_types[gen_param_index] = gen_type->di_type;
+        }
+
         fn_type->data.fn.gen_param_info = allocate<FnGenParamInfo>(fn_type_id->param_count);
         for (size_t i = 0; i < fn_type_id->param_count; i += 1) {
             FnTypeParamInfo *src_param_info = &fn_type->data.fn.fn_type_id.param_info[i];
@@ -980,14 +988,6 @@ TypeTableEntry *get_fn_type(CodeGen *g, FnTypeId *fn_type_id) {
             }
         }
 
-        if (last_arg_error_return_trace) {
-            TypeTableEntry *gen_type = get_ptr_to_stack_trace_type(g);
-            gen_param_types[gen_param_index] = gen_type->type_ref;
-            gen_param_index += 1;
-            // after the gen_param_index += 1 because 0 is the return type
-            param_di_types[gen_param_index] = gen_type->di_type;
-        }
-
         fn_type->data.fn.gen_param_count = gen_param_index;
 
         fn_type->data.fn.raw_type_ref = LLVMFunctionType(gen_return_type->type_ref,
diff --git a/src/codegen.cpp b/src/codegen.cpp
index 97cab9a69f..8b2cf2bcd4 100644
--- a/src/codegen.cpp
+++ b/src/codegen.cpp
@@ -406,6 +406,16 @@ static LLVMLinkage to_llvm_linkage(GlobalLinkageId id) {
     zig_unreachable();
 }
 
+static uint32_t get_err_ret_trace_arg_index(FnTableEntry *fn_table_entry) {
+    TypeTableEntry *fn_type = fn_table_entry->type_entry;
+    TypeTableEntry *return_type = fn_type->data.fn.fn_type_id.return_type;
+    if (return_type->id != TypeTableEntryIdErrorUnion && return_type->id != TypeTableEntryIdPureError) {
+        return UINT32_MAX;
+    }
+    bool first_arg_ret = type_has_bits(return_type) && handle_is_ptr(return_type);
+    return first_arg_ret ? 1 : 0;
+}
+
 static LLVMValueRef fn_llvm_value(CodeGen *g, FnTableEntry *fn_table_entry) {
     if (fn_table_entry->llvm_value)
         return fn_table_entry->llvm_value;
@@ -563,9 +573,10 @@ static LLVMValueRef fn_llvm_value(CodeGen *g, FnTableEntry *fn_table_entry) {
             addLLVMArgAttr(fn_table_entry->llvm_value, (unsigned)gen_index, "byval");
         }
     }
-    if (return_type->id == TypeTableEntryIdErrorUnion || return_type->id == TypeTableEntryIdPureError) {
-        unsigned gen_index = LLVMCountParamTypes(fn_llvm_type) - 1;
-        addLLVMArgAttr(fn_table_entry->llvm_value, (unsigned)gen_index, "nonnull");
+
+    uint32_t err_ret_trace_arg_index = get_err_ret_trace_arg_index(fn_table_entry);
+    if (err_ret_trace_arg_index != UINT32_MAX) {
+        addLLVMArgAttr(fn_table_entry->llvm_value, (unsigned)err_ret_trace_arg_index, "nonnull");
     }
 
     return fn_table_entry->llvm_value;
@@ -2400,8 +2411,8 @@ static LLVMValueRef ir_render_call(CodeGen *g, IrExecutable *executable, IrInstr
     TypeTableEntry *src_return_type = fn_type_id->return_type;
     bool ret_has_bits = type_has_bits(src_return_type);
     bool first_arg_ret = ret_has_bits && handle_is_ptr(src_return_type);
-    bool last_arg_err_ret_stack = src_return_type->id == TypeTableEntryIdErrorUnion || src_return_type->id == TypeTableEntryIdPureError;
-    size_t actual_param_count = instruction->arg_count + (first_arg_ret ? 1 : 0) + (last_arg_err_ret_stack ? 1 : 0);
+    bool prefix_arg_err_ret_stack = src_return_type->id == TypeTableEntryIdErrorUnion || src_return_type->id == TypeTableEntryIdPureError;
+    size_t actual_param_count = instruction->arg_count + (first_arg_ret ? 1 : 0) + (prefix_arg_err_ret_stack ? 1 : 0);
     bool is_var_args = fn_type_id->is_var_args;
     LLVMValueRef *gen_param_values = allocate<LLVMValueRef>(actual_param_count);
     size_t gen_param_index = 0;
@@ -2409,6 +2420,10 @@ static LLVMValueRef ir_render_call(CodeGen *g, IrExecutable *executable, IrInstr
         gen_param_values[gen_param_index] = instruction->tmp_ptr;
         gen_param_index += 1;
     }
+    if (prefix_arg_err_ret_stack) {
+        gen_param_values[gen_param_index] = g->cur_err_ret_trace_val;
+        gen_param_index += 1;
+    }
     for (size_t call_i = 0; call_i < instruction->arg_count; call_i += 1) {
         IrInstruction *param_instruction = instruction->args[call_i];
         TypeTableEntry *param_type = param_instruction->value.type;
@@ -2419,10 +2434,6 @@ static LLVMValueRef ir_render_call(CodeGen *g, IrExecutable *executable, IrInstr
             gen_param_index += 1;
         }
     }
-    if (last_arg_err_ret_stack) {
-        gen_param_values[gen_param_index] = g->cur_err_ret_trace_val;
-        gen_param_index += 1;
-    }
 
     ZigLLVM_FnInline fn_inline;
     switch (instruction->fn_inline) {
@@ -4578,8 +4589,9 @@ static void do_code_gen(CodeGen *g) {
         build_all_basic_blocks(g, fn_table_entry);
         clear_debug_source_node(g);
 
-        if (return_type->id == TypeTableEntryIdPureError || return_type->id == TypeTableEntryIdErrorUnion) {
-            g->cur_err_ret_trace_val = LLVMGetParam(fn, LLVMCountParamTypes(fn_table_entry->type_entry->data.fn.raw_type_ref) - 1);
+        uint32_t err_ret_trace_arg_index = get_err_ret_trace_arg_index(fn_table_entry);
+        if (err_ret_trace_arg_index != UINT32_MAX) {
+            g->cur_err_ret_trace_val = LLVMGetParam(fn, err_ret_trace_arg_index);
         } else if (fn_table_entry->calls_errorable_function) {
             g->cur_err_ret_trace_val = build_alloca(g, g->stack_trace_type, "error_return_trace", get_abi_alignment(g, g->stack_trace_type));
             size_t index_field_index = g->stack_trace_type->data.structure.fields[0].gen_index;