cgen: add support for -d trace_cgen_stmt, document it in CONTRIBUTI…

…NG.md
vlang · Sep 13, 2023 · 5d1f87c · 5d1f87c
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diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
@@ -192,20 +192,25 @@ which tests have failed and then fix them by making more changes. Just use
 run with your updated code. Use `hub ci-status --verbose` to monitor
 their status.
 
-## Flags
+## Compiler flags, useful while debugging the compiler itself:
 
 V allows you to pass custom flags using `-d my_flag` that can then be checked
 at compile time (see the documentation about
 [compile-time if](https://github.com/vlang/v/blob/master/doc/docs.md#compile-time-if)).
-There are numerous flags that can be passed when building the compiler
-with `v self` or when creating a copy of the compiler, that will help
-you when debugging.
 
-Beware that the flags must be passed when building the compiler,
-not the program, so do for example: `v -d time_parsing cmd/v` or
-`v -d trace_checker self`.
-Some flags can make the compiler very verbose, so it is recommended
-to create a copy of the compiler rather than replacing it with `v self`.
+Since the compiler is *also* an ordinary V program, there are numerous flags that can be
+passed when building the compiler itself with `v self`, or when creating a copy of the
+compiler, that will help you when debugging the compiler.
+
+Note: beware that the flags below must be passed, when building the compiler, *not the program*,
+so do for example:
+`./v -o w -d time_parsing cmd/v`
+or
+`./v -o w -d trace_checker self`
+... then use `./w file.v`, instead of `./v file.v`, to compile your program.
+
+Note: some of the flags can make the compiler *very verbose*, so it is recommended to create
+a copy of the compiler rather than replacing it with `v self`.
 
 | Flag                              | Usage                                                                                                               |
 |-----------------------------------|---------------------------------------------------------------------------------------------------------------------|
@@ -216,12 +221,21 @@ to create a copy of the compiler rather than replacing it with `v self`.
 | `print_vweb_template_expansions`  | Prints vweb compiled HTML files                                                                                     |
 | `time_checking`                   | Prints the time spent checking files and other related information                                                  |
 | `time_parsing`                    | Prints the time spent parsing files and other related information                                                   |
+|                                   |                                                                                                                     |
+| `trace_parser`                    | Prints details about parsed statements and expressions. Very verbose. Use it for panics in the parser.              |
+| `trace_ccoptions`                 | Prints options passed down to the C compiler                                                                        |
+|                                   |                                                                                                                     |
+| `trace_checker`                   | Prints details about the statements being checked. Very verbose. Use it for panics in the checker.                  |
+|                                   |                                                                                                                     |
+| `trace_gen`                       | Prints all the strings written to the generated C file. Very verbose.                                               |
+| `trace_cgen_stmt`                 | Prints details about the statements that are being processed by cgen.                                               |
+|                                   |        Use it for panics in cgen, to see the closest input V source line, that caused the panic.                    |
+|                                   |        Note: you need `v -no-parallel -d trace_cgen_stmt -o w cmd/v` to make sense of the output of that,           |
+|                                   |        otherwise by default cgen uses several threads, and the lines that are printed are out of order.             |
+|                                   |                                                                                                                     |
 | `trace_autofree`                  | Prints details about how/when -autofree puts free() calls                                                           |
 | `trace_autostr`                   | Prints details about `.str()` method auto-generated by the compiler during C generation                             |
-| `trace_ccoptions`                 | Prints options passed down to the C compiler                                                                        |
-| `trace_checker`                   | Prints details about the statements being checked                                                                   |
-| `trace_gen`                       | Prints strings written to the generated C file. Beware, this flag is very verbose                                   |
-| `trace_parser`                    | Prints details about parsed statements and expressions                                                              |
+|                                   |                                                                                                                     |
 | `trace_thirdparty_obj_files`      | Prints details about built thirdparty obj files                                                                     |
 | `trace_usecache`                  | Prints details when -usecache is used                                                                               |
 | `trace_embed_file`                | Prints details when $embed_file is used                                                                             |

diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
@@ -2006,6 +2006,14 @@ fn (mut g Gen) write_v_source_line_info(pos token.Pos) {
 }
 
 fn (mut g Gen) stmt(node ast.Stmt) {
+	$if trace_cgen_stmt ? {
+		ntype := typeof(node).replace('v.ast.', '')
+		if g.file == unsafe { nil } {
+			eprintln('cgen: <nil> | pos: ${node.pos.line_str():-39} | node: ${ntype} | ${node}')
+		} else {
+			eprintln('cgen: ${g.file.path:-30} | pos: ${node.pos.line_str():-39} | node: ${ntype} | ${node}')
+		}
+	}
 	g.inside_call = false
 	if !g.skip_stmt_pos {
 		g.set_current_pos_as_last_stmt_pos()