[llvm] 4a9cde5 - [SimpleLoopUnswitch] Invalidate the topmost loop with ExitBB as exiting.

[Florian Hahn via llvm-commits]

Author: Florian Hahn
Date: 2019-12-04T11:32:09Z
New Revision: 4a9cde5a791cd49b96993e61d0f2401b81b6212d

URL: https://github.com/llvm/llvm-project/commit/4a9cde5a791cd49b96993e61d0f2401b81b6212d
DIFF: https://github.com/llvm/llvm-project/commit/4a9cde5a791cd49b96993e61d0f2401b81b6212d.diff

LOG: [SimpleLoopUnswitch] Invalidate the topmost loop with ExitBB as exiting.

SCEV caches the exiting blocks when computing exit counts. In
SimpleLoopUnswitch, we split the exit block of the loop to unswitch.

Currently we only invalidate the loop containing that exit block, but if
that block is the exiting block for a parent loop, we have stale cache
entries. We have to invalidate the top-most loop that contains the exit
block as exiting block. We might also be able to skip invalidating the
loop containing the exit block, if the exit block is not an exiting
block of that loop.

There are also 2 more places in SimpleLoopUnswitch, that use a similar
problematic approach to get the loop to invalidate. If the patch makes
sense, I will also update those places to a similar approach (they deal
with multiple exit blocks, so we cannot directly re-use
getTopMostExitingLoop).

Fixes PR43972.

Reviewers: skatkov, reames, asbirlea, chandlerc

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D70786

Added: 
    llvm/test/Transforms/SimpleLoopUnswitch/preserve-scev-exiting-multiple-loops.ll

Modified: 
    llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
index 13e44765985f..d441c6bbf124 100644
--- a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
+++ b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
@@ -331,6 +331,20 @@ static void hoistLoopToNewParent(Loop &L, BasicBlock &Preheader,
   }
 }
 
+// Return the top-most loop containing ExitBB and having ExitBB as exiting block
+// or the loop containing ExitBB, if there is no parent loop containing ExitBB
+// as exiting block.
+static Loop *getTopMostExitingLoop(BasicBlock *ExitBB, LoopInfo &LI) {
+  Loop *TopMost = LI.getLoopFor(ExitBB);
+  Loop *Current = TopMost;
+  while (Current) {
+    if (Current->isLoopExiting(ExitBB))
+      TopMost = Current;
+    Current = Current->getParentLoop();
+  }
+  return TopMost;
+}
+
 /// Unswitch a trivial branch if the condition is loop invariant.
 ///
 /// This routine should only be called when loop code leading to the branch has
@@ -415,9 +429,10 @@ static bool unswitchTrivialBranch(Loop &L, BranchInst &BI, DominatorTree &DT,
   });
 
   // If we have scalar evolutions, we need to invalidate them including this
-  // loop and the loop containing the exit block.
+  // loop, the loop containing the exit block and the topmost parent loop
+  // exiting via LoopExitBB.
   if (SE) {
-    if (Loop *ExitL = LI.getLoopFor(LoopExitBB))
+    if (Loop *ExitL = getTopMostExitingLoop(LoopExitBB, LI))
       SE->forgetLoop(ExitL);
     else
       // Forget the entire nest as this exits the entire nest.

diff  --git a/llvm/test/Transforms/SimpleLoopUnswitch/preserve-scev-exiting-multiple-loops.ll b/llvm/test/Transforms/SimpleLoopUnswitch/preserve-scev-exiting-multiple-loops.ll
new file mode 100644
index 000000000000..badb623ff59a
--- /dev/null
+++ b/llvm/test/Transforms/SimpleLoopUnswitch/preserve-scev-exiting-multiple-loops.ll
@@ -0,0 +1,63 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+
+; We run -indvars before -simple-loop-unswitch to compute SCEV exit counts before
+; running -simple-loop-unswitch.
+; RUN:  opt -indvars -simple-loop-unswitch -S %s -verify-scev | FileCheck %s
+
+; Test for PR43972.
+
+; We have a 3 nested loops (l1 <- l2 <- l3). %for.cond.5 is the exit block of
+; l3 and the loop for it is l2. But it is also the exiting block of l1. That
+; means we have to invalidate l1 to preserve SCEV.
+
+define void @f() {
+; CHECK-LABEL: @f(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[LNOT:%.*]] = xor i1 undef, true
+; CHECK-NEXT:    br label [[FOR_COND:%.*]]
+; CHECK:       for.cond.loopexit:
+; CHECK-NEXT:    br label [[FOR_COND]]
+; CHECK:       for.cond:
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.cond1:
+; CHECK-NEXT:    br i1 true, label [[FOR_BODY]], label [[FOR_COND_LOOPEXIT:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    br i1 [[LNOT]], label [[FOR_BODY_SPLIT:%.*]], label [[FOR_COND5_SPLIT:%.*]]
+; CHECK:       for.body.split:
+; CHECK-NEXT:    br label [[LAND_RHS:%.*]]
+; CHECK:       for.cond2:
+; CHECK-NEXT:    br i1 true, label [[LAND_RHS]], label [[FOR_COND5:%.*]]
+; CHECK:       land.rhs:
+; CHECK-NEXT:    br label [[FOR_COND2:%.*]]
+; CHECK:       for.cond5:
+; CHECK-NEXT:    br label [[FOR_COND5_SPLIT]]
+; CHECK:       for.cond5.split:
+; CHECK-NEXT:    br i1 true, label [[FOR_BODY7:%.*]], label [[FOR_COND1:%.*]]
+; CHECK:       for.body7:
+; CHECK-NEXT:    ret void
+;
+entry:
+  %lnot = xor i1 undef, true
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.cond1, %entry
+  br label %for.body
+
+for.cond1:                                        ; preds = %for.cond5
+  br i1 true, label %for.body, label %for.cond
+
+for.body:                                         ; preds = %for.cond1, %for.cond
+  br label %land.rhs
+
+for.cond2:                                        ; preds = %land.rhs
+  br i1 true, label %land.rhs, label %for.cond5
+
+land.rhs:                                         ; preds = %for.cond2, %for.body
+  br i1 %lnot, label %for.cond2, label %for.cond5
+
+for.cond5:                                        ; preds = %land.rhs, %for.cond2
+  br i1 true, label %for.body7, label %for.cond1
+
+for.body7:                                        ; preds = %for.cond5
+  ret void
+}