[llvm] [AMDGPU][SIPreEmitPeephole] mustRetainExeczBranch: use BranchProbability and TargetSchedmodel (PR #109818)
Juan Manuel Martinez CaamaƱo via llvm-commits
llvm-commits at lists.llvm.org
Mon Oct 7 01:15:42 PDT 2024
================
@@ -304,11 +299,62 @@ bool SIPreEmitPeephole::getBlockDestinations(
return true;
}
+namespace {
+class BranchWeightCostModel {
+ const SIInstrInfo &TII;
+ const TargetSchedModel &SchedModel;
+ BranchProbability BranchProb;
+ static constexpr uint64_t BranchNotTakenCost = 1;
+ uint64_t BranchTakenCost;
+ uint64_t ThenCyclesCost = 0;
+
+public:
+ BranchWeightCostModel(const SIInstrInfo &TII, const MachineInstr &Branch,
+ const MachineBasicBlock &Succ)
+ : TII(TII), SchedModel(TII.getSchedModel()) {
+ assert(SchedModel.hasInstrSchedModelOrItineraries());
+
+ const MachineBasicBlock &Head = *Branch.getParent();
+ const auto *FromIt = find(Head.successors(), &Succ);
+ assert(FromIt != Head.succ_end());
+
+ BranchProb = Head.getSuccProbability(FromIt);
+ assert(!BranchProb.isUnknown());
----------------
jmmartinez wrote:
The problem with the unknown case is that we get as a probability something bigger than `1`: "UINT32_MAX / 2**31". Then, the subtraction `Denominator - Numerator` performed later would underflow.
https://github.com/llvm/llvm-project/pull/109818
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