[LLVMdev] Critical edges

[Tanya Lattner]

The problem is that you are inserting block 9 in the wrong spot. 
mf.getLastBlock() returns the block with the greatest number which may 
have nothing to do with the ordering. Why not use the end iterator 
(mf.end) to insert?

-Tanya

On Sat, 8 Jul 2006, Fernando Magno Quintao Pereira wrote:

>
> Dear guys,
>
>    I am having problem to split edges correctly. Mostly because the new
> basic blocks are creating infinite loops. Could someone help me fixing the
> code below? It is creating assembly like this one below. Block LBB1_9 was
> inserted to break the critical edge between blocks LBB1_3 and LBB1_8. But
> it changes the semantics of the original program, because, before, LBB1_8
> was falling through LBB1_4, and now it is falling on LBB1_9.
>
> LBB1_3: ;no_exit
>        lis r4, 21845
>        ori r4, r4, 21846
>        mulhw r4, r2, r4
>        addi r5, r2, -1
>        li r6, -1
>        srwi r6, r4, 31
>        add r4, r4, r6
>        mulli r4, r4, 3
>        li r6, 1
>        subf r2, r4, r2
>        cmpwi cr0, r2, 0
>        beq cr0, LBB1_9 ;no_exit
> LBB1_7: ;no_exit
>        mr r2, r6
> LBB1_8: ;no_exit
>        cmpwi cr0, r5, 0
>        add r2, r2, r3
>        bgt cr0, LBB1_5 ;no_exit.no_exit_llvm_crit_edge
> LBB1_9: ;no_exit
>        mr r2, r6
>        b LBB1_8        ;no_exit
> LBB1_4: ;no_exit.loopexit_llvm_crit_edge
>
> The code is this one below:
>
> void CriticalEdgeRemoval_Fer::split_critical_edge
>     (MachineBasicBlock & src, MachineBasicBlock & dst, MachineFunction &
> mf) {
>
>    const BasicBlock * src_bb = src.getBasicBlock();
>    MachineBasicBlock * crit_mbb = new MachineBasicBlock(src_bb);
>
>    /// modify the llvm control flow graph
>    src.addSuccessor(crit_mbb);
>    crit_mbb->addSuccessor(& dst);
>    src.removeSuccessor(& dst);
>
>    /// insert the new block into the machine function.
>    ilist<MachineBasicBlock>::iterator mbb_it = mf.getLastBlock();
>    ++mbb_it;
>    mf.getBasicBlockList().insert(mbb_it, crit_mbb);
>
>    /// insert a unconditional branch linking the new block to dst
>    const TargetMachine & tm = mf.getTarget();
>    const TargetInstrInfo * tii = tm.getInstrInfo();
>    tii->insertGoto(*crit_mbb, dst);
>
>    /// modify every branch in src that points to dst to point to the new
>    /// machine basic block instead:
>    MachineBasicBlock::iterator mii = src.end();
>    while (mii != src.begin()) {
>        mii--;
>        // if there is not more branches, finish the loop
>        if (!tii->isTerminatorInstr(mii->getOpcode())) {
>            break;
>        }
>        // Scan the operands of this machine instruction, replacing any
>        // use 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) {
>                mii->getOperand(i).setMachineBasicBlock(crit_mbb);
>            }
>        }
>    }
>
> }
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