Don't try to analyze this "backward" case. This is overly conservative

pending a correct solution.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61589 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 52 insertions, 0 deletions

diff --git a/lib/Analysis/ScalarEvolution.cpp b/lib/Analysis/ScalarEvolution.cpp
index 142b82d223..375ad5ed70 100644
--- a/lib/Analysis/ScalarEvolution.cpp
+++ b/lib/Analysis/ScalarEvolution.cpp
@@ -2905,6 +2905,11 @@ bool ScalarEvolutionsImpl::executesAtLeastOnce(const Loop *L, bool isSigned,
   return false;
 }
 
+static bool isNegative(SCEVHandle X) {
+  if (SCEVConstant *C = dyn_cast<SCEVConstant>(X))
+    return C->getValue()->getValue().isNegative();
+}
+
 /// potentialInfiniteLoop - Test whether the loop might jump over the exit value
 /// due to wrapping around 2^n.
 bool ScalarEvolutionsImpl::potentialInfiniteLoop(SCEV *Stride, SCEV *RHS,
@@ -2958,6 +2963,13 @@ HowManyLessThans(SCEV *LHS, SCEV *RHS, const Loop *L,
     if (potentialInfiniteLoop(Stride, RHS, isSigned, trueWhenEqual))
       return UnknownValue;
 
+    // We don't handle this correctly at the moment. The problem is that when
+    // the stride is negative, we're not counting how many times 'less-than' is
+    // true as we approach it, we're counting how far away we are from wrapping
+    // around the backside.
+    if (isSigned && isNegative(Stride))
+      return UnknownValue;
+
     // We know the LHS is of the form {n,+,s} and the RHS is some loop-invariant
     // m.  So, we count the number of iterations in which {n,+,s} < m is true.
     // Note that we cannot simply return max(m-n,0)/s because it's not safe to
diff --git a/test/Analysis/ScalarEvolution/2009-01-02-SignedNegativeStride.ll b/test/Analysis/ScalarEvolution/2009-01-02-SignedNegativeStride.ll
new file mode 100644
index 0000000000..07714cd1b5
--- /dev/null
+++ b/test/Analysis/ScalarEvolution/2009-01-02-SignedNegativeStride.ll
@@ -0,0 +1,40 @@
+; RUN: llvm-as < %s | opt -analyze -scalar-evolution | not grep {/u -1}
+; PR3275
+
+@g_16 = external global i16		; <i16*> [#uses=3]
+@.str = external constant [4 x i8]		; <[4 x i8]*> [#uses=0]
+
+define void @func_15() nounwind {
+entry:
+	%0 = load i16* @g_16, align 2		; <i16> [#uses=1]
+	%1 = icmp sgt i16 %0, 0		; <i1> [#uses=1]
+	br i1 %1, label %bb2, label %bb.nph
+
+bb.nph:		; preds = %entry
+	%g_16.promoted = load i16* @g_16		; <i16> [#uses=1]
+	br label %bb
+
+bb:		; preds = %bb1, %bb.nph
+	%g_16.tmp.0 = phi i16 [ %g_16.promoted, %bb.nph ], [ %2, %bb1 ]		; <i16> [#uses=1]
+	%2 = add i16 %g_16.tmp.0, -1		; <i16> [#uses=3]
+	br label %bb1
+
+bb1:		; preds = %bb
+	%3 = icmp sgt i16 %2, 0		; <i1> [#uses=1]
+	br i1 %3, label %bb1.bb2_crit_edge, label %bb
+
+bb1.bb2_crit_edge:		; preds = %bb1
+	store i16 %2, i16* @g_16
+	br label %bb2
+
+bb2:		; preds = %bb1.bb2_crit_edge, %entry
+	br label %return
+
+return:		; preds = %bb2
+	ret void
+}
+
+declare i32 @main() nounwind
+
+declare i32 @printf(i8*, ...) nounwind
+