llvm dev - Jul 2009 - [LLVMdev] patch for llc/ARM: added mechanism to move switch tables from .text -> .data; also cleanup and documentation

On Thu, Jun 25, 2009 at 6:17 PM, Bob Wilson <bob.wilson at apple.com>
wrote:
> Hi Robert,
> Evan asked me to review this patch, and I have some questions about it.  I
> apologize for not following the discussion earlier and for hitting you with
> questions after you've already gone through several revisions.
>
> LLVM provides some default behavior for handling jump tables, with the
> tables emitted separately from the code that uses them.  The ARM backend
> provides its own custom handling of jump tables so that it can emit them
> in-line with the text.  It seems to me that your patch changes the ARM
> backend to optionally handle jump tables in a way that is at least very
> similar to the default behavior, but you have implemented it without using
> the existing code.  Did you consider implementing it using the LLVM
> defaults?
>
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.

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.

> Using TargetAsmInfo::JumpTableDataSection is one piece of that, but there
> is more.  As a start, you could change ARMJITInfo::hasCustomJumpTables and
> the "Custom" argument to setOperationAction for ISD::BR_JT in the
> ARMTargetLowering constructor to be conditional on your new flag.  I'm
sure
> there is more required than that, but maybe that will get you started.
>
> Otherwise, there are some problems with your custom handling for
> out-of-line jump tables.  The most serious of these is
> in ARMAsmPrinter::printJTBlockOperand: for out-of-line jump tables, you
emit
> a ".text" directive to switch back to the text section.  If the
current
> section is not ".text", however, this will break.  I believe you
ought to
> use SwitchToTextSection instead.  Earlier in that same method, I think you
> should also use SwitchToDataSection and EmitAlignment.  But, again, rather
> than tweaking this code, I would prefer to find a way to use the existing
> LLVM code for this.
>
I changed all of these as per your request.

>
> 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.


> When you create global symbols for the jump tables, you specify a
".T"
> name:
>
> new ARMConstantPoolValue(".T", Num,
ARMCP::CPDataSegmentJumpTable);
>
> Why ".T"?
>
T as in Table, I do not care much about the naming. If you have a better
proposal I will go with that.
>
> And, by the way, I noticed that you removed a "blank" comment
line in
> ARMISelLowering.h.  I think I have done that myself, but I recently
> discovered that the extra lines are intentional.  Doxygen recognizes C++
> comments beginning with 3 slashes but you have to have at least 2 lines of
> them or doxygen will not recognize them.
>
* fixed and added artificial line break to keep this from happening again.


Happy holidays!
Robert
>
> On Jun 24, 2009, at 2:20 PM, robert muth wrote:
>
> Evan:
>
> Sorry for the late follow up, I was out of town last week.
> Enclosed please find the updated patch including all
> your suggestions and a dejagnus test.
>
> Robert
>
> On Thu, Jun 11, 2009 at 2:27 PM, Evan Cheng <evan.cheng at apple.com>
wrote:
>
>>
>> On Jun 8, 2009, at 2:42 PM, robert muth wrote:
>>
>> > On Sun, Jun 7, 2009 at 11:53 PM, Evan Cheng <evan.cheng at
apple.com>
>> > wrote:
>> >>
>> >> On Jun 7, 2009, at 6:59 AM, robert muth wrote:
>> >>
>> >>> On Sat, Jun 6, 2009 at 4:51 PM, Evan Cheng<evan.cheng
at apple.com>
>> >>> wrote:
>> >>>> +cl::opt<std::string>
FlagJumpTableSection("jumptable-section",
>> >>>> +
>> >>>> cl::init(".data.jtab"));
>> >>>> +
>> >>>
>> >>> I thought it would be nice to group all the jumptables
together.
>> >>> But as long as it stays configurable, I am fine to change
the
>> >>> default
>> >>> to ".data".
>> >>
>> >> There is already a TargetAsmInfo::JumpTableDataSection. Why
not just
>> >> use that?
>> >
>> > Nice find. I will use that and possible change the current
setting,
>> > ".const", if it does not work,
>> >
>> >>>
>> >>>> Is this necessary? Why not just put it in the normal
data section?
>> >>>> Also "outline" jumptable seems like a
strange term. Can you think
>> >>>> of
>> >>>> something else?
>> >>>
>> >>>
>> >>> Yes, that is a tough one. How about "NonInline"
instead.
>> >>
>> >> Or "OutOfLine"?
>> >
>> > That works for me.
>> >
>> >> Please add a test case as well. Thanks,
>> >
>> > I am not sure how to go about testing.
>>
>> For this please just add a test case to the llvm dejagnu tests.
>>
>> thanks,
>>
>> Evan
>>
>> >
>> > I have  a script that compiles a bunch of test
>> > programs (gnu c torture test, etc.) and then runs the executables
>> > on qemu.  I run this script with and without my flags and
>> > make sure that I do not introduce any new problems
>> > -- there are currently plenty of vararg issues.
>> >
>> > I was thinking of sending this script to the list
>> > or maybe checking it into the llvm tree.
>> >
>> > The key is that whatever tests there are they should just
>> > be run with and without the new flag.
>> > How do you run backend tests?
>> >
>> > Robert
>> >
>> >
>> >
>> >
>> >> Evan
>> >>
>> >>>
>> >>> Robert
>> >>>
>> >>>> Thanks,
>> >>>> Evan
>> >>>>
>> >>>> Sent from my iPhone
>> >>>> On Jun 2, 2009, at 6:26 PM, robert muth <robert at
muth.org> wrote:
>> >>>>
>> >>>> Hi:
>> >>>>
>> >>>> This is my first patch submission. Hopefully, this is
the proper
>> >>>> the
>> >>>> protocol.
>> >>>> Attached is a patch for the llc ARM backend:
>> >>>>
>> >>>> Added mechanism to generate switch table in a data
section
>> >>>> rather than having it interleaved with the code.
>> >>>> This is controlled by command line flags and off by
default.
>> >>>> Also, tried to document and improve the code where I
modified it.
>> >>>>
>> >>>> Robert
>> >>>>
>> >>>> <llc.patch.txt>
>> >>>>
>> >>>> _______________________________________________
>> >>>> LLVM Developers mailing list
>> >>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> >>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >>>>
>> >>>> _______________________________________________
>> >>>> LLVM Developers mailing list
>> >>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> >>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >>>>
>> >>>>
>> >>>
>> >>> _______________________________________________
>> >>> LLVM Developers mailing list
>> >>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> >>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >>
>> >> _______________________________________________
>> >> LLVM Developers mailing list
>> >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >>
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>
> <llc.patch.2.txt>_______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
>
> _______________________________________________
> LLVM Developers mailing list
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> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
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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  
fixed.

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

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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On Tue, Jul 14, 2009 at 6:48 PM, Bob Wilson <bob.wilson at apple.com>
wrote:
>
> 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
> fixed.
>
> * It would be great to change the testcase to use the new FileCheck format.
>
> 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.
>


Bob:

Thanks for cleaning this up. I like the new patch much better than the old
one.
Teaching the (abstract) ConstantValue class about jumptable indices is a
little
bit ugly but I do not see any better solution without massive refactoring.
I have added TODOs  here and elsewhere and plan to address them in future
patches. I also added a test and pcleaned up a few things.

The new patch is attached.
I also uploaded it to

 http://codereview.appspot.com/96065/show

which is more convenient for browsing.
(you will have to register with an arbitrary pair
of email address and password --if you have a gmail
account you can use that)
Cheers,

Robert

>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
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