[PATCH] D63536: [MachinePipeliner] Fix risky iterator usage R++, --R

[Jinsong Ji via Phabricator via llvm-commits]

jsji created this revision.
jsji added reviewers: bcahoon, hfinkel.
Herald added subscribers: hiraditya, nemanjai.
Herald added a project: LLVM.

When we calculateResMII, we use two loops, one with iterator

  (*RI++)->canReserveResources

to check whether we can reserve the resource, 
then later in another loop, we use

  --RI;
  (*RI)->reserveResources(*MI);

Trying to move back iterator to do reservation.

This is risky, as after several iterator `R++`, 
**--R may not move back to point to the same Resource at all,**
so we may actually test with one Resource, but reserve in another Resources,
hence overbooking some Resource.

The can cause wrong MII being calculated.


Repository:
  rL LLVM

https://reviews.llvm.org/D63536

Files:
  llvm/lib/CodeGen/MachinePipeliner.cpp
  llvm/test/CodeGen/PowerPC/sms-iterator.ll


Index: llvm/test/CodeGen/PowerPC/sms-iterator.ll
===================================================================
--- llvm/test/CodeGen/PowerPC/sms-iterator.ll
+++ llvm/test/CodeGen/PowerPC/sms-iterator.ll
@@ -5,7 +5,7 @@
 
 %0 = type { i32, [16 x double] }
 
-; CHECK: MII = 7 MAX_II = 17
+; CHECK: MII = 8 MAX_II = 18
 
 define dso_local fastcc void @_ZN3povL9polysolveEiPdS0_() unnamed_addr #0 {
   br label %1
Index: llvm/lib/CodeGen/MachinePipeliner.cpp
===================================================================
--- llvm/lib/CodeGen/MachinePipeliner.cpp
+++ llvm/lib/CodeGen/MachinePipeliner.cpp
@@ -1012,17 +1012,13 @@
     });
     for (unsigned C = 0; C < NumCycles; ++C)
       while (RI != RE) {
-        if ((*RI++)->canReserveResources(*MI)) {
+        if ((*RI)->canReserveResources(*MI)) {
+          (*RI)->reserveResources(*MI);
           ++ReservedCycles;
           break;
         }
+        RI++;
       }
-    // Start reserving resources using existing DFAs.
-    for (unsigned C = 0; C < ReservedCycles; ++C) {
-      --RI;
-      (*RI)->reserveResources(*MI);
-    }
-
     LLVM_DEBUG(dbgs() << "ReservedCycles:" << ReservedCycles
                       << ", NumCycles:" << NumCycles << "\n");
     // Add new DFAs, if needed, to reserve resources.


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