[llvm-commits] Atom alignment

[Nick Kledzik]

On Oct 15, 2012, at 1:02 PM, Shankar Kalpathi Easwaran wrote:

> Hi Nick,
> 
> Thanks for your reply.
> 
> We would need to put data into seperate sections for the optional optimization pass to work though.
What do you mean by "put into separate sections"?   Do you mean -fdata-sections where the compiler creates a unique section for each data item?  Or do you mean the ELF Reader will need to do something?  

In general, lld downplays sections as much as possible.  With the Atom mode,l each atom carries all its own attributes.  The only time sections come up is if the output format requires certain sections or the atom specifies it requires a custom section, so the Writer places atoms in the sections required.  

In other words, the Pass just re-orders atoms.  There are no sections to deal with.


> what are the tools that exist on darwin to get that information ? Does it use profile based optimization techniques ?
Currently just some dtrace scripts that record access patterns.  But there is a lot of room for improvement of the tools. 


-Nick


> How do we get a file based order, as atoms dont have 
> 
> On Mon, Oct 15, 2012 at 2:21 PM, Nick Kledzik <kledzik at apple.com> wrote:
> On Oct 15, 2012, at 11:54 AM, Shankar Kalpathi Easwaran wrote:
>> Hi,
>> 
>> I think we will also get gaps in the output if the atoms are processed in a out of order fashion.
>> 
>> Example:
>> 
>> Say there are two files that are part of the linking process 1.o, 2.o 
>> 
>> 1.o has data symbols (a, b)
>> 2.o has data symbols (c, d)
>> 
>> a, b has alignment requirements of 4 whereas c, d has alignment requirements of 16
>> 
>> So when we try to write the output with the following use cases :
>> 
>> 1) the atoms appear as a,b,c,d the output file appears properly
>> 2) the atoms appear as a,c,b,d the output file will have gaps
>> 3) whenever there is an atom movement because of symbol resolution, the output file will have gaps too
>> 
>> How do we go about without having gaps in the output file consistently and the output file using optimum space ?
> 
> My philosophy is that the default layout should be super simple and predictable (e.g. file order, then order within file, so: a, b, c, d which means gaps).
> 
> But we can write an optional optimization Pass which re-orders the data for better packing.  On embedded systems packing might be normal mode.  
> 
> On darwin, we have tools to produce "order files" which determine working sets of symbols by monitoring use at runtime.  By packing data symbols based on usage, you get fewer dirty pages at runtime.
> 
> -Nick
> 
>> 
>> Should the sections and the atom offsets be fixed after running it through the Reader.
>> 
>> Thanks
>> 
>> Shankar Easwaran
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation
>> 
>> 
>> 
>> -----Original Message-----
>> From: Sid Manning [mailto:sidneym at codeaurora.org]
>> Sent: Monday, October 15, 2012 11:23 AM
>> To: LLVM Commits; Hemant Kulkarni; Michael Spencer; Nick Kledzik; Evandro; Easwaran, Shankar
>> Subject: Atom alignment
>> 
>> 
>> DefinedAtom's alignment method is applying section alignment constraints to every atom in the chunk and this causes gaps in the output.  This change returns alignment only if the atom is a section symbol.
>> 
>> 
>> --
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation
>> 
> 
> 

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