[llvm] r307676 - [LoopUnrollRuntime] Avoid multi-exit nested loop with epilog generation

[Anna Thomas via llvm-commits]

Author: annat
Date: Tue Jul 11 10:16:33 2017
New Revision: 307676

URL: http://llvm.org/viewvc/llvm-project?rev=307676&view=rev
Log:
[LoopUnrollRuntime] Avoid multi-exit nested loop with epilog generation

The loop structure for the outer loop does not contain the epilog
preheader when we try to unroll inner loop with multiple exits and
epilog code is generated. For now, we just bail out in such cases.
Added a test case that shows the problem. Without this bailout, we would
trip on assert saying LCSSA form is incorrect for outer loop.

Modified:
    llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp
    llvm/trunk/test/Transforms/LoopUnroll/runtime-loop-multiple-exits.ll

Modified: llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp?rev=307676&r1=307675&r2=307676&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp Tue Jul 11 10:16:33 2017
@@ -470,8 +470,9 @@ bool llvm::UnrollRuntimeLoopRemainder(Lo
                                       bool UseEpilogRemainder,
                                       LoopInfo *LI, ScalarEvolution *SE,
                                       DominatorTree *DT, bool PreserveLCSSA) {
-  // for now, only unroll loops that contain a single exit
-  if (!UnrollRuntimeMultiExit && !L->getExitingBlock())
+  bool hasMultipleExitingBlocks = !L->getExitingBlock();
+  // Support only single exiting block unless UnrollRuntimeMultiExit is true.
+  if (!UnrollRuntimeMultiExit && hasMultipleExitingBlocks)
     return false;
 
   // Make sure the loop is in canonical form.
@@ -516,6 +517,13 @@ bool llvm::UnrollRuntimeLoopRemainder(Lo
   // connectEpilog.
   if (!LatchExit->getSinglePredecessor())
     return false;
+  // FIXME: We bail out of multi-exit unrolling when epilog loop is generated
+  // and L is an inner loop. This is because in presence of multiple exits, the
+  // outer loop is incorrect: we do not add the EpilogPreheader and exit to the
+  // outer loop. This is automatically handled in the prolog case, so we do not
+  // have that bug in prolog generation.
+  if (hasMultipleExitingBlocks && UseEpilogRemainder && L->getParentLoop())
+    return false;
   // Use Scalar Evolution to compute the trip count. This allows more loops to
   // be unrolled than relying on induction var simplification.
   if (!SE)

Modified: llvm/trunk/test/Transforms/LoopUnroll/runtime-loop-multiple-exits.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopUnroll/runtime-loop-multiple-exits.ll?rev=307676&r1=307675&r2=307676&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/LoopUnroll/runtime-loop-multiple-exits.ll (original)
+++ llvm/trunk/test/Transforms/LoopUnroll/runtime-loop-multiple-exits.ll Tue Jul 11 10:16:33 2017
@@ -434,3 +434,47 @@ loopexit1:
   %sext3 = phi i32 [ %shft, %header ]
   ret i32 %sext3
 }
+
+; Nested loop and inner loop is unrolled
+; FIXME: we cannot unroll with epilog remainder currently, because 
+; the outer loop does not contain the epilog preheader and epilog exit (while
+; infact it should). This causes us to choke up on LCSSA form being incorrect in
+; outer loop. However, the exit block where LCSSA fails, is infact still within
+; the outer loop. For now, we just bail out in presence of outer loop and epilog
+; loop is generated.
+; The outer loop header is the preheader for the inner loop and the inner header
+; branches back to the outer loop.
+define void @test8() {
+; EPILOG: test8(
+; EPILOG-NOT: niter
+
+; PROLOG: test8(
+; PROLOG: outerloop:
+; PROLOG-NEXT: phi i64 [ 3, %bb ], [ 0, %outerloop.loopexit ]
+; PROLOG:      %lcmp.mod = icmp eq i64
+; PROLOG-NEXT: br i1 %lcmp.mod, label %innerH.prol.loopexit, label %innerH.prol.preheader
+; PROLOG: latch.6:
+; PROLOG-NEXT: %tmp4.7 = add nsw i64 %tmp3, 8
+; PROLOG-NEXT: br i1 false, label %outerloop.loopexit.loopexit, label %latch.7
+; PROLOG: latch.7
+; PROLOG-NEXT: %tmp6.7 = icmp ult i64 %tmp4.7, 100
+; PROLOG-NEXT: br i1 %tmp6.7, label %innerH, label %exit.unr-lcssa
+bb:
+  br label %outerloop
+
+outerloop:                                              ; preds = %innerH, %bb
+  %tmp = phi i64 [ 3, %bb ], [ 0, %innerH ]
+  br label %innerH
+
+innerH:                                              ; preds = %latch, %outerloop
+  %tmp3 = phi i64 [ %tmp4, %latch ], [ %tmp, %outerloop ]
+  %tmp4 = add nuw nsw i64 %tmp3, 1
+  br i1 false, label %outerloop, label %latch
+
+latch:                                              ; preds = %innerH
+  %tmp6 = icmp ult i64 %tmp4, 100
+  br i1 %tmp6, label %innerH, label %exit
+
+exit:                                              ; preds = %latch
+  ret void
+}