[AArch64] Check Dest Resgister Liveness in CondOpt pass

[zzheng]

Summary

CondOpt pass does not check for destination register liveness of subs (alias with cmp) instructions.

Our internal test reveals such case should not be transformed:

  cmp x17, #3
  b.lt .LBB10_15
  ...
  subs x12, x12, #1
  b.gt .LBB10_1

where x12 is a liveout, becomes:

  cmp x17, #2
  b.le .LBB10_15
  ...
  subs x12, x12, #2
  b.ge .LBB10_1

[sdmitrouk]

Hi,

Somehow from instruction reference and comments I mistakenly assumed that
such instructions won't occur with liveout destinations, but now I see it
should be checked. I tried to make a test-case for a couple hours, but
had no luck here.

Thanks for catching this!

Regards,
Sergey

lib/Target/AArch64/AArch64ConditionOptimizer.cpp

130

There is INITIALIZE_PASS_BEGIN...INITIALIZE_PASS_END above (not full context, by the way), LiveIntervals should be added there as well or just drop lines from namespace llvm { to INITIALIZE_PASS_END (might be preferred).

[zzheng]

+INITIALIZE_PASS_DEPENDENCY(LiveIntervals)

[mcrosier]

Committed in r220987.

lib/Target/AArch64/AArch64ConditionOptimizer.cpp

 #include "llvm/ADT/DepthFirstIterator.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/LiveIntervalAnalysis.h"
 #include "llvm/CodeGen/MachineDominators.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"

 INITIALIZE_PASS_BEGIN(AArch64ConditionOptimizer, "aarch64-condopt",
                       "AArch64 CondOpt Pass", false, false)
 INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
+INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
 INITIALIZE_PASS_END(AArch64ConditionOptimizer, "aarch64-condopt",
                     "AArch64 CondOpt Pass", false, false)
 

 void AArch64ConditionOptimizer::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.addRequired<MachineDominatorTree>();
   AU.addPreserved<MachineDominatorTree>();
+  AU.addRequired<LiveIntervals>();
+  AU.addPreserved<LiveIntervals>();
   MachineFunctionPass::getAnalysisUsage(AU);
 }
 

     // cmn is an alias for adds with a dead destination register.
     case AArch64::ADDSWri:
     case AArch64::ADDSXri:
-      return I;
+      if (I->getOperand(0).isDead()) return I;
+      DEBUG(dbgs() << "Destination of cmp is not dead, " << *I << '\n');
+      return nullptr;
 
     // Prevent false positive case like:
     // cmp      w19, #0