Add 'force-dwarf-lexical-blocks' command line option

Hi,

Currently GCC only emits DWARF debug information (DW_TAG_lexical_block 
DIEs) for compound statements containing significant local declarations.
However, code coverage tools that process the DWARF debug information to
implement block/path coverage need more complete lexical block information.

This patch adds the necessary functionality under the control of a new
command line argument: -fforce-dwarf-lexical-blocks.

When this flag is set, a DW_TAG_lexical_block DIE will be emitted for 
every function body, loop body, switch body, case statement, if-then and 
if-else statement, even if the body is a single statement.
Likewise, a lexical block will be emitted for the first label of a 
labeled statement. This block ends at the end of the current lexical 
scope, or when a break, continue, goto or return statement is 
encountered at the same lexical scope level.
Consequently, any case in a switch statement that does not flow through 
to the next case, will have its own dwarf lexical block.

The complete change proposal contains 3 patches (attached first 2):
   1.  Add command line option -fforce-dwarf-lexical-blocks
   2.  Use of flag_force_dwarf_blocks in C.

Attached are the proposed ChangeLog additions, named according to the 
directory each one belongs to.

A third patch, extending the proposed functionality to C++ will be 
submitted in a separate message.

NOTE: This is a second submit, with changes made according to 
maintainer's sugestions.

All check-c and check-c++ tests have been run for unix target.
The only test that failed with the -fforce-dwarf-lexical-blocks set
was:
FAIL: gcc.dg/debug/dwarf2/inline2.c scan-assembler-times \\(DIE 
\\([^\n]*\\) DW_TAG_lexical_block 6
as expected.

Best regards,
Andrei Herman
Mentor Graphics Corporation
Israel branch
From 6dd5796f04ca249e8e59026208e90e7619534a80 Mon Sep 17 00:00:00 2001
From: Andrei Herman <Andrei_Herman@codesourcery.com>
Date: Sun, 1 Jun 2014 11:13:09 +0300
Subject: [PATCH 1/2]             Add command line option -fforce-dwarf-lexical-blocks.

            * gcc/c-family/c.opt: Add -fforce-dwarf-lexical-blocks flag.
            * gcc/c-family/c-opts.c (c_common_post_options): Limit its use
            to dwarf4.
            * gcc/doc/invoke.texi: Document the new option.

Signed-off-by: Andrei Herman <Andrei_Herman@codesourcery.com>
---
 gcc/c-family/c-opts.c |   13 +++++++++++++
 gcc/c-family/c.opt    |    4 ++++
 gcc/doc/invoke.texi   |   12 ++++++++++++
 3 files changed, 29 insertions(+), 0 deletions(-)

On Sun, 1 Jun 2014, Herman, Andrei wrote:

>+  /* The -fforce-dwarf-lexical-blocks option is only relevant when debug
>+     info is in DWARF4 format */
>+  if (flag_force_dwarf_blocks) {

Watch coding style: the opening '{' always goes on the next line.

>+fforce-dwarf-lexical-blocks
>+C C++ Var(flag_force_dwarf_blocks)
>+Force generation of lexical blocks in dwarf output

I don't see a good reason for this not to be supported for ObjC and ObjC++ 
as well.  Say DWARF, not dwarf.

>+@item -fforce-dwarf-lexical-blocks
>+Produce debug information (a DW_TAG_lexical_block) for every function
>+body, loop body, switch body, case statement, if-then and if-else statement,
>+even if the body is a single statement.  Likewise, a lexical block will be
>+emitted for the first label of a statement.  This block ends at the end of the
>+current lexical scope, or when a break, continue, goto or return statement is
>+encountered at the same lexical scope level.  This option is usefull for
>+coverage tools that utilize the dwarf debug information.
>+This option only applies to C/C++ code and is available when using DWARF
>+Version 4 or higher.

Use @code{} markup for keywords (if, else, break, continue, goto, return).  
"useful" not "usefull".  "DWARF" not "dwarf".

>+/* Create a block_loc struct for a statement list created on behalf of
>+   flag_force_dwarf_blocks.  We use this for label or forced c99 scopes.  */
>+
>+void
>+push_block_info (tree block, location_t loc, bool is_label)
>+{
>+  if (TREE_CODE(block) != STATEMENT_LIST)

Watch coding style: space before '(' in function and macro calls (and 
similar calls such as sizeof) (many places in this patch, not just this 
one).

