[llvm] e8b8304 - [NFC][Regalloc] Split canEvictInterference into hint and general

[Mircea Trofin via llvm-commits]

Author: Mircea Trofin
Date: 2021-11-29T16:04:03-08:00
New Revision: e8b8304d76ccefa9880bbb352d9f81f330ef1ea1

URL: https://github.com/llvm/llvm-project/commit/e8b8304d76ccefa9880bbb352d9f81f330ef1ea1
DIFF: https://github.com/llvm/llvm-project/commit/e8b8304d76ccefa9880bbb352d9f81f330ef1ea1.diff

LOG: [NFC][Regalloc] Split canEvictInterference into hint and general

There are 2 eviction queries. One is made by tryAssign, when it attempts to
free an interference occupying the hint of the candidate. The other is
during 'regular' interference resolution, where we scan over all
physical registers and try to see if we can evict live ranges in favor
of the candidate. We currently use the same logic in both cases, just
that the former never passes the cost to any subsequent query.
Technically, the 2 decisions could be implemented with different
policies.

This patch splits the 2.

RFC: https://lists.llvm.org/pipermail/llvm-dev/2021-November/153639.html

Differential Revision: https://reviews.llvm.org/D114019

Added: 
    

Modified: 
    llvm/lib/CodeGen/RegAllocGreedy.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/RegAllocGreedy.cpp b/llvm/lib/CodeGen/RegAllocGreedy.cpp
index 5a93b58e0baf..27cb613ae9d6 100644
--- a/llvm/lib/CodeGen/RegAllocGreedy.cpp
+++ b/llvm/lib/CodeGen/RegAllocGreedy.cpp
@@ -199,7 +199,8 @@ class RAGreedy : public MachineFunctionPass,
   struct RegInfo {
     LiveRangeStage Stage = RS_New;
 
-    // Cascade - Eviction loop prevention. See canEvictInterference().
+    // Cascade - Eviction loop prevention. See
+    // canEvictInterferenceBasedOnCost().
     unsigned Cascade = 0;
 
     RegInfo() = default;
@@ -410,8 +411,11 @@ class RAGreedy : public MachineFunctionPass,
   void calcGapWeights(MCRegister, SmallVectorImpl<float> &);
   Register canReassign(LiveInterval &VirtReg, Register PrevReg) const;
   bool shouldEvict(LiveInterval &A, bool, LiveInterval &B, bool) const;
-  bool canEvictInterference(LiveInterval &, MCRegister, bool, EvictionCost &,
-                            const SmallVirtRegSet &) const;
+  bool canEvictInterferenceBasedOnCost(LiveInterval &, MCRegister, bool,
+                                       EvictionCost &,
+                                       const SmallVirtRegSet &) const;
+  bool canEvictHintInterference(LiveInterval &, MCRegister,
+                                const SmallVirtRegSet &) const;
   bool canEvictInterferenceInRange(const LiveInterval &VirtReg,
                                    MCRegister PhysReg, SlotIndex Start,
                                    SlotIndex End, EvictionCost &MaxCost) const;
@@ -780,10 +784,8 @@ MCRegister RAGreedy::tryAssign(LiveInterval &VirtReg,
     if (Order.isHint(Hint)) {
       MCRegister PhysHint = Hint.asMCReg();
       LLVM_DEBUG(dbgs() << "missed hint " << printReg(PhysHint, TRI) << '\n');
-      EvictionCost MaxCost;
-      MaxCost.setBrokenHints(1);
-      if (canEvictInterference(VirtReg, PhysHint, true, MaxCost,
-                               FixedRegisters)) {
+
+      if (canEvictHintInterference(VirtReg, PhysHint, FixedRegisters)) {
         evictInterference(VirtReg, PhysHint, NewVRegs);
         return PhysHint;
       }
@@ -864,8 +866,19 @@ bool RAGreedy::shouldEvict(LiveInterval &A, bool IsHint,
   return false;
 }
 
-/// canEvictInterference - Return true if all interferences between VirtReg and
-/// PhysReg can be evicted.
+/// canEvictHintInterference - return true if the interference for VirtReg
+/// on the PhysReg, which is VirtReg's hint, can be evicted in favor of VirtReg.
+bool RAGreedy::canEvictHintInterference(
+    LiveInterval &VirtReg, MCRegister PhysReg,
+    const SmallVirtRegSet &FixedRegisters) const {
+  EvictionCost MaxCost;
+  MaxCost.setBrokenHints(1);
+  return canEvictInterferenceBasedOnCost(VirtReg, PhysReg, true, MaxCost,
+                                         FixedRegisters);
+}
+
+/// canEvictInterferenceBasedOnCost - Return true if all interferences between
+/// VirtReg and PhysReg can be evicted.
 ///
 /// @param VirtReg Live range that is about to be assigned.
 /// @param PhysReg Desired register for assignment.
@@ -873,7 +886,7 @@ bool RAGreedy::shouldEvict(LiveInterval &A, bool IsHint,
 /// @param MaxCost Only look for cheaper candidates and update with new cost
 ///                when returning true.
 /// @returns True when interference can be evicted cheaper than MaxCost.
-bool RAGreedy::canEvictInterference(
+bool RAGreedy::canEvictInterferenceBasedOnCost(
     LiveInterval &VirtReg, MCRegister PhysReg, bool IsHint,
     EvictionCost &MaxCost, const SmallVirtRegSet &FixedRegisters) const {
   // It is only possible to evict virtual register interference.
@@ -1150,8 +1163,8 @@ MCRegister RAGreedy::tryFindEvictionCandidate(
       continue;
     }
 
-    if (!canEvictInterference(VirtReg, PhysReg, false, BestCost,
-                              FixedRegisters))
+    if (!canEvictInterferenceBasedOnCost(VirtReg, PhysReg, false, BestCost,
+                                         FixedRegisters))
       continue;
 
     // Best so far.