[cfe-dev] [RFC] Setting dereferenceable flag on the implicit this parameter for non-static member functions

Hal Finkel via cfe-dev cfe-dev at lists.llvm.org
Wed Dec 6 15:57:31 PST 2017


On 12/06/2017 05:53 PM, Hal Finkel via cfe-dev wrote:
>
>
> On 12/06/2017 05:47 PM, Hal Finkel via cfe-dev wrote:
>>
>>
>> On 12/06/2017 05:09 PM, Hubert Tong wrote:
>>> I am not sure that there is a requirement that *this remains a valid 
>>> object.
>>
>> I don't think that there is, and, IIRC, we already have an open bug 
>> about this (the same applies to reference arguments, for which we 
>> also apply the dereferenceable attribute). The problem, as Nick 
>> Lewycky pointed out a some time ago,
>
> For the record, here: 
> http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20150202/257887.html

Oh, and my hope is that the only thing we need to do to fix this without 
too many performance regressions is to add some parameter attribute, 
not_freed or similar, and have FunctionAttrs do bottom-up inference on it.

  -Hal

>
>  -Hal
>
>> is that just because a pointer is dereferenceable upon entry to the 
>> function, doesn't mean that it always will remain so.
>>
>> I agree that we should fix this at some point, but it's an orthogonal 
>> issue.
>>
>>  -Hal
>>
>>> Experimenting with the -O3 behaviour of the following program (with 
>>> a powerpc64le-unknown-linux-gnu target), it seems the 
>>> dereferenceable attribute is scope based?
>>>
>>> int a[1], b[1], *cp;
>>> void dobadstuff() { delete cp; }
>>>
>>> __attribute__((__noinline__))
>>> void foo(int * __restrict__ a, int * __restrict__ b, int &c, int n) {
>>>   dobadstuff();
>>>   for (int i = 0; i < n; ++i)
>>>     if (a[i] > 0)
>>>       a[i] = c * b[i];
>>> }
>>>
>>> int main(void) {
>>>   cp = new int;
>>>   foo(a, b, *cp, 1);
>>> }
>>>
>>> Valgrind reports an invalid read.
>>>
>>>
>>> On Wed, Dec 6, 2017 at 8:06 AM, Hal Finkel via cfe-dev 
>>> <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>>>
>>>
>>>     On 12/05/2017 01:47 PM, Lei Huang via cfe-dev wrote:
>>>>     Hello,
>>>>
>>>>     In the discussion on bugzilla 30729, it is mentioned that the
>>>>     'this' pointer needs to be valid upon entry to a non-static
>>>>     method.  Does the standard guarantee this is non-null on entry?
>>>
>>>     Yes. You have to call a non-static member function on a valid
>>>     object.
>>>
>>>>      If so, is there a reason we can't use that fact to mark
>>>>     'this' as 'dereferenceable(sizeof(*this))'?
>>>
>>>     Yes, this seems like a good idea.
>>>
>>>      -Hal
>>>
>>>>
>>>>     There are LICM optimizations we can do based on the knowledge
>>>>     that 'this' is non-null on entry to a non-static member function.
>>>>
>>>>     eg.  For the following IR,  the two highlighted loads are not
>>>>     being hoisted out of the for loop because we are not able to
>>>>     guarantee that the pointer is non-null.  If the 'this' pointer
>>>>     is guaranteed to be non-null on entry and we mark it thus,
>>>>      then the 2 loads within the for-loop body can then be hoisted
>>>>     out into the loop preheader.
>>>>
>>>>     This is of course just one example of an optimization we could
>>>>     perform based on this knowledge, but there are probably a
>>>>     number of others (i.e. anything that relies on a pointer being
>>>>     'dereferenceable(N)').
>>>>
>>>>     *$ cat a.ll*
>>>>     target datalayout = "e-m:e-i64:64-n32:64"
>>>>     target triple = "powerpc64le-unknown-linux-gnu"
>>>>     %struct.S = type { <4 x i32>, <4 x i32> }
>>>>     ; Function Attrs: norecurse nounwind readonly
>>>>     define <4 x i32>
>>>>     @_ZNK1S20constShouldBeHoistedEmDv4_i(%struct.S* nocapture
>>>>     readonly %this, i64 %n, <4 x
>>>>     i32> %x) align 2 {
>>>>     entry:
>>>>      %tobool9 = icmp eq i64 %n, 0
>>>>      br i1 %tobool9, label %for.end, label %for.body.lr.ph
>>>>     <http://for.body.lr.ph>
>>>>     for.body.lr.ph <http://for.body.lr.ph>:                        
>>>>             ; preds = %entry
>>>>      %k1 = getelementptr inbounds %struct.S, %struct.S* %this, i64
>>>>     0, i32 0
>>>>      %k2 = getelementptr inbounds %struct.S, %struct.S* %this, i64
>>>>     0, i32 1
>>>>      br label %for.body
>>>>     for.body:                             ; preds = %for.body.lr.ph
>>>>     <http://for.body.lr.ph>, %if.end
>>>>      %n.addr.011 = phi i64 [ %n, %for.body.lr.ph
>>>>     <http://for.body.lr.ph> ], [ %div, %if.end ]
>>>>      %x.addr.010 = phi <4 x i32> [ %x, %for.body.lr.ph
>>>>     <http://for.body.lr.ph> ], [ %x.addr.1, %if.end ]
>>>>      %rem = and i64 %n.addr.011, 15
>>>>      %cmp = icmp eq i64 %rem, 0
>>>>      br i1 %cmp, label %if.end, label %if.then
>>>>     if.then:                              ; preds = %for.body
>>>>     *%0 = load <4 x i32>, <4 x i32>* %k1, align 16*
>>>>      %add = add <4 x i32> %0, %x.addr.010
>>>>     *%1 = load <4 x i32>, <4 x i32>* %k2, align 16*
>>>>      %xor = xor <4 x i32> %add, %1
>>>>      br label %if.end
>>>>     if.end:                             ; preds = %for.body, %if.then
>>>>      %x.addr.1 = phi <4 x i32> [ %xor, %if.then ], [ %x.addr.010,
>>>>     %for.body ]
>>>>      %div = lshr i64 %n.addr.011, 4
>>>>      %tobool = icmp eq i64 %div, 0
>>>>      br i1 %tobool, label %for.end, label %for.body
>>>>     for.end:                              ; preds = %if.end, %entry
>>>>      %x.addr.0.lcssa = phi <4 x i32> [ %x, %entry ], [ %x.addr.1,
>>>>     %if.end ]
>>>>      ret <4 x i32> %x.addr.0.lcssa
>>>>     }
>>>>
>>>>     Regards,
>>>>     Lei Huang
>>>>     LLVM Development on POWER
>>>>     Internal mail: C2/YGK/8200/MKM
>>>>     Phone: (905) 413-4419 <tel:%28905%29%20413-4419>
>>>>     TieLine: 969-4419
>>>>     E-mail: lei at ca.ibm.com <mailto:lei at ca.ibm.com>
>>>>
>>>>
>>>>
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>>>>     <http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev>
>>>
>>>     -- 
>>>     Hal Finkel
>>>     Lead, Compiler Technology and Programming Languages
>>>     Leadership Computing Facility
>>>     Argonne National Laboratory
>>>
>>>     _______________________________________________ cfe-dev mailing
>>>     list cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>
>>>     http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>     <http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev> 
>>>
>> -- 
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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