[PeepholeOptimizer] Load now may be folded into isLoadFoldBarrier instruction

[andrew.zhogin]

Summary

Patch to postpone FoldAsLoadDefCandidates array clean to the moment before next loop iteration (when IsLoadFoldBarrier instruction met).
With this change, load may be folded into store instruction (for memory-to-memory copy). Or into call instruction, or into instruction with side effects.
Targets with memory-to-memory copy may override TargetInstrInfo::optimizeLoadInstr to implement such folding.

[lhames]

Hi Andrew,

This will need a test case.

Also, would it be clearer to just test the condition at the end of the loop?

CCing Manman as a reviewer, as I'm out on vacation, and may not have a chance to check in on this for a few days.

• Lang.

[manmanren]

Is it possible to add a testing case?

Manman

[andrew.zhogin]

In D25664#696719, @manmanren wrote:

Is it possible to add a testing case?

Manman

'optimizeLoadInstr' function is used now only by X86-backend and there aren't any memory-to-memory instructions in X86 (to fold load argument into store-barrier instruction).
SystemZ-backend doesn't use this mechanics to produce MVC instruction for now.
So I couldn't create working test case for existing backends.
I think it may be usefull for SystemZ machine-level MVC instructions combining.

lib/CodeGen/PeepholeOptimizer.cpp

for (MachineBasicBlock::iterator MII = MBB.begin(), MIE = MBB.end();
       LocalMIs.insert(MI);
 
       // Skip debug values. They should not affect this peephole optimization.
       if (MI->isDebugValue())
           continue;
 
       // If we run into an instruction we can't fold across, discard
       // the load candidates.
-      if (MI->isLoadFoldBarrier())
-        FoldAsLoadDefCandidates.clear();
+      // Better to clear array a bit later - before next loop iteration.
+      // Then load may be folded into store or call
+      // or instruction with side effects.
+      bool IsLoadFoldBarrier = MI->isLoadFoldBarrier();
+      struct ScopedFoldLoadsCleaner {
+        bool Enabled;
+        SmallSet<unsigned, 16> &Array;
+        ScopedFoldLoadsCleaner(bool CleanArr, SmallSet<unsigned, 16> &Arr)
+          : Enabled(CleanArr), Array(Arr) {}
+        ~ScopedFoldLoadsCleaner() {
+          if (Enabled)
+            Array.clear();
+        }
+      } FoldLoadsArrayCleaner(IsLoadFoldBarrier, FoldAsLoadDefCandidates);
 
       if (MI->isPosition() || MI->isPHI())
         continue;
 
       if (!MI->isCopy()) {
         for (const auto &Op : MI->operands()) {
           // Visit all operands: definitions can be implicit or explicit.
           if (Op.isReg()) {