From: Tom Tromey <tom@tromey.com>
To: "Torbjörn SVENSSON via Gdb-patches" <gdb-patches@sourceware.org>
Cc: "Torbjörn SVENSSON" <torbjorn.svensson@foss.st.com>,
luis.machado@arm.com, vanekt@volny.cz,
"Yvan Roux" <yvan.roux@foss.st.com>
Subject: Re: [PATCH v2 3/4] gdb: dwarf2 generic implementation for caching function data
Date: Wed, 18 Jan 2023 11:47:04 -0700 [thread overview]
Message-ID: <878rhz1xzr.fsf@tromey.com> (raw)
In-Reply-To: <20221118155252.113476-4-torbjorn.svensson@foss.st.com> (=?utf-8?Q?=22Torbj=C3=B6rn?= SVENSSON via Gdb-patches"'s message of "Fri, 18 Nov 2022 16:52:52 +0100")
>>>>> Torbjörn SVENSSON via Gdb-patches <gdb-patches@sourceware.org> writes:
> When there is no dwarf2 data for a register, a function can be called
> to provide the value of this register. In some situations, it might
> not be trivial to determine the value to return and it would cause a
> performance bottleneck to do the computation each time.
> This patch allows the called function to have a "cache" object that it
> can use to store some metadata between calls to reduce the performance
> impact of the complex logic.
> +struct dwarf2_frame_fn_data
> +{
This struct should have some kind of introductory comment, as should the
fields.
> + struct value *(*fn) (frame_info_ptr this_frame, void **this_cache,
> + int regnum);
Seems like this should use the fn_prev_register typedef. (But see below.)
> + void *data;
> + struct dwarf2_frame_fn_data* next;
Wrong "*" placement.
> +void *dwarf2_frame_get_fn_data (frame_info_ptr this_frame, void **this_cache,
> + fn_prev_register fn)
Wrong formatting.
> +void *dwarf2_frame_allocate_fn_data (frame_info_ptr this_frame,
> + void **this_cache,
> + fn_prev_register fn, unsigned long size)
Wrong formatting.
> +{
> + struct dwarf2_frame_fn_data *fn_data = nullptr;
> + struct dwarf2_frame_cache *cache
> + = dwarf2_frame_cache (this_frame, this_cache);
> +
> + /* First try to find an existing object. */
> + void *data = dwarf2_frame_get_fn_data (this_frame, this_cache, fn);
> + if (data)
> + return data;
> +
> + /* No object found, lets create a new instance. */
> + fn_data = FRAME_OBSTACK_ZALLOC (struct dwarf2_frame_fn_data);
> + fn_data->fn = fn;
> + fn_data->data = frame_obstack_zalloc (size);
> + fn_data->next = cache->fn_data;
> + cache->fn_data = fn_data;
> +
> + return fn_data->data;
This API seems a bit weird to me.
It seems like the 'fn' parameter is never really used. It's maybe just
a sort of cookie. But if so, I think it would be better to just use a
'const void *' or 'const char *' or something like that. (A string is
nice because then it can also be seen in the debugger and give a clue
where it came from.)
Ok, I dug up the follow-up patch and indeed it is just a cookie. I
think naming it as such and changing the type would make this more
clear.
Also in the follow-up I see that it calls dwarf2_frame_fn_data first.
So if you're going to go that route, then it seems that
dwarf2_frame_allocate_fn_data does not need to find an existing
object -- it can just assert there isn't one.
> +
> +/* Allocate a new instance of the function unique data. */
> +
> +extern void *dwarf2_frame_allocate_fn_data (frame_info_ptr this_frame,
> + void **this_cache,
> + fn_prev_register fn,
> + unsigned long size);
> +
> +/* Retrieve the function unique data for this frame. */
> +
> +extern void *dwarf2_frame_get_fn_data (frame_info_ptr this_frame,
> + void **this_cache,
> + fn_prev_register fn);
IMO both of these could use a longer comment. From this it's impossible
to tell what the point of them is.
thanks,
Tom
next prev parent reply other threads:[~2023-01-18 18:47 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-18 15:52 [PATCH 0/4] v2 gdb/arm: Fixes for Cortex-M stack unwinding Torbjörn SVENSSON
2022-11-18 15:52 ` [PATCH v2 1/4] gdb/arm: Update active msp/psp when switching stack Torbjörn SVENSSON
2022-11-21 14:04 ` Luis Machado
2022-11-18 15:52 ` [PATCH v2 2/4] gdb/arm: Ensure that stack pointers are in sync Torbjörn SVENSSON
2022-11-21 14:04 ` Luis Machado
2022-11-18 15:52 ` [PATCH v2 3/4] gdb: dwarf2 generic implementation for caching function data Torbjörn SVENSSON
2022-11-18 16:01 ` Torbjorn SVENSSON
2022-12-20 21:04 ` Tom Tromey
2022-11-21 21:16 ` Luis Machado
2022-11-29 15:19 ` Torbjorn SVENSSON
2022-11-29 16:24 ` Tomas Vanek
2022-11-30 10:16 ` Torbjorn SVENSSON
2022-11-30 10:19 ` Luis Machado
2022-12-08 1:11 ` Luis Machado
2022-12-19 19:28 ` [PING] " Torbjorn SVENSSON
2022-12-20 21:02 ` Tom Tromey
2022-12-28 16:16 ` Torbjorn SVENSSON
2023-01-05 20:53 ` Torbjorn SVENSSON
2023-01-14 6:54 ` Joel Brobecker
2023-01-18 18:47 ` Tom Tromey [this message]
2023-01-19 10:31 ` Torbjorn SVENSSON
2022-11-18 15:52 ` [PATCH v2 4/4] gdb/arm: Use new dwarf2 function cache Torbjörn SVENSSON
2022-11-21 21:04 ` Luis Machado
2022-11-29 15:19 ` Torbjorn SVENSSON
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