From 523573e90ddb6db3b58d7a04b7dbe90fa7366dc2 Mon Sep 17 00:00:00 2001
From: Philip Reames <listmail@philipreames.com>
Date: Mon, 17 Jan 2022 11:39:19 -0800
Subject: [PATCH] [LoopDeletion] Revert 3af8a11 and add test coverage for
 breakage

This reverts 3af8a11 because I'd used an upper bound where an lower bound was required.  The included reduced test case demonstrates the issue.
---
 llvm/lib/Transforms/Scalar/LoopDeletion.cpp   | 29 +++++-------
 .../LoopDeletion/D108848-regression.ll        | 47 +++++++++++++++++++
 2 files changed, 58 insertions(+), 18 deletions(-)
 create mode 100644 llvm/test/Transforms/LoopDeletion/D108848-regression.ll

diff --git a/llvm/lib/Transforms/Scalar/LoopDeletion.cpp b/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
index 5814e2f043d5..d70ff60a1aac 100644
--- a/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
@@ -407,25 +407,18 @@ breakBackedgeIfNotTaken(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
   if (!L->getLoopLatch())
     return LoopDeletionResult::Unmodified;
 
-  auto *BTC = SE.getSymbolicMaxBackedgeTakenCount(L);
-  if (BTC->isZero()) {
-    // SCEV knows this backedge isn't taken!
-    breakLoopBackedge(L, DT, SE, LI, MSSA);
-    ++NumBackedgesBroken;
-    return LoopDeletionResult::Deleted;
-  }
-
-  // If SCEV leaves open the possibility of a zero trip count, see if
-  // symbolically evaluating the first iteration lets us prove the backedge
-  // unreachable.
-  if (isa<SCEVCouldNotCompute>(BTC) || !SE.isKnownNonZero(BTC))
-    if (canProveExitOnFirstIteration(L, DT, LI)) {
-      breakLoopBackedge(L, DT, SE, LI, MSSA);
-      ++NumBackedgesBroken;
-      return LoopDeletionResult::Deleted;
+  auto *BTCMax = SE.getConstantMaxBackedgeTakenCount(L);
+  if (!BTCMax->isZero()) {
+    auto *BTC = SE.getBackedgeTakenCount(L);
+    if (!BTC->isZero()) {
+      if (!isa<SCEVCouldNotCompute>(BTC) && SE.isKnownNonZero(BTC))
+        return LoopDeletionResult::Unmodified;
+      if (!canProveExitOnFirstIteration(L, DT, LI))
+        return LoopDeletionResult::Unmodified;
     }
-
-  return LoopDeletionResult::Unmodified;
+  }
+  breakLoopBackedge(L, DT, SE, LI, MSSA);
+  return LoopDeletionResult::Deleted;
 }
 
 /// Remove a loop if it is dead.
diff --git a/llvm/test/Transforms/LoopDeletion/D108848-regression.ll b/llvm/test/Transforms/LoopDeletion/D108848-regression.ll
new file mode 100644
index 000000000000..68cb32aaa284
--- /dev/null
+++ b/llvm/test/Transforms/LoopDeletion/D108848-regression.ll
@@ -0,0 +1,47 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -loop-deletion -S < %s | FileCheck %s
+
+@a = global i1 false, align 4
+
+define i32 @main() {
+; CHECK-LABEL: @main(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[D_015:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    call void @foo()
+; CHECK-NEXT:    [[DOTB:%.*]] = load i1, i1* @a, align 4
+; CHECK-NEXT:    [[CONV1:%.*]] = select i1 [[DOTB]], i32 -128, i32 1
+; CHECK-NEXT:    [[CMP2_NOT:%.*]] = icmp ugt i32 [[CONV1]], [[D_015]]
+; CHECK-NEXT:    br i1 [[CMP2_NOT]], label [[CLEANUP:%.*]], label [[FOR_INC8:%.*]]
+; CHECK:       for.inc8:
+; CHECK-NEXT:    [[INC9:%.*]] = add nuw nsw i32 [[D_015]], 1
+; CHECK-NEXT:    [[EXITCOND_NOT:%.*]] = icmp eq i32 [[INC9]], 19
+; CHECK-NEXT:    br label [[CLEANUP]]
+; CHECK:       cleanup:
+; CHECK-NEXT:    store i1 true, i1* @a, align 4
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  br label %for.body
+
+for.body:
+  %d.015 = phi i32 [ 0, %entry ], [ %inc9, %for.inc8 ]
+  call void @foo()
+  %.b = load i1, i1* @a, align 4
+  %conv1 = select i1 %.b, i32 -128, i32 1
+  %cmp2.not = icmp ugt i32 %conv1, %d.015
+  br i1 %cmp2.not, label %cleanup, label %for.inc8
+
+for.inc8:
+  %inc9 = add nuw nsw i32 %d.015, 1
+  %exitcond.not = icmp eq i32 %inc9, 19
+  br i1 %exitcond.not, label %cleanup, label %for.body
+
+cleanup:
+  store i1 true, i1* @a, align 4
+  ret i32 0
+}
+
+declare void @foo()
+