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<div class="moz-cite-prefix">On 06/01/2016 03:48 PM, Peter
Collingbourne via llvm-dev wrote:<br>
</div>
<blockquote
cite="mid:CAPQLkRiny1WmJ_4_t4W3nSBHtbi+T=qw9w_hWb=JwU1TF4eCNg@mail.gmail.com"
type="cite">
<div dir="ltr">
<div>Hi all,</div>
<div><br>
</div>
The bitset metadata currently used in LLVM has a few problems:
<div><br>
</div>
<div>1. It has the wrong name. The name "bitset" refers to an
implementation detail of one use of the metadata (i.e. its
original use case, CFI). This makes it harder to understand,
as the name makes no sense in the context of virtual call
optimization.</div>
<div>2. It is represented using a global named metadata node,
rather than being directly associated with a global. This
makes it harder to manipulate the metadata when rebuilding
global variables, summarise it as part of ThinLTO and drop
unused metadata when associated globals are dropped. For this
reason, CFI does not currently work correctly when both CFI
and vcall opt are enabled, as vcall opt needs to rebuild
vtable globals, and fails to associate metadata with the
rebuilt globals. As I understand it, the same problem could
also affect ASan, which rebuilds globals with a red zone.</div>
<div><br>
</div>
<div>I would like to solve both of these problems in the
following way:</div>
<div><br>
</div>
<div>1. Rename the metadata to "type metadata". This new name
reflects how the metadata is currently being used (i.e. to
represent type information for CFI and vtable opt).</div>
<div>2. Attach metadata directly to the globals that it pertains
to, rather than using the "llvm.bitsets" global metadata node
as we are doing now. This would be done using the newly
introduced capability to attach metadata to global variables
(r271348 and r271358). Passes which manipulate globals can
easily copy metadata between globals with the
GlobalObject::copyMetadata function, which would be taught to
understand type metadata.</div>
<div><br>
</div>
<div>To give an example of how this would look, suppose that we
have the following declarations:</div>
<div><br>
</div>
<div>class A {</div>
<div> virtual void f() {}</div>
<div>};<br clear="all">
<div><br>
</div>
<div>class B : public A {</div>
<div> virtual void f() {}</div>
<div> virtual void g() {}</div>
<div>};<br>
</div>
<div><br>
</div>
<div>The vtables for A and B would be represented in IR like
this:</div>
<div><br>
</div>
<div>@_ZTV1A = constant [3 x i8*] [i8* ..., i8* ..., i8*
@A::f], !type !0</div>
<div>@_ZTV1B = constant [4 x i8*] [i8* ..., i8* ..., i8*
@B::f, i8* @B::g], type !0, !type !1</div>
<div><br>
</div>
<div>!0 = {i64 16, !"A"}</div>
<div>!1 = {i64 16, !"B"}</div>
<div><br>
</div>
<div>The metadata !0 indicates that the attached global has an
address point for the type A at byte offset 16, and metadata
!1 indicates that the attached global has an address point
for the type B at byte offset 16. We attach !0 to _ZTV1A,
which indicates that the vtable for A has a valid address
point for A at offset 16, and attach both !0 and !1 to
_ZTV1B, which indicates that the vtable for B has a valid
address point for both A and B at offset 16.</div>
</div>
</div>
</blockquote>
Can you define "address point"? I don't recognize the term and thus
didn't understand the example.<br>
<br>
FTR, I have no stance on the proposal. <br>
<blockquote
cite="mid:CAPQLkRiny1WmJ_4_t4W3nSBHtbi+T=qw9w_hWb=JwU1TF4eCNg@mail.gmail.com"
type="cite">
<div dir="ltr">
<div>
<div><br>
</div>
<div>I also plan to apply this renaming to existing passes and
intrinsics that use the "bitset" name.</div>
<div><br>
</div>
<div>Thanks,</div>
<div>-- <br>
</div>
<div class="gmail_signature" data-smartmail="gmail_signature">
<div dir="ltr">--
<div>Peter</div>
</div>
</div>
</div>
</div>
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</blockquote>
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