[PATCH 3/7] Force to insert software single step breakpoint

[Yao Qi <qiyaoltc at gmail dot com>]

GDB doesn't insert software single step breakpoint if the instruction
branches to itself, so that the program can't stop after command "si".

(gdb) b 32
Breakpoint 2 at 0x8680: file git/gdb/testsuite/gdb.base/branch-to-self.c, line 32.
(gdb) c
Continuing.

Breakpoint 2, main () at gdb/git/gdb/testsuite/gdb.base/branch-to-self.c:32
32	  asm (".Lhere: " BRANCH_INSN " .Lhere"); /* loop-line */
(gdb) si
infrun: clear_proceed_status_thread (Thread 3991.3991)
infrun: proceed (addr=0xffffffff, signal=GDB_SIGNAL_DEFAULT)
infrun: step-over queue now empty
infrun: resuming [Thread 3991.3991] for step-over
infrun: skipping breakpoint: stepping past insn at: 0x8680
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Sending packet: $Z0,8678,4#f3...Packet received: OK
infrun: skipping breakpoint: stepping past insn at: 0x8680
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Sending packet: $Z0,b6fe86c8,4#82...Packet received: OK
infrun: resume (step=1, signal=GDB_SIGNAL_0), trap_expected=1, current thread [Thread 3991.3991] at 0x868

breakpoint.c:should_be_inserted thinks the breakpoint shouldn't be
inserted, which is wrong.  This patch restrict the condition that only
return false if breakpoint is NOT single step breakpoint.

gdb:

2016-03-23  Yao Qi  <yao.qi@linaro.org>

	* breakpoint.c (should_be_inserted): Don't return 0 if single
	step breakpoint is inserted at the address we're stepping over.
	* gdbarch.sh (software_single_step): Update comments.
	* gdbarch.h: Regenerated.
---
 gdb/breakpoint.c | 9 ++++++++-
 gdb/gdbarch.h    | 5 ++++-
 gdb/gdbarch.sh   | 5 ++++-
 3 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index f99a7ab..9ecfb07 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -2219,9 +2219,16 @@ should_be_inserted (struct bp_location *bl)
     return 0;
 
   /* Don't insert a breakpoint if we're trying to step past its
-     location.  */
+     location except single step breakpoint, because the single step
+     breakpoint may be inserted at the location we're trying to step
+     if the instruction branches to itself.  However, the instruction
+     won't be executed at all and it may break the semantics of the
+     instruction, for example, the instruction is a conditional
+     branch or updates some flags.  We can't fix it unless GDB is able
+     to emulate the instruction or switch to displaced stepping.  */
   if ((bl->loc_type == bp_loc_software_breakpoint
        || bl->loc_type == bp_loc_hardware_breakpoint)
+      && bl->owner->type != bp_single_step
       && stepping_past_instruction_at (bl->pspace->aspace,
 				       bl->address))
     {
diff --git a/gdb/gdbarch.h b/gdb/gdbarch.h
index 252fc4b..ba3d4ca 100644
--- a/gdb/gdbarch.h
+++ b/gdb/gdbarch.h
@@ -650,7 +650,10 @@ extern void set_gdbarch_addr_bits_remove (struct gdbarch *gdbarch, gdbarch_addr_
    target can single step.  If not, then implement single step using breakpoints.
   
    A return value of 1 means that the software_single_step breakpoints
-   were inserted; 0 means they were not. */
+   were inserted; 0 means they were not.  Multiple breakpoints may be
+   inserted for some instructions such as conditional branch.  However,
+   each implementation must always evaluate the condition and only put
+   the breakpoint at the branch destination if the condition is true. */
 
 extern int gdbarch_software_single_step_p (struct gdbarch *gdbarch);
 
diff --git a/gdb/gdbarch.sh b/gdb/gdbarch.sh
index 37f59b7..8192370 100755
--- a/gdb/gdbarch.sh
+++ b/gdb/gdbarch.sh
@@ -609,7 +609,10 @@ m:CORE_ADDR:addr_bits_remove:CORE_ADDR addr:addr::core_addr_identity::0
 # target can single step.  If not, then implement single step using breakpoints.
 #
 # A return value of 1 means that the software_single_step breakpoints
-# were inserted; 0 means they were not.
+# were inserted; 0 means they were not.  Multiple breakpoints may be
+# inserted for some instructions such as conditional branch.  However,
+# each implementation must always evaluate the condition and only put
+# the breakpoint at the branch destination if the condition is true.
 F:int:software_single_step:struct frame_info *frame:frame
 
 # Return non-zero if the processor is executing a delay slot and a
-- 
1.9.1