[PATCH] remove registers_changed from switch_to_thread

Yao Qi
2012-07-26T14:45:42Z

Hi,
When reading the code, I find registers_changed is called from
switch_to_thread, which confuses me a little bit. We have
regcache, and each thread has its own regcache. Why do we have
to invalidate all regcaches during thread switch?

The line of calling registers_changed in switch_to_thread is quite
old, it was written in gdb 4.10 (1993). At that time, we didn't
have regcache, and register state is a global state. There is also
a global variable 'registers_pid' associated with the global
register state. When thread switches, 'registers_pid' should be
updated. That is the reason why registers_changed should
be called in switch_to_thread. So we don't have to do it now.
This patch is to remove this line. Regression tested on x86_64
native and gdbserver. OK to apply?

Note that this patch hightly reduced the allocation and
deallocation of regcache, especially in multi-threaded program.
Here are some statistics data I collected when running testsuite.

(gdb) PASS: gdb.multi/base.exp: check remove-inferiors
maintenance info statistics^M
Name Allocate Free ^M
frame info 19 8 ^M
-regcache 36 35 ^M
+regcache 8 7

(gdb) PASS: gdb.threads/create-fail.exp: iteration 3: run till end
maintenance info statistics^M
Name Allocate Free ^M
frame info 156 78 ^M
-regcache 269 269 ^M
+regcache 103 103 ^M

(gdb) PASS: gdb.threads/create-fail.exp: iteration 4: run till end
maintenance info statistics^M
Name Allocate Free ^M
frame info 156 78 ^M
-regcache 315 315 ^M
+regcache 96 96 ^M

gdb:

2012-07-26 Yao Qi <yao@codesourcery.com>

* thread.c (switch_to_thread): Don't call registers_changed.
---
gdb/thread.c | 1 -
1 files changed, 0 insertions(+), 1 deletions(-)

diff --git a/gdb/thread.c b/gdb/thread.c
index ff5220d..7e8eec5 100644
--- a/gdb/thread.c
+++ b/gdb/thread.c
@@ -987,7 +987,6 @@ switch_to_thread (ptid_t ptid)

inferior_ptid = ptid;
reinit_frame_cache ();
- registers_changed ();

/* We don't check for is_stopped, because we're called at times
while in the TARGET_RUNNING state, e.g., while handling an
--
1.7.7.6