[LoopUnswitch] Fix a simple bug which disables loop unswitch for select statement

[wmi]

Summary

This is to fix PR34257. rL309059 takes an early return when FindLIVLoopCondition fails to find a loop invariant condition. This is wrong and it will disable loop unswitch for select. The patch fixes the bug.

[aivchenk]

Thanks for the fix. I debugged it a little bit yesterday and found the same suspicious place, but was not sure whether that was intended in order to fix the initial wrong optimization or not

[wmi]

Ping.

lib/Transforms/Scalar/LoopUnswitch.cpp

if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
  
       // If this isn't branching on an invariant condition, we can't unswitch
       // it.
       if (BI->isConditional()) {
         // See if this, or some part of it, is loop invariant.  If so, we can
         // unswitch on it if we desire.
         Value *LoopCond = FindLIVLoopCondition(BI->getCondition(),
                                                currentLoop, Changed).first;
-        if (!LoopCond || EqualityPropUnSafe(*LoopCond))
-          continue;
-
-        if (UnswitchIfProfitable(LoopCond, ConstantInt::getTrue(Context), TI)) {
+        if (LoopCond && !EqualityPropUnSafe(*LoopCond) &&
+            UnswitchIfProfitable(LoopCond, ConstantInt::getTrue(Context), TI)) {
           ++NumBranches;
           return true;
         }
       }
     } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
       Value *SC = SI->getCondition();
       Value *LoopCond;
       OperatorChain OpChain;

test/Transforms/LoopUnswitch/unswitch-select.ll

+; REQUIRES: asserts
+; RUN: opt < %s -loop-unswitch -disable-output -stats 2>&1| FileCheck %s
+
+; Check the select statement in the loop will be unswitched.
+; CHECK: 1 loop-unswitch - Number of selects unswitched
+define i32 @test(i1 zeroext %x, i32 %a) local_unnamed_addr #0 {
+entry:
+  br label %while.cond
+
+while.cond:                                       ; preds = %while.body, %entry
+  %i.0 = phi i32 [ 0, %entry ], [ %inc, %while.body ]
+  %s.0 = phi i32 [ %a, %entry ], [ %add, %while.body ]
+  %cmp = icmp slt i32 %i.0, 10000
+  br i1 %cmp, label %while.body, label %while.end
+
+while.body:                                       ; preds = %while.cond
+  %cond = select i1 %x, i32 %a, i32 %i.0
+  %add = add nsw i32 %s.0, %cond
+  %inc = add nsw i32 %i.0, 1
+  br label %while.cond
+
+while.end:                                        ; preds = %while.cond
+  %s.0.lcssa = phi i32 [ %s.0, %while.cond ]
+  ret i32 %s.0.lcssa
+}
+