[LLVMdev] LLD: Layout-after and layout-before (original) (raw)

Shankar Easwaran shankare at codeaurora.org
Fri Mar 21 16:10:36 PDT 2014

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_ _<mailto: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?

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