[llvm] r297993 - [LoopUnroll] Don't peel loops where the latch isn't the exiting block

[Michael Kuperstein via llvm-commits]

Author: mkuper
Date: Thu Mar 16 16:07:48 2017
New Revision: 297993

URL: http://llvm.org/viewvc/llvm-project?rev=297993&view=rev
Log:
[LoopUnroll] Don't peel loops where the latch isn't the exiting block

Peeling assumed this doesn't happen, but didn't check it.
This fixes PR32178.

Differential Revision: https://reviews.llvm.org/D30757

Added:
    llvm/trunk/test/Transforms/LoopUnroll/peel-loop-irreducible.ll
Modified:
    llvm/trunk/lib/Transforms/Utils/LoopUnrollPeel.cpp

Modified: llvm/trunk/lib/Transforms/Utils/LoopUnrollPeel.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/LoopUnrollPeel.cpp?rev=297993&r1=297992&r2=297993&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/LoopUnrollPeel.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/LoopUnrollPeel.cpp Thu Mar 16 16:07:48 2017
@@ -56,6 +56,13 @@ static bool canPeel(Loop *L) {
   if (!L->getExitingBlock() || !L->getUniqueExitBlock())
     return false;
 
+  // Don't try to peel loops where the latch is not the exiting block.
+  // This can be an indication of two different things:
+  // 1) The loop is not rotated.
+  // 2) The loop contains irreducible control flow that involves the latch.
+  if (L->getLoopLatch() != L->getExitingBlock())
+    return false;
+
   return true;
 }
 

Added: llvm/trunk/test/Transforms/LoopUnroll/peel-loop-irreducible.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopUnroll/peel-loop-irreducible.ll?rev=297993&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/LoopUnroll/peel-loop-irreducible.ll (added)
+++ llvm/trunk/test/Transforms/LoopUnroll/peel-loop-irreducible.ll Thu Mar 16 16:07:48 2017
@@ -0,0 +1,36 @@
+; RUN: opt < %s -S -loop-unroll -unroll-force-peel-count=1 | FileCheck %s
+
+; Check we don't peel loops where the latch is not the exiting block.
+; CHECK-LABEL: @invariant_backedge_irreducible
+; CHECK: entry:
+; CHECK: br label %header
+; CHECK-NOT: peel
+; CHECK: header:
+; CHECK: br i1 {{.*}} label %latch, label %exiting
+; CHECK: latch:
+; CHECK: br i1 {{.*}} label %header, label %exiting
+; CHECK: exiting:
+; CHECK: br i1 {{.*}} label %latch, label %exit
+
+define i32 @invariant_backedge_irreducible(i32 %a, i32 %b) {
+entry:
+  br label %header
+
+header:
+  %i = phi i32 [ 0, %entry ], [ %inc, %latch ]
+  %cmp.phi = phi i1 [ false, %entry ], [ %cmp, %latch ]
+  br i1 %cmp.phi, label %latch, label %exiting
+
+latch:
+  %inc = add i32 %i, 1
+  %cmp = icmp slt i32 %i, 1000
+  br i1 %cmp, label %header, label %exiting
+
+exiting:
+  %cmp.exiting = phi i1 [ %cmp.phi, %header ], [ %cmp, %latch ]
+  br i1 %cmp.exiting, label %latch, label %exit
+
+exit:
+  ret i32 0
+}
+