[LLVMdev] LLD: Layout-after and layout-before

[Rui Ueyama]

Thank you for quick responses!

As to dead stripping, if dead stripping is the only pass we need
bi-directional edges, we might want the dead stripping pass to construct
internal data structure by reversing the graph to construct layout-before
edges from layout-after edges. This should be less error prone than
maintaining two reverse-directional edges throughout all passes. Of course
it will make time for dead stripping proportional to the number of all
atoms, rather than live ones. It looks traversing graph is surprisingly
cheep so I guess it wouldn't matter, but it needs investigation.

It's interesting that ELF no longer uses layout-before's. I agree that that
would be simpler than using both layout-after and layout-before.

I'll try to modify the dead-stripping pass as I described above. Any
concerns?

On Fri, Mar 21, 2014 at 4:10 PM, Shankar Easwaran
<shankare at codeaurora.org>wrote:

>  Hi Rui,
>
> ELF uses layout-after and in-group references now. It no longer uses
> layout-before.
>
> The reason that two references are used are to make sure garbage
> collection treats the whole group of atoms together when it wants to
> Garbage collect an atom.
>
> Thanks
>
> Shankar Easwaran
>
>
> On 3/21/2014 5:45 PM, Rui Ueyama wrote:
>
>  +llvmdev
>
> On Fri, Mar 21, 2014 at 3:43 PM, Rui Ueyama <ruiu at google.com> wrote:
>
>> Hi all,
>>
>>  I'm trying to debug an issue that LLD sometimes get into an infinite
>> loop in setChainRoot() in LayoutPass.cpp. It looks like the cause is either
>> buildPrecededByTable() handles layoutBefore edges in a wrong way or we
>> construct a contradictory layout-before/layout-after graph.
>>
>>  At this point I started thinking that I'm wasting time on data
>> structure that's more complicated than it needs to be. LayoutPass.cpp is I
>> think the most complicated piece of code in our code base and is also hard
>> to debug. If we can simplify it we totally should do.
>>
>>  So, I'm planning to remove one of layout-before or layout-after edges
>> from the graph. Currently, in LLD, if node X has an outgoing layout-before
>> edge to Y, Y always has an outgoing layout-after edge to X. In other words
>> it's doubly-linked. Doubly-linked edge is useful if you need bi-directional
>> access, however, we don't need it in LayoutPass. We only need one of two.
>>
>>  Removing one of layout-before/layout-after edges has three benefits:
>>
>>   1. Reduces memory usage and runtime overhead
>>  2. Simpler code and algorithm
>>  3. No need to maintain consistency between layout-before/layout-after
>> edges, which is often a cause of nasty bugs.
>>
>>  Does this sound good?
>>
>
>
>
> --
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by the Linux Foundation
>
>
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