This is the mail archive of the mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Suspected bugs in ptr_difference_const & split_address_to_core_and_offset

I experienced an issue in our port, which I suspected due to bugs
in ptr_difference_const & split_address_to_core_and_offset. Basically,
ptr_difference_const, called by ivopts pass, tries to evaluate 
whether e1 & e2 differ by a const. In this example,

e1 is (addr_expr (mem_ref (ssa_name1, 8))
e2 is just ssa_name1.

It is obvious to me that ptr_difference_const should return true. But
it calls split_address_to_core_and_offset to split e1 to some base pointer
and offset. split_addess_to_core_and_offset in turn calls get_inner_reference
to do it. get_inner_reference cannot handle (mem_ref (ssa_name1, 8)),
it just returns the same expression back. 

get_inner_reference function
	case MEM_REF:
	  /* Hand back the decl for MEM[&decl, off].  */
	  if (TREE_CODE (TREE_OPERAND (exp, 0)) == ADDR_EXPR)
	      tree off = TREE_OPERAND (exp, 1);
	      if (!integer_zerop (off))
		  double_int boff, coff = mem_ref_offset (exp);
		  boff = coff.alshift (BITS_PER_UNIT == 8
				       ? 3 : exact_log2 (BITS_PER_UNIT),
		  bit_offset += boff;
	      exp = TREE_OPERAND (TREE_OPERAND (exp, 0), 0);
	  goto done;

Then in ptr_difference_const, we get core1 as (mem_ref (ssa_name1, 8))
and toffset1 is empty. ptr_difference_const will return false as result.

There is another possible bug in ptr_difference_const. If one of toffset1
& toffset2 is true, why it returns false?  The comment doesn't make sense
to me. In this example, toffset1 should be 8 and toffset2 should be empty.
No way it should return false.

  else if (toffset1 || toffset2)
     /* If only one of the offsets is non-constant, the difference cannot
        be a constant.  */
      return false;

Any comment? I would like to submit a patch for it. The problem is I don't
have an reproducible example on x86 or other public targets. I ran through
the x86-64 tests and didn't hit a single case that meets this condition.
e1 is (addr_expr (mem_ref (ssa_name1, 8))
e2 is just ssa_name1.
Not sure about other targets though.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]