[LLVMdev] patch for llc/ARM: added mechanism to move switch tables from .text -> .data; also cleanup and documentation

Bob Wilson bob.wilson at apple.com
Tue Jul 14 15:48:51 PDT 2009

On Jul 2, 2009, at 10:48 AM, robert muth wrote:
> I spend over a day trying to follow your suggestion. In the end I  
> was not successful. Here is what Iearned:
> After setting
> ARMJITInfo::hasCustomJumpTables -> true
> setOperationAction for ISD::BR_JT -> Expand
> I needed to add a "brind" definition to ARMInstrInfo.td
> I picked "bx" but to do a proper job one would have to take older  
> architectures
> into account..
> The next thing was to to set
>  setOperationAction for ISD::JumpTable -> Custom
> and implement the corresponding code.
> This code is very similar to my "outline" code as it has to  
> materialize
> the table start address.
>  I never quite got this  to work, though.

Sorry for the slow response.  Since it wasn't clear what problems you  
ran into, and since I wasn't very familiar with LLVM's handling of  
jump tables, I wanted to have a look at it myself to see what the  
issues were.

I was able to make it work after fixing one problem: the branch folder  
removes "dead" jump tables and it doesn't scan the constant pool for  
jump table references.  I had to add some code to the branch folder  
and the MachineConstantPoolValue class to fix that.  I haven't yet  
done much testing of this but it looks like it's basically working.

Since I had to write the code to try this out, I'm sending my revised  
patch back to you.  There are still a few issues to sort out before  
committing this:

* I followed your lead and added a "brind" pattern using "bx".  I  
think this works on older architectures, too.  What issue were you  
thinking of with regard to supporting older architectures?  I didn't  
even look at Thumb or Thumb2, but we'll need something for them.

* I saw some earlier discussion about the command-line option name.   
You had "outline-jumptables" and Evan had suggested "OutOfLine".  I  
don't particularly like either one (sorry, Evan).  How about "no- 
inline-jumptables"?  I prefer that because it captures the sense that  
this option is disabling an ARM-specific feature (the inline  
jumptables) and changing back to the default.

* I didn't look at the JIT code emitter at all.  That will need to be  

* It would be great to change the testcase to use the new FileCheck  

I've attached my revised patch to show you how I made this work.  I've  
also attached a copy of your last patch with embedded comments (search  
for "bob>").  Let me know what you think and if you can take a look at  
addressing some of the open issues above.

> Presumably what the generic code generator
> would to is to use this table start multiple the "switch index" by 4  
> load the target address and jump there. Not a very  big saving in my  
> mind.
> And it is offset by having to touch ARMInstrInfo.td.
> Also note, that the generic code generator must make some assumptions
> about the format of each jumptable entry (e.g. 32bit absolute  
> addresses) so this cannot be changed easily afterwards.
> Having the default jumtables and the out of line version diverge too  
> much
> and have different code path may also not be such a good idea.

I'm not so much concerned about savings in the quantity of code as I  
am in keeping the ARM backend from diverging too far from the other  
LLVM backends.  Sometimes we have to do that but it is an ongoing  
maintenance burden, and in this case, it seems like we can easily  
avoid it.

It also helps leverage the existing code.  For example, I didn't look  
too closely, but with my patch, compiling with -relocation-model=pic  
seemed to do the right thing.

> Why did you change the default value for JumpTableDataSection?  Jump  
> tables really should not go into .data.  That is a security hole.   
> They should be read-only.
> I changed that too so the data goes into the default .rodata now.
> I am not sure where .rodata lives. if it lives in the text *segment*
> you get better protection but the code wont be PIC if .rodata
> goes into the data segment it is the other way round.

If you compile with -relocation-model=pic, then the jump table  
contains offsets relative to the start of the table.  The tables live  
in .rodata (or even in .text itself, but not inline in the code) which  
is in the same segment as .text and thus the code is still PIC.

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