Re: buildbot vs --enable-targets=all

[Luis Machado <luis.machado@arm.com>]

On 8/3/22 12:49, Mark Wielaard wrote:
> Hi Luis,
> 
> On Wed, 2022-08-03 at 12:35 +0100, Luis Machado wrote:
>> On 8/3/22 11:38, Mark Wielaard wrote:
>>> On Mon, 2022-07-25 at 21:59 -0400, Jeffrey Walton wrote:
>>>> On Mon, Jul 25, 2022 at 8:26 AM Mark Wielaard <mark@klomp.org>
>>>> wrote:
>>>>> On fedora-s390x and debian-ppc64 one of the gdb selftests fails
>>>>> https://builder.sourceware.org/buildbot/#builders/75/builds/783
>>>>> https://builder.sourceware.org/buildbot/#builders/76/builds/772
>>>>>
>>>>> Running selftest arm-record.
>>>>> Process record and replay target doesn't support syscall number
>>>>> -2036195
>>>>> Process record does not support instruction 0x7f70ee1d at
>>>>> address 0x0.
>>>>> Self test failed: self-test failed at ../../binutils-
>>>>> gdb/gdb/arm-
>>>>> tdep.c:14407
>>>>>
>>>>> Which is:
>>>>>
>>>>>     /* 32-bit Thumb-2 instructions.  */
>>>>>     {
>>>>>       arm_insn_decode_record arm_record;
>>>>>
>>>>>       memset (&arm_record, 0, sizeof (arm_insn_decode_record));
>>>>>       arm_record.gdbarch = gdbarch;
>>>>>
>>>>>       static const uint16_t insns[] = {
>>>>>         /* 1d ee 70 7f     mrc    15, 0, r7, cr13, cr0, {3} */
>>>>>         0xee1d, 0x7f70,
>>>>>       };
>>>>>
>>>>>       enum bfd_endian endian = gdbarch_byte_order_for_code
>>>>> (arm_record.gdbarch);
>>>>>       instruction_reader_thumb reader (endian, insns);
>>>>>       int ret = decode_insn (reader, &arm_record, THUMB2_RECORD,
>>>>>                              THUMB2_INSN_SIZE_BYTES);
>>>>>
>>>>>       SELF_CHECK (ret == 0);
>>>>>       SELF_CHECK (arm_record.mem_rec_count == 0);
>>>>>       SELF_CHECK (arm_record.reg_rec_count == 1);
>>>>>       SELF_CHECK (arm_record.arm_regs[0] == 7);
>>>>>     }
>>>>>
>>>>> This seems a big endian issue given the instructions are given
>>>>> as two
>>>>> 16bit numbers.
>>>>
>>>> [...]
>>>>> For ARM, this does not look right (to me):
>>>>>       static const uint16_t insns[] = {
>>>>>         /* 1d ee 70 7f     mrc    15, 0, r7, cr13, cr0, {3} */
>>>>>         0xee1d, 0x7f70,
>>>>>       };
>>>>
>>>> I think you are supposed to use .inst.n and .inst.w because they
>>>> handle endianness properly.
>>>
>>> I couldn't figure out how to do that. Could you give an example?
>>> It looks like the instruction_reader_thumb only takes an array of
>>> uint16_t (even though the instructions are 32bit long). But I might
>>> be
>>> looking at the wrong code.
>>>
>>> So the following fixes it for me on s390x and ppc64
>>>
>>> diff --git a/gdb/arm-tdep.c b/gdb/arm-tdep.c
>>> index d4c5beb5e06..ef0da73398d 100644
>>> --- a/gdb/arm-tdep.c
>>> +++ b/gdb/arm-tdep.c
>>> @@ -14471,7 +14471,7 @@ arm_record_test (void)
>>>    
>>>        static const uint16_t insns[] = {
>>>          /* 1d ee 70 7f    mrc    15, 0, r7, cr13, cr0, {3} */
>>> -      0xee1d, 0x7f70,
>>> +      0x7f70, 0xee1d,
>>>        };
>>>    
>>>        enum bfd_endian endian = gdbarch_byte_order_for_code
>>> (arm_record.gdbarch);
>>>
>>> But obviously that breaks things on little-endian architectures.
>>> We could define the insns[] differently using
>>>
>>>    #if _BYTE_ORDER == _LITTLE_ENDIAN
>>>         0xee1d, 0x7f70,
>>>    #else
>>>         0x7f70, 0xee1d,
>>>    #endif
>>>
>>> But that might not be what you meant.
>>>
>> I wonder what's going wrong here given we're using
>> gdbarch_byte_order_for_code to read things.
>>
>> Technically it should read in the right order.
> 
> Yes, if the code bytes were in the right order. Like when you read them
> from disk or from a process running in that byte order. But here they
> are constructed in memory in the byte order of the host architecture as
> an uint16_t array. So when using the gdbarch_byte_order_for_code order
> to reconstruct those instructions it is not using the host byte order
> (if they don't match).
> 
> I think what Jeffrey means is that there is a different way to
> construct the bytes in memory using .inst.n and .inst.w, but I don't
> fully understand how that works.
> 
> Cheers,
> 
> Mark

Ah, I see. And s390/ppc being potentially big endian, this fails.

I managed to reproduce this on my end by forcing GDB to use big endian. Could you please file a PR for this one and cc me?

I'll try to come up with a fix.