When SCEV can determine the loop test is X < X, set ExactBECount=0.

When ExactBECount is a constant, use it for MaxBECount.
When MaxBECount cannot be computed, replace it with ExactBECount.
Fixes PR9424.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127342 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 34 insertions, 0 deletions

diff --git a/test/Analysis/ScalarEvolution/2011-03-09-ExactNoMaxBECount.ll b/test/Analysis/ScalarEvolution/2011-03-09-ExactNoMaxBECount.ll
new file mode 100644
index 0000000000..9f17e27577
--- /dev/null
+++ b/test/Analysis/ScalarEvolution/2011-03-09-ExactNoMaxBECount.ll
@@ -0,0 +1,34 @@
+; RUN: opt -indvars  %s
+; PR9424: Attempt to use a SCEVCouldNotCompute object!
+; The inner loop computes the Step and Start of the outer loop.
+; Call that Vexit. The outer End value is max(2,Vexit), because
+; the condition "icmp %4 < 2" does not guard the outer loop.
+; SCEV knows that Vexit has range [2,4], so End == Vexit == Start.
+; Now we have ExactBECount == 0. However, MinStart == 2 and MaxEnd == 4.
+; Since the stride is variable and may wrap, we cannot compute
+; MaxBECount. SCEV should override MaxBECount with ExactBECount.
+
+define void @bar() nounwind {
+entry:
+  %. = select i1 undef, i32 2, i32 1
+  br label %"5.preheader"
+
+"4":                                              ; preds = %"5.preheader", %"4"
+  %0 = phi i32 [ 0, %"5.preheader" ], [ %1, %"4" ]
+  %1 = add nsw i32 %0, 1
+  %2 = icmp sgt i32 %., %1
+  br i1 %2, label %"4", label %"9"
+
+"9":                                              ; preds = %"4"
+  %3 = add i32 %6, 1
+  %4 = add i32 %3, %1
+  %5 = icmp slt i32 %4, 2
+  br i1 %5, label %"5.preheader", label %return
+
+"5.preheader":                                    ; preds = %"9", %entry
+  %6 = phi i32 [ 0, %entry ], [ %4, %"9" ]
+  br label %"4"
+
+return:                                           ; preds = %"9"
+  ret void
+}