[llvm-commits] [llvm] r101075 - in /llvm/trunk: lib/Target/X86/X86InstrInfo.cpp test/CodeGen/X86/brcond.ll

[Dale Johannesen]

On Apr 12, 2010, at 4:27 PMPDT, Dan Gohman wrote:

> 
> On Apr 12, 2010, at 4:22 PM, Dale Johannesen wrote:
> 
>> 
>> On Apr 12, 2010, at 4:13 PMPDT, Chris Lattner wrote:
>> 
>>> 
>>> On Apr 12, 2010, at 3:19 PM, Bill Wendling wrote:
>>> 
>>>> Author: void
>>>> Date: Mon Apr 12 17:19:57 2010
>>>> New Revision: 101075
>>> 
>>> hi Bill, is it possible for this to go in target independent code, e.g. branch folding?
>>> 
>>> -Chris
>> 
>> Branch folding is already trying:
>> 
>>   // If the prior block branches here on true and somewhere else on false, and
>>   // if the branch condition is reversible, reverse the branch to create a
>>   // fall-through.
>>   if (PriorTBB == MBB) {
>>     SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
>>     if (!TII->ReverseBranchCondition(NewPriorCond)) {
>>       TII->RemoveBranch(PrevBB);
>>       TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond);
>>       MadeChange = true;
>>       ++NumBranchOpts;
>>       goto ReoptimizeBlock;
>>     }
>>   }
>> 
>> Why doesn't it work in this case?
> 
> I was curious too. CodeGenPrepare has split a critical edge for a PHI
> node, however after register allocation it turns out that no copy was
> needed, so the jmp is actually in a separate basic block from the
> jne, jp.


In that case, branch folding should be removing the block that consists of only an unconditional branch:

      // If this block is just an unconditional branch to CurTBB, we can
      // usually completely eliminate the block.  The only case we cannot
      // completely eliminate the block is when the block before this one
      // falls through into MBB and we can't understand the prior block's branch
      // condition.
      if (MBB->empty()) {
         <too long to quote, but appears to do what the comment says>

so why doesn't *that* work?