>+tree
>+pop_block_info (location_t &loc)

It's not documented in codingconventions.html, but I think it's preferred 
to avoid returning values through reference arguments (see e.g. 
<https://gcc.gnu.org/ml/gcc-patches/2013-11/msg00198.html>).

>+{
>+  block_loc  tl = NULL;

Excess space between "block_loc" and "tl".

>@@ -4679,7 +4712,7 @@ c_parser_compound_statement_nostart (c_parser *parser)
>    expressions being rejected later.  */
> 
> static void
>-c_parser_label (c_parser *parser)
>+c_parser_label (c_parser *parser, bool prev_label)

You're adding a new argument - you need to update the comment above this 
function to explain the semantics of this argument.

In general, make sure that new functions have comments above them that 
explain the semantics of the arguments (by name) and any return value.

>+/* If current scope is a label scope, pop it from block info stack
>+   and close it's compound statement.  */

"its" not "it's".

I will be on vacation until July 13.
I'll have access to my e-mail occasionally.

If you could send me please your comments, both style and content, 
pertaining to all three patches related to this subject, I will make all 
the needed changes and I could submit a new version, as soon as I get back.

Thanks and regards,
Andrei Herman
Mentor Graphics Corporation
Israel branch

On 6/20/2014 12:09 AM, Joseph S. Myers wrote:
> On Sun, 1 Jun 2014, Herman, Andrei wrote:
>
>> +  /* The -fforce-dwarf-lexical-blocks option is only relevant when debug
>> +     info is in DWARF4 format */
>> +  if (flag_force_dwarf_blocks) {
>
> Watch coding style: the opening '{' always goes on the next line.
>
>> +fforce-dwarf-lexical-blocks
>> +C C++ Var(flag_force_dwarf_blocks)
>> +Force generation of lexical blocks in dwarf output
>
> I don't see a good reason for this not to be supported for ObjC and ObjC++
> as well.  Say DWARF, not dwarf.
>
>> +@item -fforce-dwarf-lexical-blocks
>> +Produce debug information (a DW_TAG_lexical_block) for every function
>> +body, loop body, switch body, case statement, if-then and if-else statement,
>> +even if the body is a single statement.  Likewise, a lexical block will be
>> +emitted for the first label of a statement.  This block ends at the end of the
>> +current lexical scope, or when a break, continue, goto or return statement is
>> +encountered at the same lexical scope level.  This option is usefull for
>> +coverage tools that utilize the dwarf debug information.
>> +This option only applies to C/C++ code and is available when using DWARF
>> +Version 4 or higher.
>
> Use @code{} markup for keywords (if, else, break, continue, goto, return).
> "useful" not "usefull".  "DWARF" not "dwarf".
>
>> +/* Create a block_loc struct for a statement list created on behalf of
>> +   flag_force_dwarf_blocks.  We use this for label or forced c99 scopes.  */
>> +
>> +void
>> +push_block_info (tree block, location_t loc, bool is_label)
>> +{
>> +  if (TREE_CODE(block) != STATEMENT_LIST)
>
> Watch coding style: space before '(' in function and macro calls (and
> similar calls such as sizeof) (many places in this patch, not just this
> one).
>
>> +tree
>> +pop_block_info (location_t &loc)
>
> It's not documented in codingconventions.html, but I think it's preferred
> to avoid returning values through reference arguments (see e.g.
> <https://gcc.gnu.org/ml/gcc-patches/2013-11/msg00198.html>).
>
>> +{
>> +  block_loc  tl = NULL;
>
> Excess space between "block_loc" and "tl".
>
>> @@ -4679,7 +4712,7 @@ c_parser_compound_statement_nostart (c_parser *parser)
>>     expressions being rejected later.  */
>>
>> static void
>> -c_parser_label (c_parser *parser)
>> +c_parser_label (c_parser *parser, bool prev_label)
>
> You're adding a new argument - you need to update the comment above this
> function to explain the semantics of this argument.
>
> In general, make sure that new functions have comments above them that
> explain the semantics of the arguments (by name) and any return value.
>
>> +/* If current scope is a label scope, pop it from block info stack
>> +   and close it's compound statement.  */
>
> "its" not "it's".
>