[PATCH] D32261: [LoopUnroll] Don't try to unroll non-rotated loops

[Davide Italiano via Phabricator via llvm-commits]

davide created this revision.

LoopUnroll doesn't seem to be in a state to handle this, and we end discovering later, when we end up with a broken dominator :(
(see https://bugs.llvm.org/show_bug.cgi?id=28103#c3)

Fixes PR28103.


https://reviews.llvm.org/D32261

Files:
  lib/Transforms/Utils/LoopUnroll.cpp
  test/Transforms/LoopUnroll/not-rotated.ll


Index: test/Transforms/LoopUnroll/not-rotated.ll
===================================================================
--- /dev/null
+++ test/Transforms/LoopUnroll/not-rotated.ll
@@ -0,0 +1,21 @@
+; RUN: opt -loop-unroll -verify-dom-info %s
+; REQUIRE: asserts
+
+define void @tinkywinky() {
+entry:
+  br label %header1
+header1:
+  %indvars.iv = phi i64 [ 1, %body2 ], [ 0, %entry ]
+  %exitcond = icmp ne i64 %indvars.iv, 1
+  br i1 %exitcond, label %body1, label %exit
+body1:
+  br i1 true, label %body2, label %sink
+body2:
+  br i1 true, label %header1, label %body3
+body3:
+  br label %sink
+sink:
+  br label %body2
+exit:
+  ret void
+}
Index: lib/Transforms/Utils/LoopUnroll.cpp
===================================================================
--- lib/Transforms/Utils/LoopUnroll.cpp
+++ lib/Transforms/Utils/LoopUnroll.cpp
@@ -335,6 +335,17 @@
     return false;
   }
 
+  // If the loop is not rotated the latch is not one of the exiting blocks.
+  // Bail out on non-rotated loops.
+  SmallVector<BasicBlock *, 8> ExitingBlocks;
+  L->getExitingBlocks(ExitingBlocks);
+  bool NotRotated =
+      llvm::all_of(ExitingBlocks, [&LatchBlock](const BasicBlock *BB) {
+        return LatchBlock != BB;
+      });
+  if (NotRotated)
+    return false;
+
   if (TripCount != 0)
     DEBUG(dbgs() << "  Trip Count = " << TripCount << "\n");
   if (TripMultiple != 1)


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