[LLVMdev] MBB Critical edges

[Fernando Magno Quintao Pereira]

Thanks, Evan.

     Actually I've solved my problem with some hints from Dale and Anton. 
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.

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.

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