Improve CodeLoopSinking compilation time

Add a heuristic to check the candidates size on every iteration before
rerunning the liveness analysis. Skip the rerun if the candidates size
is too small to achieve the needed register pressure

Author: admitric

IGC/Compiler/CISACodeGen/CodeSinking.cpp

@@ -1083,6 +1083,14 @@ namespace IGC {
             RPE->rerunLivenessAnalysis(*F, &AffectedBBs);
         };
 
+        auto getSizeInRegs = [&](const SmallVectorImpl<Instruction *> &Insts)
+        {
+            auto SIMD = numLanes(RPE->bestGuessSIMDSize(F));
+            ValueSet InstsSet(Insts.begin(), Insts.end());
+            unsigned int SizeInBytes = RPE->estimateSizeInBytes(InstsSet, *F, SIMD, &WI);
+            return RPE->bytesToRegisters(SizeInBytes);
+        };
+
         bool EverChanged = false;
 
         // Find LIs in preheader that would definitely reduce
@@ -1105,6 +1113,7 @@ namespace IGC {
         uint MaxLoopPressure = InitialLoopPressure;
 
         bool AchievedNeededRegpressure = false;
+        bool RecomputeMaxLoopPressure = false;
 
         do
         {
@@ -1159,9 +1168,21 @@ namespace IGC {
                     LocalInstSet.clear();
                 }
 
+                if (MaxLoopPressure - getSizeInRegs(SinkCandidates) > NeededRegpressure)
+                {
+                    // Heuristic to save recalculation of liveness
+                    // The size of the candidates set is not enough to reach the needed regpressure
+                    // even if we sinked everything
+                    PrintDump("Running one more iteration without recalculating liveness...\n");
+                    RecomputeMaxLoopPressure = true;
+                    continue;
+                }
+
                 rerunLiveness();
                 MaxLoopPressure = getMaxRegCountForLoop(L);
+                RecomputeMaxLoopPressure = false;
                 PrintDump("New max loop pressure = " << MaxLoopPressure << "\n");
+
                 if ((MaxLoopPressure < NeededRegpressure)
                         && (Mode == LoopSinkMode::SinkWhileRegpressureIsHigh))
                 {
@@ -1199,6 +1220,12 @@ namespace IGC {
             return false;
         }
 
+        if (RecomputeMaxLoopPressure)
+        {
+            rerunLiveness();
+            MaxLoopPressure = getMaxRegCountForLoop(L);
+        }
+
         PrintDump("New max loop pressure = " << MaxLoopPressure << "\n");
 
         bool NeedToRollback = false;