[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 15:53:36 PST 2017
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
-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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>>
>> --
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
>> _______________________________________________ cfe-dev mailing
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>>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
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--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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