This is the mail archive of the gcc-patches@gcc.gnu.org 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]

Re: [PATCH]Fix computation of offset in ivopt


On 10/18/2013 02:10 PM, Richard Biener wrote:
> On Fri, Oct 18, 2013 at 2:02 PM, Bernd Schmidt <bernds@codesourcery.com> wrote:
>> On 10/18/2013 01:18 PM, Richard Biener wrote:
>>
>>> Index: gcc/fold-const.c
>>> ===================================================================
>>> --- gcc/fold-const.c (revision 203267)
>>> +++ gcc/fold-const.c (working copy)
>>> @@ -7270,8 +7270,8 @@ fold_plusminus_mult_expr (location_t loc, enum tre
>>>        HOST_WIDE_INT int01, int11, tmp;
>>>        bool swap = false;
>>>        tree maybe_same;
>>> -      int01 = TREE_INT_CST_LOW (arg01);
>>> -      int11 = TREE_INT_CST_LOW (arg11);
>>> +      int01 = int_cst_value (arg01);
>>> +      int11 = int_cst_value (arg11);
>>>
>>> this is not correct - it will mishandle all large unsigned numbers.
>>>
>>> The real issue is that we rely on twos-complement arithmetic to work
>>> when operating on pointer offsets (because we convert them all to
>>> unsigned sizetype).  That makes interpreting the offsets or expressions
>>> that compute offsets hard (or impossible in some cases), as you can
>>> see from the issues in IVOPTs.
>>
>> I still have patches to keep pointer types in ivopts (using a new
>> POINTER_PLUSV_EXPR). Would that help in this case? Last time I posted
>> them they met an unenthusiastic reception so I've never bothered to
>> repost them.
> 
> Can you point me to that patch?  Or shortly elaborate on "keep pointer types
> in ivopts"?  I think this issue is about decomposing offset computations into
> a constant and a variable part, which after foldings messed up the unsigned
> computation can result in "odd" constant offset parts.  So it's rather
> because the offset operand of POINTER_PLUS_EXPR is fixed as
> sizetype.

Okay, my patch doesn't address that part, it only ensures the pointer
base values are kept and arithmetic on them is done using POINTER_PLUS.

The original patch was here.
http://gcc.gnu.org/ml/gcc-patches/2012-03/msg01104.html


Bernd


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