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List:       llvm-dev
Subject:    Re: [LLVMdev] big bitcode files producing bad ARM asm
From:       Nick Kledzik <kledzik () apple ! com>
Date:       2011-04-26 3:34:07
Message-ID: 5D9AD289-D51F-4A57-ADF3-309DCD45D55B () apple ! com
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On Apr 25, 2011, at 8:17 PM, Alexander MacDonald wrote:
> On 25 Apr 2011, at 18:20, Jakob Stoklund Olesen wrote:
> > On Apr 25, 2011, at 6:01 PM, Alexander MacDonald wrote:
> > 
> > > I have a rather large bitcode file which when run through "llc -march arm -O0" \
> > > produces an asm file of about 500Mb. Trying to assemble this file with the ios \
> > > assembler on osx gives me lots of "branch out of range" errors thanks to jump \
> > > instructions overflowing the +/-32Mb relative jump limit. 
> > > I've tried running llc with the hidden "-arm-long-calls" option, which solves \
> > > the problem but forces everything to be an indirect branch. That feels a bit \
> > > like overkill, does anybody have a suggestion for what the right solution might \
> > > be?
> > 
> > I don't think any other solutions are currently supported.
> > 
> > One problem is that the linker can move functions around as it pleases, so there \
> > is no way of knowing which functions are going to be far away.
> 
> But the linker will fix branches that become "long-calls" after it's shuffled \
> things around right? so it would still be reasonable to try to get LLVM to at least \
> codegen a single object file correctly, assuming that the codegen phase has some \
> knowledge of roughly how big the branches will have to be when it is generating the \
> asm, which on second thought it probably doesn't unless it knows the size of all \
> functions before writing out the asm (I'm not too familiar with the codegen phase).

If the problem is that you have zillions of small functions (as opposed to some MB \
sized functions), then this is really a limitation of the mach-o object format.  The \
iOS linker tool will happily synthesize branch islands when the target of a bl is too \
far away (that is, it will add an island of code between functions that is just jump \
instruction.  The bl branches to the island which jumps to the final target).  

In ARM mach-o, if the target of a bl is in the same translation unit, the assembler \
uses a local relocation and adjusts the instruction to branch to the target address.  \
If the address is too far, there is no way to encode it ;-(    You might think \
switching to an external relocation would work, but because mach-o has no RELA \
relocations, the addend is encoded in the instruction.  And, unfortunately, the \
pc-rel instructions like bl, they are encoded as the target of the bl instruction.  \
So an addend of zero means the instruction looks like it branches to address zero, \
which means the instruction must itself within branch range of zero. So you are \
limited in how big your .o file can be.

You might get lucky and be able to just chop up the big assembly text file into \
pieces and assemble each one...

-Nick


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