[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

1 files changed, 63 insertions, 0 deletions

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 00000000000..badb623ff59
--- /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
+}