Re: [PATCH] handle pathological anti-ranges in gimple_fold_builtin_memory_op (PR 81908)

[Jeff Law <law at redhat dot com>]

On 08/24/2017 09:03 AM, Martin Sebor wrote:
> On 08/24/2017 08:03 AM, Richard Biener wrote:
>> On Wed, Aug 23, 2017 at 9:42 PM, Martin Sebor <msebor@gmail.com> wrote:
>>> Bug 81908 is about a -Wstringop-overflow warning for a Fortran
>>> test triggered by a recent VRP improvement.  A simple test case
>>> that approximates the warning is:
>>>
>>>   void f (char *d, const char *s, size_t n)
>>>   {
>>>     if (n > 0 && n <= SIZE_MAX / 2)
>>>       n = 0;
>>>
>>>     memcpy (d, s, n);   // n in ~[1, SIZE_MAX / 2]
>>>   }
>>>
>>> Since the only valid value of n is zero the call to memcpy can
>>> be considered a no-op (a value of n > SIZE_MAX is in excess of
>>> the maximum size of the largest object and would surely make
>>> the call crash).
>>>
>>> The important difference between the test case and Bug 81908
>>> is that in the latter, the code is emitted by GCC itself from
>>> what appears to be correct source (looks like it's the result
>>> of the loop distribution pass).  I believe the warning for
>>> the test case above and for other human-written code like it
>>> is helpful, but warning for code emitted by GCC, even if it's
>>> dead or unreachable, is obviously not (at least not to users).
>>>
>>> The attached patch enhances the gimple_fold_builtin_memory_op
>>> function to eliminate this patholohgical case by making use
>>> of range information to fold into no-ops calls to memcpy whose
>>> size argument is in a range where the only valid value is zero.
>>> This gets rid of the warning and improves the emitted code.
>>>
>>> Tested on x86_64-linux.
>>
>> @@ -646,6 +677,12 @@ gimple_fold_builtin_memory_op
>> (gimple_stmt_iterator *gsi,
>>    tree destvar, srcvar;
>>    location_t loc = gimple_location (stmt);
>>
>> +  if (tree valid_len = only_valid_value (len))
>> +    {
>> +      /* LEN is in range whose only valid value is zero.  */
>> +      len = valid_len;
>> +    }
>> +
>>    /* If the LEN parameter is zero, return DEST.  */
>>    if (integer_zerop (len))
>>
>> just enhance this check to
>>
>>   if (integer_zerop (len)
>>       || size_must_be_zero_p (len))
>>
>> ?  'only_valid_value' looks too generic for this.
> 
> Sure.
> 
> FWIW, the reason I did it this was because my original patch
> returned error_mark_node for entirely invalid ranges and had
> the caller replace the call with a trap.  I decided not to
> include that part in this fix to keep it contained.
Seems reasonable.  Though I would suggest going forward with trap
replacement for clearly invalid ranges as a follow-up.  Once you do trap
replacement, the input operands all become dead as does all the code
after the trap and the outgoing edges in the CFG.

That often exposes a fair amount of cleanup.  Furthermore on targets
that have conditional traps, the controlling condition often turns into
a conditional trap.  In all, you get a lot of nice cascading effects
when you turn something that is clearly bogus into a trap.

gimple-ssa-isolate-erroneous-paths probably has the infrastructure you
need, including a code you can crib to detect PHI arguments which would
cause bogus behavior and allow you to isolate that specific path.



> 
>>
>> +  if (!wi::fits_uhwi_p (min) || !wi::fits_uhwi_p (max))
>> +    return NULL_TREE;
>> +
>>
>> why?
> 
> Only because I never remember what APIs are safe to use with
> what input.
> 
>> +  if (wi::eq_p (min, wone)
>> +      && wi::geu_p (max + 1, ssize_max))
>>
>>    if (wi::eq_p (min, 1)
>>       && wi::gtu_p (max, wi::max_value (prec, SIGNED)))
>>
>> your ssize_max isn't signed size max, and max + 1 might overflow to zero.
> 
> You're right that the addition to max would be better done
> as subtraction from the result of (1 << N).  Thank you.
> 
> If (max + 1) overflowed then (max == TYPE_MAX) would have
> to hold which I thought could never be true for an anti
> range. (The patch includes tests for this case.)  Was I
> wrong?
Couldn't we have an anti range like ~[TYPE_MAX,TYPE_MAX]?  Or am I
misunderstanding something.

> 
> Attached is an updated version with the suggested changes
> plus an additional test to verify the absence of warnings.
The patch is OK.

I'll note this is another use of anti-ranges.  I'd really like to see
Aldy's work on the new range API move forward and get rid of anti-ranges.

THanks,
jeff