Re: [PATCH] [gdb/testsuite] Fix gdb.dwarf2/dw2-entry-points.exp on ppc64le

[Guinevere Larsen <blarsen@redhat.com>]

On 11/01/2024 14:08, Tom de Vries wrote:
> On 1/11/24 13:55, Guinevere Larsen wrote:
>> On 11/01/2024 13:42, Tom de Vries wrote:
>>> On ppc64le-linux, I run into:
>>> ...
>>> (gdb) bt^M
>>>   #0  0x00000000100006dc in foobar (J=2)^M
>>>   #1  0x000000001000070c in prog ()^M
>>> (gdb) FAIL: gdb.dwarf2/dw2-entry-points.exp: bt foo
>>> ...
>>>
>>> The test-case attemps to emulate additional entry points of a 
>>> function, with
>>> function bar having entry points foo and foobar:
>>> ...
>>> (gdb) p bar
>>> $1 = {void (int, int)} 0x1000064c <bar>
>>> (gdb) p foo
>>> $2 = {void (int, int)} 0x10000698 <foo>
>>> (gdb) p foobar
>>> $3 = {void (int)} 0x100006d0 <foobar>
>>> ...
>>>
>>> However, when setting a breakpoint on the entry point foo:
>>> ...
>>> (gdb) b foo
>>> Breakpoint 1 at 0x100006dc
>>> ...
>>> it ends up in foobar instead of in foo, due to prologue skipping, and
>>> consequently the backtrace show foobar instead foo.
>>>
>>> The problem is that the test-case does not emulate an actual 
>>> prologue at each
>>> entry point.
>>>
>>> Fix this by disabling the prologue skipping when setting a 
>>> breakpoint, using
>>> "break *foo".
>>
>> Sorry if my question doesn't make sense, but wouldn't we also want to 
>> verify that prologue skipping works correctly in a function with 
>> multiple entrypoints?
>>
>
> Agreed, ideally we'd also verify it in this test-case.
>
> However, the commit introducing support for this also adds a test-case 
> gdb.fortran/entry-point.exp which does verify that part of the 
> functionality with compiler-generated entry points and prologues.
>
>> I agree that this looks like a testsuite fail rather than a real GDB 
>> bug, but I feel like we'll be losing a valuable test if we don't try 
>> it at any point, especially now that it is easy to explicitly mark an 
>> instruction as EPILOGUE_END in the dwarf assembler.
>>
>
> I suppose you mean prologue_end (and I hadn't thought of that, thanks 
> for the suggestion), but yes, we could do this.  It would require 
> adding a .debug_line section in the dwarf assembly, something that is 
> currently missing.
oops, yes sorry, prologue_end. Working on the epilogue_begin stuff has 
me mixing the 2 words all the time hahaha
>
> But given that the other test-case already tests this functionality, 
> I'm not sure if it's worth the effort.

Yeah, I think I agree with you here that if a testcase already verifies 
that, going through the trouble of adding the whole lines section is not 
worth it.

FWIW, patch doesn't introduce any regressions on my x86, f39 machine,

Tested-By: Guinevere Larsen <blarsen@redhat.com>

-- 
Cheers,
Guinevere Larsen
She/Her/Hers