[llvm-dev] [CodeGen] PeepholeOptimizer: optimizing condition dependent instrunctions

[Evgeny Astigeevich via llvm-dev]

Hi,

I find it's quite strange how condition dependent instructions are processed in PeepholeOptimizer::runOnMachineFunction:

01577       if ((isUncoalescableCopy(*MI) &&
01578            optimizeUncoalescableCopy(MI, LocalMIs)) ||
01579           (MI->isCompare() && optimizeCmpInstr(MI, &MBB)) ||
01580           (MI->isSelect() && optimizeSelect(MI, LocalMIs))) {
01581         // MI is deleted.
01582         LocalMIs.erase(MI);
01583         Changed = true;
01584         continue;
01585       }
01586
01587       if (MI->isConditionalBranch() && optimizeCondBranch(MI)) {
01588         Changed = true;
01589         continue;
01590       }

CmpInstr, SelectInstr and CondBranch are processed separately. It's assumed that CmpInstr and SelectInstr are deleted but CondBranch is not.
In fact CmpInstr is always connected to SelectInstr or CondBranch or both of them. So if such connection exists it should be processed as a whole.
For example, there are cases when CMP+BRC can be replaced by BR. The same is true for CMP+SEL.

The main problem I have is that I have to find corresponding CmpInstr and to repeat analysis of it in optimizeSelect and in optimizeCondBranch.

Any thoughts why it's implemented in such way.

Kind regards,
Evgeny Astigeevich
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