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

[Davide Italiano via Phabricator via llvm-commits]

davide updated this revision to Diff 96426.
davide added a reviewer: efriedma.
davide added a comment.

Updated the check as per Eli's suggestion. PTAL.


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 +1,26 @@
+; PR28103
+; Bail out if the two successors are not the header
+; and another bb outside of the loop. This case is not
+; properly handled by LoopUnroll, currently.
+
+; RUN: opt -loop-unroll -verify-dom-info %s
+; REQUIRE: asserts
+
+define void @tinkywinky(i1 %patatino) {
+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 %patatino, label %body2, label %sink
+body2:
+  br i1 %patatino, 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
@@ -318,6 +318,10 @@
     return false;
   }
 
+  // The current loop unroll pass can only unroll loops with a single latch
+  // that's a conditional branch exiting the loop.
+  // FIXME: The implementation can be extended to work with more complicated
+  // cases, e.g. loops with multiple latches.
   BasicBlock *Header = L->getHeader();
   BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator());
 
@@ -328,6 +332,17 @@
     return false;
   }
 
+  auto CheckSuccessors = [&](unsigned S1, unsigned S2) {
+    return BI->getSuccessor(S1) == Header && !L->contains(BI->getSuccessor(S2));
+
+  };
+
+  if (!CheckSuccessors(0, 1) && !CheckSuccessors(1, 0)) {
+    DEBUG(dbgs() << "Can't unroll; only loops with one conditional latch"
+                    " exiting the loop can be unrolled\n");
+    return false;
+  }
+
   if (Header->hasAddressTaken()) {
     // The loop-rotate pass can be helpful to avoid this in many cases.
     DEBUG(dbgs() <<


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