Re: [RFA/commit] (SPARC/LEON) fix incorrect array return value printed by "finish"

vladimir.mezentsev
2018-04-24T07:18:41Z

On 04/23/2018 10:45 AM, Joel Brobecker wrote:
> Consider the code in the gdb.ada/array_return.exp testcase, which
> defines a function returning an array of 2 integers:
>
> type Data_Small is array (1 .. 2) of Integer;
> function Create_Small return Data_Small;
>
> When doing a "finish" from inside function Create_Small, we expect
> GDB to tell us that the return value was "(1, 1)". However, it currently
> prints the wrong value:
>
> (gdb) finish
> Run till exit from #0 pck.create_small () at /[...]/pck.adb:5
> p () at /[...]/p.adb:10
> 10 Large := Create_Large;
> Value returned is $1 = (0, 0)

Â I never usedÂ  ada and it looks like a bug in ada compiler not in gdb.
Probably ada generates incorrect code for function which returns a small
array.

The similar cÂ  test works:
% cat r.c
#include <stdio.h>

typedef int __attribute__ ((vector_size (2 * sizeof(int)))) I2;

I2 func() {
Â Â Â  I2 a;
Â Â Â  a[0] = 11;
Â Â Â  a[1] = 12;
Â Â Â  return a;
}

int main() {
Â Â Â  I2 x = func();
Â Â Â  printf("%d %d\n", x[0], x[1]);
Â Â Â  return 0;
}

% gcc -g r.c
% ./a.out
11 12

% /export/home/vmezents/git/obj_binutils-gdb/gdb/gdb ./a.out
GNU gdb (GDB) 8.0.50.20170512-git
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License GPLv3+: GNU GPL version 3 or later
<http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.Â  Type "show copying"
and "show warranty" for details.
This GDB was configured as "sparc64-unknown-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
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For help, type "help".
Type "apropos word" to search for commands related to "word"...
Resuming the execution of threads of all processes is on.
Reading symbols from ./a.out...done.
(gdb) b main
Breakpoint 1 at 0x1006a8: file r.c, line 13.
(gdb) run
Starting program: /export/home/vmezents/BUGs/25502809/a.out

Breakpoint 1, main () at r.c:13
13Â Â Â  Â Â Â  I2 x = func();
(gdb) print func()
$1 = {11, 12}

See alsoÂ  my comment below.

> This is a regression which I traced back to the following commit...
>
> | commit 1933fd8ee01ad2e74a9c6341bc40f54962a8f889
> | Date: Fri May 19 03:06:19 2017 -0700
> | Subject: gdb: fix TYPE_CODE_ARRAY handling in sparc targets
>
> ... which, despite what the subject says, is not really about
> TYPE_CODE_ARRAY handling, which is a bit of an implementation detail,
> but about the GNU vectors extension.
>
> The author of the patch equated TYPE_CODE_ARRAY with vectors, which
> is not correct. Vectors are TYPE_CODE_ARRAY types with the TYPE_VECTOR
> flag set. So at the very minimum, the patch should have been checking
> for both TYPE_CODE_ARRAY and TYPE_VECTOR.
>
> But, that's not the only thing that did not seem right to me. When
> looking at the ABI, and at the summary of the implementation in GCC
> of the calling conventions for that architecture:
>
> size argument return value
>
> small integer <4 int. reg. int. reg.
> word 4 int. reg. int. reg.
> double word 8 int. reg. int. reg.
>
> _Complex small integer <8 int. reg. int. reg.
> _Complex word 8 int. reg. int. reg.
> _Complex double word 16 memory int. reg.
>
> vector integer <=8 int. reg. FP reg.
> vector integer >8 memory memory
>
> float 4 int. reg. FP reg.
> double 8 int. reg. FP reg.
> long double 16 memory memory
>
> _Complex float 8 memory FP reg.
> _Complex double 16 memory FP reg.
> _Complex long double 32 memory FP reg.
>
> vector float any memory memory
>
> aggregate any memory memory

Â I think the 4 from last 5 are incorrect (See for example
https://www.gnu.org/software/gcc/gcc-3.4/sparc-abi.html)
It should be:

_Complex float 8 reg FP reg.
_Complex double 16 reg FP reg.
vector float <=16 reg reg
aggregate <=16 reg reg

Â It will be good to add comments with the correct table intoÂ
gdb/sparc-tdep.c.

-Vladimir

>
> The nice thing about the patch above is that it nicely factorized
> the code that determines how arguments are passed/returns. The bad
> news is that the implementation, particularly for the handling of
> arrays and vectors, doesn't seem to match the summary above. Hence,
> the regression we observed.
>
> So what I did was review and re-implement some of the predicate functions
> according to the summary above. Because dejagnu crashes all our Solaris
> machines real bad, I can't run the dejagnu testsuite there. So what I did
> was test the patch with AdaCore's testsuite against leon3-elf, no
> regression. I verified that this fixes the regression above while
> at the same time still passing gdb.base/gnu_vector.exp (I transposed
> that testcase to our testsuite), which is the testcase that was cited
> in the commit above as seeing some FAIL->PASS improvements.
>
> This patch also removes one assertion...
>
> gdb_assert (sparc_integral_or_pointer_p (type)
> || (TYPE_CODE (type) == TYPE_CODE_ARRAY && len <= 8));
>
> ... because that assertion is really the "negative" of the other conditions
> written in the same "if, else if, else [assert]" block in this function.
> To me, this assertion forces us to maintain two versions of the same code,
> and is an unnecessary burden. In particular, the above is not the
> correct condition, and the ABI summary table above shows that we need
> a more complex condition to describe the situations where we expect
> arguments to be passed by register.
>
> gdb/ChangeLog:
>
> * sparc-tdep.c (sparc_structure_return_p): Re-implement to
> match the ABI as summarized in GCC's gcc/config/sparc/sparc.c.
> (sparc_arg_by_memory_p): Renamed from sparc_arg_on_registers_p.
> Re-implement to match the ABI as summarized in GCC's
> gcc/config/sparc/sparc.c. All callers updated.
> (sparc32_store_arguments): Remove assertion.
>
> OK to commit in master?
>
> Thanks!