sometimes free stuff from Zig IR pass 1

Total bytes used in stage1 std lib tests:
3.418 -> 3.198 GiB (saving 225 MiB)

There's still this from pass 1 not getting freed:
Const: 6909049 items, 72 bytes each, total 474.407 MiB

This is due to 2 things hanging on to references to IrAnalyze pointers:
 * ZigVar->owner_exec->analysis
 * LazyValue->ira

The LazyValue one could be solved by memoizing the results after the
lazy value is resolved, and then it could unref the IrAnalyze.

ZigVars that are determined to be comptime const, could have their
const_value set to that value, instead of using the mem_slot_index
mechanism. This would prevent an IrAnalyze ref in some cases.

Author: andrewrk

src/all_types.hpp

@@ -1565,7 +1565,7 @@ struct ZigFn {
     // in the case of async functions this is the implicit return type according to the
     // zig source code, not according to zig ir
     ZigType *src_implicit_return_type;
-    IrExecutable ir_executable;
+    IrExecutable *ir_executable;
     IrExecutable analyzed_executable;
     size_t prealloc_bbc;
     size_t prealloc_backward_branch_quota;
@@ -2204,6 +2204,8 @@ struct ZigVar {
     bool src_is_const;
     bool gen_is_const;
     bool is_thread_local;
+    bool is_comptime_memoized;
+    bool is_comptime_memoized_value;
 };
 
 struct ErrorTableEntry {

src/analyze.cpp

@@ -3275,14 +3275,15 @@ static void get_fully_qualified_decl_name(CodeGen *g, Buf *buf, Tld *tld, bool i
 }
 
 ZigFn *create_fn_raw(CodeGen *g, FnInline inline_value) {
-    ZigFn *fn_entry = allocate<ZigFn>(1);
+    ZigFn *fn_entry = allocate<ZigFn>(1, "ZigFn");
+    fn_entry->ir_executable = allocate<IrExecutable>(1, "IrExecutablePass1");
 
     fn_entry->prealloc_backward_branch_quota = default_backward_branch_quota;
 
     fn_entry->analyzed_executable.backward_branch_count = &fn_entry->prealloc_bbc;
     fn_entry->analyzed_executable.backward_branch_quota = &fn_entry->prealloc_backward_branch_quota;
     fn_entry->analyzed_executable.fn_entry = fn_entry;
-    fn_entry->ir_executable.fn_entry = fn_entry;
+    fn_entry->ir_executable->fn_entry = fn_entry;
     fn_entry->fn_inline = inline_value;
 
     return fn_entry;
@@ -4610,7 +4611,7 @@ static void analyze_fn_ir(CodeGen *g, ZigFn *fn, AstNode *return_type_node) {
     assert(!fn_type->data.fn.is_generic);
     FnTypeId *fn_type_id = &fn_type->data.fn.fn_type_id;
 
-    ZigType *block_return_type = ir_analyze(g, &fn->ir_executable,
+    ZigType *block_return_type = ir_analyze(g, fn->ir_executable,
             &fn->analyzed_executable, fn_type_id->return_type, return_type_node);
     fn->src_implicit_return_type = block_return_type;
 
@@ -4706,15 +4707,15 @@ static void analyze_fn_body(CodeGen *g, ZigFn *fn_table_entry) {
     assert(!fn_type->data.fn.is_generic);
 
     ir_gen_fn(g, fn_table_entry);
-    if (fn_table_entry->ir_executable.first_err_trace_msg != nullptr) {
+    if (fn_table_entry->ir_executable->first_err_trace_msg != nullptr) {
         fn_table_entry->anal_state = FnAnalStateInvalid;
         return;
     }
     if (g->verbose_ir) {
         fprintf(stderr, "\n");
         ast_render(stderr, fn_table_entry->body_node, 4);
         fprintf(stderr, "\nfn %s() { // (IR)\n", buf_ptr(&fn_table_entry->symbol_name));
-        ir_print(g, stderr, &fn_table_entry->ir_executable, 4, IrPassSrc);
+        ir_print(g, stderr, fn_table_entry->ir_executable, 4, IrPassSrc);
         fprintf(stderr, "}\n");
     }
 
@@ -6453,20 +6454,31 @@ Error type_resolve(CodeGen *g, ZigType *ty, ResolveStatus status) {
 }
 
 bool ir_get_var_is_comptime(ZigVar *var) {
+    if (var->is_comptime_memoized)
+        return var->is_comptime_memoized_value;
+
+    var->is_comptime_memoized = true;
+
     // The is_comptime field can be left null, which means not comptime.
-    if (var->is_comptime == nullptr)
-        return false;
+    if (var->is_comptime == nullptr) {
+        var->is_comptime_memoized_value = false;
+        return var->is_comptime_memoized_value;
+    }
     // When the is_comptime field references an instruction that has to get analyzed, this
     // is the value.
     if (var->is_comptime->child != nullptr) {
         assert(var->is_comptime->child->value->type->id == ZigTypeIdBool);
-        return var->is_comptime->child->value->data.x_bool;
+        var->is_comptime_memoized_value = var->is_comptime->child->value->data.x_bool;
+        var->is_comptime = nullptr;
+        return var->is_comptime_memoized_value;
     }
     // As an optimization, is_comptime values which are constant are allowed
     // to be omitted from analysis. In this case, there is no child instruction
     // and we simply look at the unanalyzed const parent instruction.
     assert(var->is_comptime->value->type->id == ZigTypeIdBool);
-    return var->is_comptime->value->data.x_bool;
+    var->is_comptime_memoized_value = var->is_comptime->value->data.x_bool;
+    var->is_comptime = nullptr;
+    return var->is_comptime_memoized_value;
 }
 
 bool const_values_equal_ptr(ZigValue *a, ZigValue *b) {