llvm dev - Oct 2013 - [LLVMdev] [Debug Info + LTO] Type Uniquing for C types?

On Fri, Oct 11, 2013 at 12:07 PM, Eric Christopher <echristo at
gmail.com>wrote:
> >
> > Since we don't have ODR, we may have macros defined differently
for a
> struct
> > in a .h file,
> > thus having two versions of the struct from two different CU. It seems
> that
> > we can't assume
> > structs with the same name and defined in the same file/line/column
are
> the
> > same.
> >
>
> Ah right sorry, I remember this. Also, macros are evil, just ask the
> modules guys :)
>
> > Hashing the types can happen either at the front end or at IR level.
> That is
> > our first design choice :)
> >
>
> Sorta :)
>
> > I think we should try not to hash the types for non-LTO builds at the
> front
> > end or at IR level, since it does not give us
> > any benefit given that we are hashing them at the back end.
> >
> > One advantage of hashing it at IR level is that we can just hash the
> MDNodes
> > that affect the
> > type MDNode, at front end, the AST contains more information and
should
> be
> > harder to hash.
>
> It depends upon the goals. If the goal is to make debug information
> post-link smaller then just using the type hashing machinery for
> structs will be sufficient.

By "the type hashing machinery for structs", are you referring to the
type
hashing at the back end?

> However, if it's to save space during an
> LTO link then we'll want to do it in the front end.
>
Yes, my purpose here is to save memory space in number of MDNodes (also #
of DIEs) generated in a LTO build.
Type hashing at the DIE level can reduce the dwarf size.

Manman

>
> Doug: Have a link for how you do the C type merging for modules?
>
> -eric
>
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>> It depends upon the goals. If the goal is to make debug information
>> post-link smaller then just using the type hashing machinery for
>> structs will be sufficient.
>
>
> By "the type hashing machinery for structs", are you referring to
the type
> hashing at the back end?
>
I am, yes, since there's no other place we do currently.
>>
>> However, if it's to save space during an
>> LTO link then we'll want to do it in the front end.
>
>
> Yes, my purpose here is to save memory space in number of MDNodes (also #
of
> DIEs) generated in a LTO build.
> Type hashing at the DIE level can reduce the dwarf size.
>
I agree with both of these statements.

I also agree with the desire to help LTO memory consumption so we'll
need something from the front end for this since we'd like to continue
to use the folding set to do the uniquing.

-eric

On Fri, Oct 11, 2013 at 12:40 PM, Eric Christopher <echristo at
gmail.com>wrote:
> >> It depends upon the goals. If the goal is to make debug
information
> >> post-link smaller then just using the type hashing machinery for
> >> structs will be sufficient.
> >
> >
> > By "the type hashing machinery for structs", are you
referring to the
> type
> > hashing at the back end?
> >
>
> I am, yes, since there's no other place we do currently.
>
> >>
> >> However, if it's to save space during an
> >> LTO link then we'll want to do it in the front end.
> >
> >
> > Yes, my purpose here is to save memory space in number of MDNodes
(also
> # of
> > DIEs) generated in a LTO build.
> > Type hashing at the DIE level can reduce the dwarf size.
> >
>
> I agree with both of these statements.
>
> I also agree with the desire to help LTO memory consumption so we'll
> need something from the front end for this since we'd like to continue
> to use the folding set to do the uniquing.
>
Hi Eric,

Assume that we need to do type hashing (i.e. assume Doug's rules for
merging C types do not apply), would you
like it to be done on AST types or IR debug info metadata types?

One possibility is to hash the IR debug info types in DIBuilder::finalize.
When we are creating the MD nodes, we can provide a simple identifier that
is unique within the DIBuider. In finailize(),
we update the identifier to include a hash value (prepend the type name),
so types constructed by one DIBuilder will be unique
from types constructed by another DIBUilder unless they are structurally
equivalent by having the same hash.

We can then use the folding set to do the uniquing across CUs.

Thanks,
Manman

>
> -eric
>
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