[patch] Misc changes to LTOCodeGenerator

[Bob Wilson]

Shuxin,

It is hard to review this patch because you've combined 3 separate changes.  Please split this up and resubmit the patches so we can consider each change separately.

On Aug 12, 2013, at 2:32 PM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:

> Ping...
> 
> It actually took me lots of effort to convince people inside Apple as to It is *terribly* bad idea to expose
> control of libLTO.{dylib,so} to linkers, and I hope in the long run, we will ditch those damn lto_xxx APIs,
> or just expose to those who just want to plugin a home-brew linker for hobby project...
> 
> It seems I need to dual effort to convince people as well...
> 
> Why it is terrible to use those lto_xxx APIs in the projects he/she want to make money out of it, and is
> release in separate SW?
> 
> Considering design a PC with 3 components involved : CPU, graphic adapter, display. Suppose the
> adapater is installed on old PCI bus, and display can hook to VGA port.
> 
>   Design 1: put CPU in computer case, and graphic-adapter stay with display, they two form monitor.
>                  Dig a hole on the computer case to install a socket for PCI bus.
>                  And the monitor hook to the external-PCI-socket?
> 
>   Design 2: CPU and adapter stay in computer case,  which communicate with display with VCG.
> 
>  Design 1 sound like a disaster right? Linker directly calling lto_xxx APIs are similar to the design-1
> in many ways.
> 
>  In our case, things are even messy. We support at least two linkers, each has its esoteric way to
> "control" the libLTO. (Do we have unified way to save the merged-linker?).   Adding control
> to linkers is just like cancer grow....
> 
> 
> On 8/8/13 5:22 PM, Shuxin Yang wrote:
>> Hi,
>> 
>> This patch is to :
>> 1) restructure the LTOCodegenerator little bit such that partitioning can comfortably kick in.
>>   (The code about partitioning is not include in this patch)
>> 2) introduce workdir to better manage intermediate files generated during the LTO compilation.
>> 3) tries to unify some debugging functionalities of Apple-linker and GNU-gold.
>> 
>> For the first goal, I introduce several classes:
>>  o. IPOPartition: To depict a partition.
>>  o. PostIPODriver: The driver for the Post-IPO compilation.
>>      We are going to support several variants of the drivers, all of them will derive
>>      from this base class.
>> 
>>  o. IPOFile : To manage misc info about an intermediate file generated during the IPO or
>>               PostIPO phases, like file-name, absolute-path, last-err-code, last-error-string.
>>               It is not a be wrapper of file IOs.
>>  o. and other misc classes.
>> 
>>  They are designed in a way such the the top-level driver LTOCodeGenerator will not see much
>> difference with or without partition.  This is achieved by viewing a merged module as
>> single degenerated partition if the partition is not enabled.
>> 
>> The workdir
>> ===========
>>  To better manage the intermediate files (*.bc file, obj file etc), we put all intermediate
>> files into a temp workdir.  If the path of workdir is not specified, the workdir is created
>> under $PWD, with the name leading with "llvm-ipo".
>> 
>>  This workdir will be erased if the user dose not instruct the compiler to keep it.
>> 
>>  In order to properly erase the workdir, I have to introduce one API:
>>    lto_codegen_get_files_need_remove().
>> 
>>  Apple-ld will not see this change, as Apple-linker communicates with libLTO.dylib with
>> memory buffer instead of object files. After the libLTO.dylib hands the buffer the
>> linker, it can safely erase the workdir, i.e. it is up to libLTO.dylib it erase the workdir.
>> 
>>  The GNU-gold linker's adaptor (tools/gold/gold-plugin.cpp) is modified slightly in order
>> to accommodate this change.
>> 
>>  TODO: I don't know to install a sig-handler to erase the workdir if the linker quit
>>  prematurely. Currently, the delete-on-signal support-routine ignores directory. I don't
>>  know if this is intentional.
>> 
>> Misc
>> =====
>>   Now that we are able to keep intermediate file, the "obj-path" flag in
>> GNU-gold-linker adaptor is not useful any more, and is removed by this patch.
>> 
>>   Apple-ld's -Wl,-save-temps is not useful either. On the other hand, it will
>> become meaningless if partition is enabled. The reason is that -Wl,-save-temps
>> is to instruct linker to save "merged-module". However, the partitioning,
>> the merged-module is "merged" module at beginning; it become "remaining module"
>> right after partition.
>> 
>>   Maintaining two linker is annoying, I'd like to shift such functionality to
>> the LTO driver.
>> 
>> Thanks
>> Shuxin
> 
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