[IndVarSimplify] Never sinking if none operand is used in or after loop exit block

[xjtuhu]

Summary

IndVarSimplify::sinkUnusedInvariants would sink a instruction from a loop's preheader block to exit block to reduce register pressure. But the optimization is not conservative, it could increase register pressure conversely if none operand is used in or after loop exit block, because sinking could extend operands' liveness range hence increase register pressure.

[lebedev.ri]

This looks pretty ad-hoc to me.
I believe we consider this to be a canonicalization transform that will be undone by other passes if profitable.

[reames]

Change rejected. As Roman mentions, this is more of a canonicalization than an optimization. If you want to do actual scheduling, please use the MI layer.

p.s. The wording "If none operand is used" does not parse for me. I think I get the intent of the change, but I'd suggest rewording any future discussion of this.

[xjtuhu]

OK, I see. Thanks for giving the advice!

llvm/lib/Transforms/Scalar/IndVarSimplify.cpp

for (Use &U : I->uses()) {
         break;
       }
     }
 
     // If there is, the def must remain in the preheader.
     if (UsedInLoop)
       continue;
 
+    // If none operand is used in or after the loop exit block, sinking the
+    // instruction from pre-header to exit block will extend operands'
+    // live range, hence could increase register pressure. The instruction
+    // should not be sinked for conservativeness.
+    bool OperandsUsedAfterLoop = false;
+    for (Value *Op : I->operands()) {
+      if (isa<Constant>(Op)) {
+        continue;
+      }
+      for (Use &U : Op->uses()) {
+        Instruction *User = cast<Instruction>(U.getUser());
+        BasicBlock *UseBB = User->getParent();
+        if (PHINode *P = dyn_cast<PHINode>(User)) {
+          unsigned i =
+            PHINode::getIncomingValueNumForOperand(U.getOperandNo());
+          UseBB = P->getIncomingBlock(i);
+        }
+        if (DT->dominates(ExitBlock, UseBB)) {
+          OperandsUsedAfterLoop = true;
+          break;
+        }
+      }
+    }
+    if (!OperandsUsedAfterLoop)
+      continue;
+
     // Otherwise, sink it to the exit block.
     Instruction *ToMove = &*I;
     bool Done = false;
 
     if (I != Preheader->begin()) {
       // Skip debug info intrinsics.
       do {
         --I;