[llvm][CodeGen] Prevent endless loop when running the post-RA-sched pass.

[csstormq]

Summary

Skip the stage that no functional units are required in ScoreboardHazardRecognizer::getHazardType. If not, this function will always return Hazard by default, leading to endless loop when running the post-RA-sched pass.

Originally, I defined an itinerary like this:

InstrItinData<
  IIC_MOVI,
  [
    InstrStage<1, []>,
    InstrStage<1, [FU_ALU]>
  ]>,

indicates that the second stage occupies FU_ALU for 1 cycle and that this stage starts 2 cycles from the issue point.

In our case, we also make use of the TargetInstrInfo::getInstrLatency to compute the instruction latency of a given instruction. Then, the SchedulePostRATDList::ListScheduleTopDown will never exit once it is called.

Later, I have modified this itinerary like this:

InstrItinData<
  IIC_MOVI,
  [
    InstrStage<0, [], 1>,
    InstrStage<1, [FU_ALU]>
  ]>,

Then, the SchedulePostRATDList::ListScheduleTopDown works well.

So, whether this problem described above is really a problem? Maybe it's my falult.

[myhsu]

I believe we also write tests for timeout bugs, for instance https://github.com/llvm/llvm-project/blob/main/llvm/test/Transforms/InstCombine/phi-timeout.ll. Could you also add one?

[csstormq]

Thanks, @myhsu . I'm sorry that I have no idea how to add some test case like phi-timeout.ll. For more clarity, I have updated the summary above with more information.

llvm/lib/CodeGen/ScoreboardHazardRecognizer.cpp

for (unsigned int i = 0; i < IS->getCycles(); ++i) {
       if (StageCycle >= (int)RequiredScoreboard.getDepth()) {
         assert((StageCycle - Stalls) < (int)RequiredScoreboard.getDepth() &&
                "Scoreboard depth exceeded!");
         // This stage was stalled beyond pipeline depth, so cannot conflict.
         break;
       }
 
       InstrStage::FuncUnits freeUnits = IS->getUnits();
+      if (!freeUnits) {
+        // No functional units are required by this stage, so cannot conflict.
+        // In addition, prevent endless loop when running the post-RA-sched
+        // pass.
+        continue;
+      }
+
       switch (IS->getReservationKind()) {
       case InstrStage::Required:
         // Required FUs conflict with both reserved and required ones
         freeUnits &= ~ReservedScoreboard[StageCycle];
         [[fallthrough]];
       case InstrStage::Reserved:
         // Reserved FUs can conflict only with required ones.
         freeUnits &= ~RequiredScoreboard[StageCycle];