From: Wojciech Matyjewicz <wmatyjewicz@fastmail.fm>
Date: Mon, 11 Feb 2008 18:37:34 +0000 (+0000)
Subject: If the LHS of the comparison is a loop-invariant we also want to move it
X-Git-Url: http://plrg.eecs.uci.edu/git/?a=commitdiff_plain;h=a089b104218c46803a1af374f6c441df885e54c2;p=oota-llvm.git

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 cbfc56373ae..122dba3e6ac 100644
--- 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
index 00000000000..3cdb1e3c768
--- /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
+}