Simon Marchi
11bb5c41eb
The reg_buffer constructor zero-initializes (value-initializes, in C++
speak) the gdb_bytes of the m_registers array. This is not necessary,
as these bytes are only meaningful if the corresponding register_status
is REG_VALID. If the corresponding register_status is REG_VALID, then
they will have been overwritten with the actual register data when
reading the registers from the system into the reg_buffer.
Fix that by removing the empty parenthesis following the new expression,
meaning that the bytes will now be default-initialized, meaning they'll
be left uninitialized. For reference, this is explained here:
https://en.cppreference.com/w/cpp/language/new#Construction
These new expressions were added in 835dcf9261 ("Use std::unique_ptr
in reg_buffer"). As mentioned in that commit message, the use of
value-initialisation was done on purpose to keep existing behavior, but
now there is some data that suggest it would be beneficial not to do it,
which is why I suggest changing it.
This doesn't make a big difference on typical architectures where the
register buffer is not that big. However, on ROCm (AMD GPU), the
register buffer is about 65000 bytes big, so the reg_buffer constructor
shows up in profiling. If you want to make some tests and profile it on
a standard system, it's always possible to change:
- m_registers.reset (new gdb_byte[m_descr->sizeof_raw_registers] ());
+ m_registers.reset (new gdb_byte[65000] ());
and run a program that constantly hits a breakpoint with a false
condition. For example, by doing this change and running the following
program:
static void break_here () {}
int main ()
{
for (int i = 0; i < 100000; i++)
break_here ();
}
with the following GDB incantation:
/usr/bin/time ./gdb -nx --data-directory=data-directory -q test -ex "b break_here if 0" -ex r -batch
I get, for value-intializing:
11.75user 7.68system 0:18.54elapsed 104%CPU (0avgtext+0avgdata 56644maxresident)k
And for default-initializing:
6.83user 8.42system 0:14.12elapsed 108%CPU (0avgtext+0avgdata 56512maxresident)k
gdb/ChangeLog:
* regcache.c (reg_buffer::reg_buffer): Default-initialize
m_registers array.
Change-Id: I5071a4444dee0530ce1bc58ebe712024ddd2b158
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