Re: Broken prologue skipping with non-returning function

Jonathan Larmour
2008-09-19T19:01:43Z

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Daniel Jacobowitz wrote:
> On Fri, Sep 19, 2008 at 03:32:59PM +0100, Jonathan Larmour wrote:
>> We end up with a .loc for both lines 6 and 7 with no intervening
>> instructions. gdb's symtab.c:find_pc_sect_line() looks for when the pc
>> changes to something different and thus ends up returning a symtab_and_line
>> indicating that the line at that pc is at the 'if' and runs from the start
>> of the function to the ldr after the .loc 1 9 0.
>
> skip_prologue_using_sal is supposed to detect this. We have a
> patch to improve it in our internal tree that we haven't gotten round
> to yet. Here it is; I do not remember what the language_asm check was
> really about, except that I'm sure it came up running the gdb
> testsuite, so removing it and running asm-source.exp would probably
> explain it.

Thanks! The current arm-tdep.c doesn't presently use
skip_prologue_using_sal() however. At a guess that's also lurking in your
internal tree, but nevermind, I'm attaching a patch assuming that's useful.

With both of these (and my tentative patch reverted) I can confirm it works
as expected.

If it helps, I have write after approval perms, and a valid current FSF
copyright assignment, including disclaimer with my current employer. I
noticed I need to update my email address in the MAINTAINERS file which I
can do too.

I can check in your change too. If so, presumably you already have a
ChangeLog entry you'd like me to use to ease your merges?

Jifl

2008-09-19 Jonathan Larmour <jifl@eCosCentric.com>

* arm-tdep.c (arm_skip_prologue): Call skip_prologue_using_sal
instead of determining symbol and line info directly.

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[-- Attachment #2: arm.skip.prologue.using.sal.patch --]
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--- arm-tdep.c.old 2008-09-19 17:01:32.000000000 +0100
+++ arm-tdep.c 2008-09-19 17:23:42.000000000 +0100
@@ -519,43 +519,40 @@ arm_skip_prologue (struct gdbarch *gdbar
{
unsigned long inst;
CORE_ADDR skip_pc;
- CORE_ADDR func_addr, func_end = 0;
- char *func_name;
+ CORE_ADDR func_addr, limit_pc;
struct symtab_and_line sal;

/* If we're in a dummy frame, don't even try to skip the prologue. */
if (deprecated_pc_in_call_dummy (pc))
return pc;

- /* See what the symbol table says. */
-
- if (find_pc_partial_function (pc, &func_name, &func_addr, &func_end))
- {
- struct symbol *sym;
-
- /* Found a function. */
- sym = lookup_symbol (func_name, NULL, VAR_DOMAIN, NULL);
- if (sym && SYMBOL_LANGUAGE (sym) != language_asm)
- {
- /* Don't use this trick for assembly source files. */
- sal = find_pc_line (func_addr, 0);
- if ((sal.line != 0) && (sal.end < func_end))
- return sal.end;
- }
- }
-
- /* Can't find the prologue end in the symbol table, try it the hard way
- by disassembling the instructions. */
-
+ /* See if we can determine the end of the prologue via the symbol table.
+ If so, then return either PC, or the PC after the prologue, whichever
+ is greater. */
+ if (find_pc_partial_function (pc, NULL, &func_addr, NULL))
+ {
+ CORE_ADDR post_prologue_pc = skip_prologue_using_sal (func_addr);
+ if (post_prologue_pc != 0)
+ return max (pc, post_prologue_pc);
+ }
+
+ /* Can't determine prologue from the symbol table, need to examine
+ instructions. */
+
+ /* Find an upper limit on the function prologue using the debug
+ information. If the debug information could not be used to provide
+ that bound, then use an arbitrary large number as the upper bound. */
/* Like arm_scan_prologue, stop no later than pc + 64. */
- if (func_end == 0 || func_end > pc + 64)
- func_end = pc + 64;
+ limit_pc = skip_prologue_using_sal (pc);
+ if (limit_pc == 0)
+ limit_pc = pc + 64; /* Magic. */
+

/* Check if this is Thumb code. */
if (arm_pc_is_thumb (pc))
- return thumb_analyze_prologue (gdbarch, pc, func_end, NULL);
+ return thumb_analyze_prologue (gdbarch, pc, limit_pc, NULL);

- for (skip_pc = pc; skip_pc < func_end; skip_pc += 4)
+ for (skip_pc = pc; skip_pc < limit_pc; skip_pc += 4)
{
inst = read_memory_unsigned_integer (skip_pc, 4);