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Re: [PATCH, GCC/ARM/gcc-7-branch] Backport PR71607


Prakhar Bahuguna <prakhar.bahuguna@arm.com> writes:
> On 31/05/2017 14:11:43, Richard Sandiford wrote:
>> Prakhar Bahuguna <prakhar.bahuguna@arm.com> writes:
>> > On 31/05/2017 09:19:40, Richard Sandiford wrote:
>> >> const_ints are supposed to be stored in sign-extended form, so a 32-bit
>> >> integer with the MSB set should be 0xffffffff80000000|x instead of
>> >> 0x80000000|x.  It's a bug if you have one where that isn't true.
>> >> 
>> >> In the patch it looks like this could come from:
>> >> ...these two splits, where the GEN_INTs should probably be:
>> >> 
>> >>   gen_int_mode (..., SImode);
>> >> 
>> >> instead.
>> >
>> > Hi Richard, thanks for the tip. Is there a test case that could produce an
>> > incorrect result? I've attempted to create one using negative doubles and
>> > floats but haven't succeeded.
>> 
>> Just to check, are you testing with --enable-checking=yes,rtl?
>> 
>> When the values you tried were split, did you get the sign-extended form
>> or the zero-extended form?
>> 
>> Thanks,
>> Richard
>
> I've now rebuilt with --enable-checking=yes,rtl and it appears that the split
> values are being correctly sign-extended in the rtl and appear correctly in the
> assembly.
>
> However, if you believe it is safer to use gen_int_mode(), I'll respin the
> patch accordingly.

Yeah, I think it would be safer.  But if they were already correctly
sign-extended, then what did you mean by:

  Also the pattern for splitting 32-bit immediates had to be changed, it
  was not accepting unsigned 32-bit unsigned integers with the MSB
  set. I believe const_int_operand expects the mode of the operand to be
  set to VOIDmode and not SImode. I have only changed it in the patterns
  that were affecting this code, though I suggest looking into changing
  it in the rest of the ARM backend.

Thanks,
Richard


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