[Mesa-dev] [PATCH] intel/compiler: Print floating point values upto precision 8

[Sagar Ghuge]

On 10/22/18 10:34 AM, Matt Turner wrote:
> On Fri, Oct 5, 2018 at 11:15 AM Sagar Ghuge <sagar.ghuge at intel.com> wrote:
>>
>> avoid misinterpretation of encoded immediate values in instruction and
>> disassembled output.
>>
>> Signed-off-by: Sagar Ghuge <sagar.ghuge at intel.com>
>> ---
>> While encoding the immediate floating point values in instruction we use
>> values upto precision 8, but while disassembling, we print precision to
>> 6 places, which round up the value and gives wrong interpretation for
>> encoded immediate constant. Printing it upto precision 8 will help in
>> reassembling it again.
> 
> Let's put that in the commit message.
> 
>>  src/intel/compiler/brw_disasm.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/src/intel/compiler/brw_disasm.c b/src/intel/compiler/brw_disasm.c
>> index 322f4544df..7cbbc080b3 100644
>> --- a/src/intel/compiler/brw_disasm.c
>> +++ b/src/intel/compiler/brw_disasm.c
>> @@ -1293,7 +1293,7 @@ imm(FILE *file, const struct gen_device_info *devinfo, enum brw_reg_type type,
>>        format(file, "0x%08xV", brw_inst_imm_ud(devinfo, inst));
>>        break;
>>     case BRW_REGISTER_TYPE_F:
>> -      format(file, "%-gF", brw_inst_imm_f(devinfo, inst));
>> +      format(file, "%.8fF", brw_inst_imm_f(devinfo, inst));
> 
> I'm not sure 8 digits is sufficient to get an exact representation
> that the assembler can "round-trip". This page [1] indicates that 9
> digits are necessary for binary->decimal->binary round-trips.
> 
I was also not sure about it, [1] article is nice. 
 
> NIR takes another approach:
> 
> vec1 32 ssa_0 = load_const (0x3f800000 /* 1.000000 */)
> 
> What do you think about printing the binary representation and the
> floating-point value? That way humans can easily read one number and
> the assembler can easily read the other :)
> 
I think we can just print F and DF to 9 and 17 precision respectively to avoid
output alignment issue.

> Also, I think the DF case immediately after this should be handled as well.
> 
Yes,  I was planning to handle it when I shift to 64 bit datatypes. But I can club both in single
patch. 
> [1] https://www.exploringbinary.com/number-of-digits-required-for-round-trip-conversions/
>