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

[Hal Finkel via cfe-dev]

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
> 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, %if.end
>  %n.addr.011 = phi i64 [ %n, %for.body.lr.ph ], [ %div, %if.end ]
>  %x.addr.010 = phi <4 x i32> [ %x, %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
> TieLine: 969-4419
> E-mail: lei at ca.ibm.com
>
>
>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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