[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 11:50:56 PST 2017
On 12/06/2017 01:32 PM, Martin J. O'Riordan wrote:
>
> I think that the key concept goes all the way back to the original C++
> Standard (C++98), where section 5.2.2 “Function Call” states:
>
> The first expression in the postfix expression is then called the
> /object expression/, and the call
>
> is as a member of the object pointed to or referred to. In the case of
> an implicit class member access, the
>
> implied object is the one pointed to by this. [/Note: /a member
> function call of the form f() is interpreted
>
> as (*this).f() (see 9.3.1). ]
>
> A NULL pointer does not point to an object, so two things make a NULL
> ‘this’ invalid - first the /object expression/does not refer to an
> object and is thus undefined; and second, the note which clarifies
> that it is equivalent to ‘(*this).’ means that if ‘this’ is NULL, then
> it is a NULL pointer dereference which is already undefined behaviour
> elsewhere in the Standard. I can’t remember if “notes” are normative.
>
> In “very old C++”, i.e. prior to the introduction of static member
> functions (circa 1987), the following idiom was not unusual to get the
> semantic intent of a static member provide that ‘this’ was neither
> implicitly nor explicitly used:
>
> class T {
>
> public:
>
> void wishIwasStatic();
>
> };
>
> ...
>
> ((T*)0)->wishIwasStatic();
>
> Such functions had access to all object of type ‘T’, but without the
> need for ‘friend’ declarations. But this was only a stop-gap until
> the introduction of static member functions was devised.
>
> I very much doubt that pre-C++98 code such as this is still part of
> any production application, and if it is, it really ought to be rewritten.
>
Regardless, this is not legal C++ code, and we don't need to support it.
> Making ‘this’ not de-referenceable seems to me to be a really good
> idea semantically, and if it yields performance advantages too, then
> this is a really good thing. Perhaps it might be a good idea to ping
> either Bjarne Stroustrup or the C++ Standards committee to be sure -
> though I expect many of the committee’s members are also participants
> in this forum.
>
Yes, many of us are here :-)
-Hal
> MartinO
>
> *From:*cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] *On Behalf Of
> *Hal Finkel via cfe-dev
> *Sent:* 06 December 2017 13:07
> *To:* Lei Huang <lei at ca.ibm.com>; cfe-dev at lists.llvm.org
> *Cc:* LLVM on Power <powerllvm at ca.ibm.com>
> *Subject:* Re: [cfe-dev] [RFC] Setting dereferenceable flag on the
> implicit this parameter for non-static member functions
>
> 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 <mailto:lei at ca.ibm.com>
>
>
>
>
>
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>
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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