Re: [patch] More thorough checking in reg_fits_class_p

[Richard Sandiford <rdsandiford at googlemail dot com>]

Richard Earnshaw <rearnsha@arm.com> writes:
> On 26/04/12 14:20, Jim MacArthur wrote:
>> The current code in reg_fits_class_p appears to be incorrect; since 
>> offset may be negative, it's necessary to check both ends of the range 
>> otherwise an array overrun or underrun may occur when calling 
>> in_hard_reg_set_p. in_hard_reg_set_p should also be checked for each 
>> register in the range of regno .. regno+offset.
>> 
>> A negative offset can occur on a big-endian target. For example, on 
>> AArch64, subreg_regno_offset (0, DImode, 0, TImode) returns -1.
>> 
>> We discovered this problem while developing unrelated code for 
>> big-endian support in the AArch64 back end.
>> 
>> I've tested this with an x86 bootstrap which shows no errors, and with 
>> our own AArch64 back end.
>> 
>> Ok for trunk?
>> 
>> gcc/Changelog entry:
>> 
>> 2012-04-26 Jim MacArthur<jim.macarthur@arm.com>
>>       * recog.c (reg_fits_class_p): Check every register between regno and
>>         regno+offset is in the hard register set.
>> 
>
> OK.
>
> R.
>
>> 
>> reg-fits-class-9
>> 
>> 
>> diff --git a/gcc/recog.c b/gcc/recog.c
>> index 8fb96a0..825bfb1 100644
>> --- a/gcc/recog.c
>> +++ b/gcc/recog.c
>> @@ -2759,14 +2759,28 @@ bool
>>  reg_fits_class_p (const_rtx operand, reg_class_t cl, int offset,
>>  		  enum machine_mode mode)
>>  {
>> -  int regno = REGNO (operand);
>> +  unsigned int regno = REGNO (operand);
>>  
>>    if (cl == NO_REGS)
>>      return false;
>>  
>> -  return (HARD_REGISTER_NUM_P (regno)
>> -	  && in_hard_reg_set_p (reg_class_contents[(int) cl],
>> -				mode, regno + offset));
>> +  /* We should not assume all registers in the range regno to regno + offset are
>> +     valid just because each end of the range is.  */
>> +  if (HARD_REGISTER_NUM_P (regno) && HARD_REGISTER_NUM_P (regno + offset))
>> +    {
>> +      unsigned int i;
>> +
>> +      unsigned int start = MIN (regno, regno + offset);
>> +      unsigned int end = MAX (regno, regno + offset);
>> +      for (i = start; i <= end; i++)
>> +	{
>> +	  if (!in_hard_reg_set_p (reg_class_contents[(int) cl],
>> +				  mode, i))
>> +	    return false;
>> +	}

This doesn't look right to me.  We should still only need to check
in_hard_reg_set_p for one register number.  I'd have expected
something like:

  return (HARD_REGISTER_NUM_P (regno)
	  && HARD_REGISTER_NUM_P (regno + offset)
	  && in_hard_reg_set_p (reg_class_contents[(int) cl],
				mode, regno + offset));

instead.

Richard