Re: [RFC] pb unwinding from pthread_cond_wait on ppc-linux (RFA?)

Joel Brobecker
2004-12-09T16:00:25Z

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> Precisely! That's what I thought it would be. It's trying to load lr
> with the address of @+16, so that the function can access PIC data
> using PC-relative displacement.

Daniel, you never stop to impress me.

> (Does this obsolete the "branch in first three insns" check? I'm not
> sure if there are other possible reasons for that.)

Here is a new patch that implements your suggestion. Indeed, I could
then remove the "branch in first three insns" check...

2004-12-09 Joel Brobecker <brobecker@gnat.com>

* rs6000-tdep.c (bl_to_blrl_insn_p): New function.
(skip_prologue): Stop unconditionaly skipping "bl" instructions
that are within the first 3 instruction. Instead, Skip that "bl"
instruction iff the destination instruction is a "blrl".

Tested on powerpc-linux. No regression.

How does it look?

--
Joel

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Index: rs6000-tdep.c
===================================================================
RCS file: /cvs/src/src/gdb/rs6000-tdep.c,v
retrieving revision 1.233
diff -u -p -r1.233 rs6000-tdep.c
--- rs6000-tdep.c 25 Nov 2004 02:48:27 -0000 1.233
+++ rs6000-tdep.c 9 Dec 2004 15:40:46 -0000
@@ -823,6 +823,30 @@ store_param_on_stack_p (unsigned long op
return 0;
}

+/* Assuming that INSN is a "bl" instruction located at PC, return
+ nonzero if the destination of the branch is a "blrl" instruction.
+
+ This sequence is sometimes found in certain function prologues.
+ It allows the function to load the LR register with a value that
+ they can use to access PIC data using PC-relative offsets. */
+
+static int
+bl_to_blrl_insn_p (CORE_ADDR pc, int insn)
+{
+ const int opcode = 18;
+ const CORE_ADDR dest = branch_dest (opcode, insn, pc, -1);
+ int dest_insn;
+
+ if (dest == -1)
+ return 0; /* Should never happen, but just return zero to be safe. */
+
+ dest_insn = read_memory_integer (dest, 4);
+ if ((dest_insn & 0xfc00ffff) == 0x4c000021) /* blrl */
+ return 1;
+
+ return 0;
+}
+
static CORE_ADDR
skip_prologue (CORE_ADDR pc, CORE_ADDR lim_pc, struct rs6000_framedata *fdata)
{
@@ -1047,19 +1071,9 @@ skip_prologue (CORE_ADDR pc, CORE_ADDR l
to save fprs??? */

fdata->frameless = 0;
- /* Don't skip over the subroutine call if it is not within
- the first three instructions of the prologue and either
- we have no line table information or the line info tells
- us that the subroutine call is not part of the line
- associated with the prologue. */
- if ((pc - orig_pc) > 8)
- {
- struct symtab_and_line prologue_sal = find_pc_line (orig_pc, 0);
- struct symtab_and_line this_sal = find_pc_line (pc, 0);

- if ((prologue_sal.line == 0) || (prologue_sal.line != this_sal.line))
- break;
- }
+ if (bl_to_blrl_insn_p (pc, op))
+ continue;

op = read_memory_integer (pc + 4, 4);