[Bug tdep/18144] New: the skip prologue functionality doesn't work well for same assembly functions

[mihail.nistor at freescale dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=18144

Bug ID: 18144
Summary: the skip prologue functionality doesn't work well for
same assembly functions
Product: gdb
Version: HEAD
Status: NEW
Severity: normal
Priority: P2
Component: tdep
Assignee: unassigned at sourceware dot org
Reporter: mihail.nistor at freescale dot com

Created attachment 8198
--> https://sourceware.org/bugzilla/attachment.cgi?id=8198&action=edit
an example, gdb and gcc information

The gdb always sets the break-point to an assembly function (or a symbol) at
the next line when the ASM file is compiled with debug information (-g option)
even if the function does not have prologue instructions.

Let's have an example:

.global _start
_start:
nop
nop
ret

The gdb will put the break-point at the second nop instruction - that is wrong
- when the user uses the break-point _start command.
In this scenario, the gdb should put the break-point at the first nop
instruction.

[mihail.nistor at freescale dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=18144

Mihail-Marian Nistor <mihail.nistor at freescale dot com> changed:

What |Removed |Added
----------------------------------------------------------------------------
Target| |aarch64
CC| |mihail.nistor at freescale dot com
Summary|the skip prologue |[aarch64] the skip prologue
|functionality doesn't work |functionality doesn't work
|well for same assembly |well for same assembly
|functions |functions

[keiths at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=18144

Keith Seitz <keiths at redhat dot com> changed:

What |Removed |Added
----------------------------------------------------------------------------
CC| |keiths at redhat dot com
Component|tdep |breakpoints
Severity|normal |enhancement

--- Comment #1 from Keith Seitz <keiths at redhat dot com> ---
In the attachment is a reproducer:

---- from readme.txt in the attachment ----
1) The command line to compile the assembly file with debug information
aarch64-none-elf-gcc -O0 -g -nostartfiles -o main.elf main.S

2) You can see below the steps in order to reproduce the aarch64 gdb problem.
aarch64-none-elf-gdb main.elf
(gdb) b _start
Breakpoint 1 at 0x400028: file main.S, line 4.

Observed:
The breakpoint was set at the second nop instruction.
Expected:
The breakpoint should be set at the first nop instruction.
---- end readme.txt ----

The problem here is that when you use the break command and give it a linespec
location, it *will* attempt to skip prologue instructions. Always.

However, if you give it an address location instead, gdb will *not* attempt to
skip the prologue. I guess some of us old-timers have the proper idiom hammered
into our fingers already.

Using "break *_start" should better suit your needs when debugging assembler
source files.

I am recategorizing this to "enhancement" because I think it might be desirable
to change the behavior based on the source language, e.g., if the source
language of a breakpoint location is assembler, do not do prologue-skipping. I
can see arguments both ways for this, so I will wait for maintainers to weigh
in on the issue.

[weimin.pan at oracle dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=18144

weimin.pan at oracle dot com changed:

What |Removed |Added
----------------------------------------------------------------------------
CC| |weimin.pan at oracle dot com

--- Comment #2 from weimin.pan at oracle dot com ---
It works fine now with current git master:

(gdb) disass _start
Dump of assembler code for function _start:
0x0000000000400244 <+0>: nop
0x0000000000400248 <+4>: nop
0x000000000040024c <+8>: ret
End of assembler dump.
(gdb) b _start
Breakpoint 1 at 0x400244

[palves at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=18144

Pedro Alves <palves at redhat dot com> changed:

What |Removed |Added
----------------------------------------------------------------------------
CC| |palves at redhat dot com

--- Comment #3 from Pedro Alves <palves at redhat dot com> ---
GDB stopped skipping the prologue of asm sources with:

commit 6e22494e5076e4d3c0b2c2785883162f83db499e
Author: Jan Kratochvil <***@redhat.com>
AuthorDate: Fri Jun 26 15:11:14 2015 +0200

Do not skip prologue for asm (.S) files

[weimin.pan at oracle dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=18144

weimin.pan at oracle dot com changed:

What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution|--- |FIXED

--- Comment #4 from weimin.pan at oracle dot com ---
Fix verified.