If the LHS of the comparison is a loop-invariant we also want to move it

to the RHS. This simple change allows to compute loop iteration count
for loops with condition similar to the one in the testcase (which seems
to be quite common).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46959 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Analysis/ScalarEvolution.cpp b/lib/Analysis/ScalarEvolution.cpp
index cbfc56373aea72c834376e2d902daa15b5cb9956..122dba3e6ac08b4b8bd0476a47e17069327d5e6c 100644 (file)
--- a/lib/Analysis/ScalarEvolution.cpp
+++ b/lib/Analysis/ScalarEvolution.cpp
@@ -1819,8 +1819,8 @@ SCEVHandle ScalarEvolutionsImpl::ComputeIterationCount(const Loop *L) {
 
   // At this point, we would like to compute how many iterations of the 
   // loop the predicate will return true for these inputs.
-  if (isa<SCEVConstant>(LHS) && !isa<SCEVConstant>(RHS)) {
-    // If there is a constant, force it into the RHS.
+  if (LHS->isLoopInvariant(L) && !RHS->isLoopInvariant(L)) {
+    // If there is a loop-invariant, force it into the RHS.
     std::swap(LHS, RHS);
     Cond = ICmpInst::getSwappedPredicate(Cond);
   }

diff --git a/test/Analysis/ScalarEvolution/2008-02-11-ReversedCondition.ll b/test/Analysis/ScalarEvolution/2008-02-11-ReversedCondition.ll
new file mode 100644 (file)
index 0000000..3cdb1e3
--- /dev/null
+++ b/test/Analysis/ScalarEvolution/2008-02-11-ReversedCondition.ll
@@ -0,0 +1,15 @@
+; RUN: llvm-as < %s | opt -scalar-evolution -analyze | not grep Unpredictable
+
+define void @foo(i32 %n) {
+entry:
+       br label %header
+header:
+       %i = phi i32 [ 0, %entry ], [ %i.inc, %next ]
+       %cond = icmp sgt i32 %n, %i
+       br i1 %cond, label %next, label %return
+next:
+        %i.inc = add i32 %i, 1
+       br label %header
+return:
+       ret void
+}