[LLVMdev] MBB Critical edges

[Evan Cheng]

On Aug 17, 2007, at 3:08 PM, Fernando Magno Quintao Pereira wrote:

>
> Thanks, Evan.
>
>      Actually I've solved my problem with some hints from Dale and  
> Anton.

Ok, cool. I didn't see their replies. Like I said, my email client is  
acting funny.

> At least I think I've solved it. I had to add one method to
> TargetInstrInfo to tell me when an instruction is an indirect jump -
> TargetInstrInfo::tii.isIndirectJump(opcode). When that is the case, I
> update the jump table using:
>
>      // Change jumps to go to the new basic block:
>      if(isJumpTable) {
>          mf.getJumpTableInfo()->ReplaceMBBInJumpTables(& dst,  
> crit_mbb);
>      }
>
> where dst was the old basic block, and crit_mbb is the new one,  
> created to
> remove a critical edge. I think this should solve the problem. If you
> want, I can give you the pass to break critical edges of machine  
> blocks,
> so that you guys can see if it is worth adding to LLVM's main  
> branch. Up
> to now, as far as I know, there is no pass to remove critical edges of
> machine functions in LLVM.

Can you explain briefly what it does? What benefits do you expect?

>
> By the way, with this change, my register allocator is passing all the
> tests that LLVM with default RA (linearscan) is passing, although I am
> still using LLVM v1.8.

Very nice. Does this include all the external tests (specs, etc.)? Is  
it possible for you to move to tot?

Evan

>
> best,
>
> Fernando
>
>> Sorry about the tardiness of my reply. My mail client has playing
>> tricks with me. :-)
>>
>> I am assuming the issue has nothing to do the branch to jumptable
>> instructions but rather the MachineJumpTableInfo associated with  
>> every
>> MachineFunction? If so, please take a look at BranchFoldiing.cpp for
>> an example.
>>
>> Evan
>>
>> On Aug 10, 2007, at 12:30 PM, Fernando Magno Quintao Pereira wrote:
>>
>>>
>>> Hi all,
>>>
>>>     I have a pass to break critical edges of Machine Basic Blocks,
>>> but I
>>> just discovered a bug (when compiling code for x86). The problem is
>>> 'jumpl
>>> *%reg'. I don't know how to update the jump table for this type of
>>> instruction. The code that I had (see below) does not update the  
>>> jump
>>> table, and the actual branch keeps jumping to the old basic block,
>>> instead
>>> of the new. Could someone help me to fix this? If it works fine,  
>>> I can
>>> send a patch for this pass.
>>>
>>>     /// modify every branch in src that points to dst to point
>>>     /// to the new machine basic block "crit_mbb" instead:
>>>     MachineBasicBlock::iterator mii = src.end();
>>>     bool found_branch = false;
>>>     while (mii != src.begin()) {
>>>         mii--;
>>>         // if there are no more branches, finish the loop
>>>         if (!tii->isTerminatorInstr(mii->getOpcode())) {
>>>             break;
>>>         }
>>>         // Scan the operands of this branch, replacing any
>>>         // uses of dst with crit_mbb.
>>>         for (unsigned i = 0, e = mii->getNumOperands(); i != e; + 
>>> +i) {
>>>             if (mii->getOperand(i).isMachineBasicBlock() &&
>>>                  mii->getOperand(i).getMachineBasicBlock() == &  
>>> dst) {
>>>                 found_branch = true;
>>>                 mii->getOperand(i).setMachineBasicBlock(crit_mbb);
>>>             }
>>>         }
>>>     }
>>>
>>> Thanks a lot,
>>>
>>> Fernando
>>> _______________________________________________
>>> 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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