LoopVectorizer: Don't assert on the absence of induction variables

A computable loop exit count does not imply the presence of an induction
variable. Scalar evolution can return a value for an infinite loop.

Fixes PR15926.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181495 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 36 insertions, 1 deletions

diff --git a/lib/Transforms/Vectorize/LoopVectorize.cpp b/lib/Transforms/Vectorize/LoopVectorize.cpp
index e7da695..b16b371 100644
--- a/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -2523,7 +2523,8 @@ bool LoopVectorizationLegality::canVectorizeInstrs() {
 
   if (!Induction) {
     DEBUG(dbgs() << "LV: Did not find one integer induction var.\n");
-    assert(getInductionVars()->size() && "No induction variables");
+    if (Inductions.empty())
+      return false;
   }
 
   return true;
diff --git a/test/Transforms/LoopVectorize/infiniteloop.ll b/test/Transforms/LoopVectorize/infiniteloop.ll
new file mode 100644
index 0000000..f6ab564
--- /dev/null
+++ b/test/Transforms/LoopVectorize/infiniteloop.ll
@@ -0,0 +1,34 @@
+; RUN: opt -S -indvars -loop-vectorize -force-vector-width=2  < %s | FileCheck %s
+
+target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:32:32-n8:16:32-S128"
+
+@a = common global i64 0, align 8
+@x = common global i32 0, align 4
+
+; We used to assert on this loop because we could not find an induction
+; variable but assumed there must be one. Scalar evolution returned a exit
+; count for the loop below and from there on we assumed that there must be an
+; induction variable. This is not a valid assumption:
+;   // getExitCount - Get the expression for the number of loop iterations for
+;   // which this loop is *guaranteed not to exit* via ExitingBlock. Otherwise
+;   // return SCEVCouldNotCompute.
+; For an infinite loop SE can return any number.
+
+; CHECK: fn1
+define void @fn1()  {
+entry:
+  store i64 0, i64* @a, align 8
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %inc1 = phi i64 [ 0, %entry ], [ %inc, %for.body ]
+  store volatile i32 0, i32* @x, align 4
+  %inc = add nsw i64 %inc1, 1
+  %cmp = icmp sgt i64 %inc1, -2
+  br i1 %cmp, label %for.body, label %for.end
+
+for.end:                                          ; preds = %for.body
+  %inc.lcssa = phi i64 [ %inc, %for.body ]
+  store i64 %inc.lcssa, i64* @a, align 8
+  ret void
+}