[MachinePipeliner] Fix risky iterator usage R++, --R

[jsji]

Summary

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.

[jsji]

Ping... Any comments or feedback, especially for Hexagon? @bcahoon

[bcahoon]

Looks good to me. Thanks for the fix!

Thanks,
Brendon

llvm/trunk/lib/CodeGen/MachinePipeliner.cpp

while (!FuncUnitOrder.empty()) {
     SmallVectorImpl<ResourceManager *>::iterator RE = Resources.end();
     LLVM_DEBUG({
       dbgs() << "Trying to reserve resource for " << NumCycles
              << " cycles for \n";
       MI->dump();
     });
     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.
     for (unsigned C = ReservedCycles; C < NumCycles; ++C) {
       LLVM_DEBUG(if (SwpDebugResource) dbgs()
                  << "NewResource created to reserve resources"
                  << "\n");
       ResourceManager *NewResource = new ResourceManager(&MF.getSubtarget());

llvm/trunk/test/CodeGen/PowerPC/sms-iterator.ll

 ; REQUIRES: asserts
 ; RUN: llc < %s -mtriple=powerpc64le-unknown-linux-gnu -verify-machineinstrs\
 ; RUN:       -mcpu=pwr9 --ppc-enable-pipeliner -debug-only=pipeliner 2>&1 \
 ; RUN:       >/dev/null | FileCheck %s
 
 %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
 
 1:                                                ; preds = %1, %0
   br i1 undef, label %2, label %1
 
 2:                                                ; preds = %1