Re: [PATCH v3 1/2] gdb: dwarf2 generic implementation for caching function data

[Simon Marchi <simark@simark.ca>]

On 1/19/23 05:29, Torbjörn SVENSSON via Gdb-patches wrote:
> v2 -> v3:
> 
> Addressed comments from Tom in
> https://sourceware.org/pipermail/gdb-patches/2023-January/195882.html
> 
> 
> ---
> 
> 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.
> 
> The cache object is unique for each function and frame, so if there are
> more than one function pointer stored in the dwarf2_frame_cache->reg
> array, then the appropriate pointer will be supplied (the type is not
> known by the dwarf2 implementation).
> 
> dwarf2_frame_get_fn_data can be used to retrieve the function unique
> cache object.
> dwarf2_frame_allocate_fn_data can be used to allocate and retrieve the
> function unique cache object.
> 
> Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>
> Signed-off-by: Yvan Roux <yvan.roux@foss.st.com>

Hi,

My main question is: this approach requires each custom prev_register
function to set up and manage its own cache.  Did you consider caching
values at a higher level?

Perhaps:

 1. In dwarf2_frame_prev_register, just for DWARF2_FRAME_REG_FN
 2. In dwarf2_frame_prev_register, but for all kinds of registers
 3. In frame_unwind_register_value, which would cache all register values
    for all frames regardless of the unwinder

I don't know if other ways of unwinding would benefit that much from
caching, but I guess it wouldn't hurt.  For instance, evaluating DWARF
expressions is probably generally not super expensive, but it might
still benefit from getting cached.  And it would be nice to write the
caching code just once and have it work transparently for everything.

> @@ -262,4 +264,35 @@ extern int dwarf2_fetch_cfa_info (struct gdbarch *gdbarch, CORE_ADDR pc,
>  				  const gdb_byte **cfa_start_out,
>  				  const gdb_byte **cfa_end_out);
>  
> +
> +/* Allocate a new instance of the function unique data.
> +
> +   The main purpose of this custom function data object is to allow caching the
> +   value of expensive lookups in the prev_register implementation.

I would replace "in the prev_register implementation" with "in
prev_register implementations".  There isn't only one prev_register
implementation AFAIK.

Simon