Do not generate debug info for compiler generated VLAs

Hi,

in February Jakub made a change to the gimplifier in order to generate debug 
info for bounds of VLAs.  This has a significant impact on the code itself in 
Ada (typically +2% code size and +1% compilation time at -O0) because the Ada 
compiler itself generates a lot of VLAs for various constructs; now gigi puts 
DECL_IGNORED_P on these VLAs so the gimplifier shouldn't overrule it.

Bootstrapped/regtested on x86_64-suse-linux, OK for mainline?  What about the 
4.5 branch?


2010-10-13  Eric Botcazou  <ebotcazou@adacore.com>

	* gimplify.c (gimplify_type_sizes) <ARRAY_TYPE>: If the type is to be
	ignored for debug info purposes, do not clear the DECL_IGNORED_P flag
	on the bounds of its domain.

On Thu, Oct 14, 2010 at 1:16 AM, Eric Botcazou <ebotcazou@adacore.com> wrote:
> Hi,
>
> in February Jakub made a change to the gimplifier in order to generate debug
> info for bounds of VLAs.  This has a significant impact on the code itself in
> Ada (typically +2% code size and +1% compilation time at -O0) because the Ada
> compiler itself generates a lot of VLAs for various constructs; now gigi puts
> DECL_IGNORED_P on these VLAs so the gimplifier shouldn't overrule it.
>
> Bootstrapped/regtested on x86_64-suse-linux, OK for mainline?  What about the
> 4.5 branch?

Hm, it looks sensible, but it's of course odd that if we don't have an
explicit DECL_IGNORED_P flag (like for a non-type-decl TYPE_NAME
or for a NULL TYPE_NAME) that we still emit the info.  In fact it's odd
that the Ada frontend creates a TYPE_DECL for the artificial VLAs
at all (does it do so only to make this flag available?).

I don't see any better way to deal with this though (other than stealing
a bit for all types and making sure we do not unify them).  I suppose
it's not enough to set DECL_IGNORED_P on the VLAs itself?

Anyway, I think that the DECL_IGNORED_P doucmentation in tree.h
needs to be amended for this special case (like, "For a TYPE_DECL,
debug information for its type will be as simple as possible." or a
better wording that I can't come up with right now).

So I'd say, ok for trunk and the branch with the tree.h change.

Thanks,
Richard.

> 2010-10-13  Eric Botcazou  <ebotcazou@adacore.com>
>
>        * gimplify.c (gimplify_type_sizes) <ARRAY_TYPE>: If the type is to be
>        ignored for debug info purposes, do not clear the DECL_IGNORED_P flag
>        on the bounds of its domain.
>
> --
> Eric Botcazou
>

> Hm, it looks sensible, but it's of course odd that if we don't have an
> explicit DECL_IGNORED_P flag (like for a non-type-decl TYPE_NAME
> or for a NULL TYPE_NAME) that we still emit the info.

The C testcase doesn't pass if you don't clear the flag for unnamed types.

> In fact it's odd that the Ada frontend creates a TYPE_DECL for the
> artificial VLAs at all (does it do so only to make this flag available?).

gigi creates a TYPE_DECL for almost every type for internal reasons that are 
related to the global unsharing thing.  Then it uses (an extended version of) 
the TYPE_DECL_IS_STUB trick in dwarf2out.c to avoid emitting DW_TAG_typedef.

> I don't see any better way to deal with this though (other than stealing
> a bit for all types and making sure we do not unify them).  I suppose
> it's not enough to set DECL_IGNORED_P on the VLAs itself?

The check is already for DECL_IGNORED_P on the VLA, not on its TYPE_DOMAIN.

> Anyway, I think that the DECL_IGNORED_P doucmentation in tree.h
> needs to be amended for this special case (like, "For a TYPE_DECL,
> debug information for its type will be as simple as possible." or a
> better wording that I can't come up with right now).

We want no debug info at all for the type though; I think that dwarf2out.c 
already ensures that.