[PATCH] D47624: [SimpleLoopUnswitch] Invalidate SCEV after making structural changes to a loop

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.
mkazantsev added reviewers: chandlerc, uabelho, reames.
Herald added a subscriber: javed.absar.

This patch makes Simple Loop Unswitch invalidate cached SCEV information for the
modified loop and its outer loops properly. Fix for PR37651.

This patch has also exposed the fact that sometimes simple loop unswitch produces
invalid loops. It is a separate issue to fix, we will address it later.


https://reviews.llvm.org/D47624

Files:
  lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
  test/Transforms/SimpleLoopUnswitch/invalidate_scev.ll


Index: test/Transforms/SimpleLoopUnswitch/invalidate_scev.ll
===================================================================
--- test/Transforms/SimpleLoopUnswitch/invalidate_scev.ll
+++ test/Transforms/SimpleLoopUnswitch/invalidate_scev.ll
@@ -0,0 +1,34 @@
+; RUN: opt -iv-users -simple-loop-unswitch -indvars -S < %s | FileCheck %s
+; PR37651
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+
+; Make sure that SCEV gets invalidated properly after simple unswitching.
+define void @f1() {
+
+; CHECK-LABEL: @f1
+
+vector.ph:
+  br label %bb3
+
+bb3:                                              ; preds = %bb13, %vector.ph
+  %_tmp5329 = phi i8 [ %_tmp54, %bb13 ], [ undef, %vector.ph ]
+  %_tmp40 = icmp eq i8 undef, 0
+  br label %bb11
+
+bb11:                                             ; preds = %bb12, %bb3
+  br i1 %_tmp40, label %bb12, label %bb13
+
+bb12:                                             ; preds = %bb11
+  br i1 false, label %bb11, label %bb13
+
+bb13:                                             ; preds = %bb12, %bb11
+  %_tmp54 = add nsw i8 %_tmp5329, 1
+  %_tmp57 = icmp eq i8 %_tmp54, 0
+  br i1 %_tmp57, label %bb14.loopexit, label %bb3
+
+bb14.loopexit:                                    ; preds = %bb13
+  ret void
+}
Index: lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
===================================================================
--- lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
+++ lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
@@ -2006,9 +2006,17 @@
       LPM.markLoopAsDeleted(*L);
   };
 
+  assert(!L->isInvalid() && "Unswitching invalid loop?");
   bool Changed =
       unswitchLoop(*L, DT, LI, AC, TTI, NonTrivial, UnswitchCB);
 
+  // We might have done structural changes to a CFG that may break cached
+  // invariants in SCEV for this and any outer loop.
+  // FIXME: Is it OK that unswitching produces invalid loops?!
+  if (Changed && !L->isInvalid())
+    if (auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>())
+      SEWP->getSE().forgetTopmostLoop(L);
+
   // If anything was unswitched, also clear any cached information about this
   // loop.
   LPM.deleteSimpleAnalysisLoop(L);


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