Fix for PR2607: SCEV miscomputing the loop count for loops with an

SGT exit condition.  Essentially, the correct way to flip an inequality
in 2's complement is the not operator, not the negation operator.
That said, the difference only affects cases involving INT_MIN.

Also, enhance the pre-test search logic to be a bit smarter about
inequalities flipped with a not operator, so it can eliminate the smax
from the iteration count for simple loops.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@54184 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Analysis/ScalarEvolution.cpp b/lib/Analysis/ScalarEvolution.cpp
index 737d9b3823e6471aacb4c7221f935b34ed37625b..14c1389b43435b83b943f13ec86564d936cf0987 100644 (file)
--- a/lib/Analysis/ScalarEvolution.cpp
+++ b/lib/Analysis/ScalarEvolution.cpp
@@ -1976,8 +1976,8 @@ SCEVHandle ScalarEvolutionsImpl::ComputeIterationCount(const Loop *L) {
     break;
   }
   case ICmpInst::ICMP_SGT: {
-    SCEVHandle TC = HowManyLessThans(SE.getNegativeSCEV(LHS),
-                                     SE.getNegativeSCEV(RHS), L, true);
+    SCEVHandle TC = HowManyLessThans(SE.getNotSCEV(LHS),
+                                     SE.getNotSCEV(RHS), L, true);
     if (!isa<SCEVCouldNotCompute>(TC)) return TC;
     break;
   }
@@ -2724,7 +2724,11 @@ bool ScalarEvolutionsImpl::executesAtLeastOnce(const Loop *L, bool isSigned,
 
   if (!PreCondLHS->getType()->isInteger()) return false;
 
-  return LHS == getSCEV(PreCondLHS) && RHS == getSCEV(PreCondRHS);
+  SCEVHandle PreCondLHSSCEV = getSCEV(PreCondLHS);
+  SCEVHandle PreCondRHSSCEV = getSCEV(PreCondRHS);
+  return (LHS == PreCondLHSSCEV && RHS == PreCondRHSSCEV) ||
+         (LHS == SE.getNotSCEV(PreCondRHSSCEV) &&
+          RHS == SE.getNotSCEV(PreCondLHSSCEV));
 }
 
 /// HowManyLessThans - Return the number of times a backedge containing the

diff --git a/test/Analysis/ScalarEvolution/2008-07-29-SGTTripCount.ll b/test/Analysis/ScalarEvolution/2008-07-29-SGTTripCount.ll
new file mode 100644 (file)
index 0000000..307439f
--- /dev/null
+++ b/test/Analysis/ScalarEvolution/2008-07-29-SGTTripCount.ll
@@ -0,0 +1,27 @@
+; RUN: llvm-as < %s | opt -analyze -scalar-evolution -disable-output \
+; RUN:   -scalar-evolution-max-iterations=0 | \
+; RUN: grep -F "( -1 + ( -1 *  %j)) iterations"
+; PR2607
+
+define i32 @_Z1aj(i32 %j) nounwind  {
+entry:
+       icmp sgt i32 0, %j              ; <i1>:0 [#uses=1]
+       br i1 %0, label %bb.preheader, label %return
+
+bb.preheader:          ; preds = %entry
+       br label %bb
+
+bb:            ; preds = %bb, %bb.preheader
+       %i.01 = phi i32 [ %1, %bb ], [ 0, %bb.preheader ]               ; <i32> [#uses=1]
+       add i32 %i.01, -1               ; <i32>:1 [#uses=3]
+       icmp sgt i32 %1, %j             ; <i1>:2 [#uses=1]
+       br i1 %2, label %bb, label %return.loopexit
+
+return.loopexit:               ; preds = %bb
+       br label %return
+
+return:                ; preds = %return.loopexit, %entry
+       %i.0.lcssa = phi i32 [ 0, %entry ], [ %1, %return.loopexit ]            ; <i32> [#uses=1]
+       ret i32 %i.0.lcssa
+}
+