Fix ScalarEvolution::ComputeExitLimitFromCond for 'or' conditions.

Fixes PR16130 - clang produces incorrect code with loop/expression at -O2.

This is a 2+ year old bug that's now holding up the release. It's a
case where we knowingly made aggressive assumptions about undefined
behavior. These assumptions are wrong when SCEV is computing a
subexpression that does not directly control the branch. With this
fix, we avoid making assumptions in those cases but still optimize the
common case. SCEV's trip count computation for exits controlled by
'or' expressions is now analagous to the trip count computation for
loops with multiple exits. I had already fixed the multiple exit case
to be conservative.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182989 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 6 insertions, 4 deletions

diff --git a/include/llvm/Analysis/ScalarEvolution.h b/include/llvm/Analysis/ScalarEvolution.h
index 306549f..349447f 100644
--- a/include/llvm/Analysis/ScalarEvolution.h
+++ b/include/llvm/Analysis/ScalarEvolution.h
@@ -453,7 +453,8 @@ namespace llvm {
     ExitLimit ComputeExitLimitFromCond(const Loop *L,
                                        Value *ExitCond,
                                        BasicBlock *TBB,
-                                       BasicBlock *FBB);
+                                       BasicBlock *FBB,
+                                       bool IsSubExpr);
 
     /// ComputeExitLimitFromICmp - Compute the number of times the backedge of
     /// the specified loop will execute if its exit condition were a conditional
@@ -461,7 +462,8 @@ namespace llvm {
     ExitLimit ComputeExitLimitFromICmp(const Loop *L,
                                        ICmpInst *ExitCond,
                                        BasicBlock *TBB,
-                                       BasicBlock *FBB);
+                                       BasicBlock *FBB,
+                                       bool IsSubExpr);
 
     /// ComputeLoadConstantCompareExitLimit - Given an exit condition
     /// of 'icmp op load X, cst', try to see if we can compute the
@@ -483,7 +485,7 @@ namespace llvm {
     /// HowFarToZero - Return the number of times an exit condition comparing
     /// the specified value to zero will execute.  If not computable, return
     /// CouldNotCompute.
-    ExitLimit HowFarToZero(const SCEV *V, const Loop *L);
+    ExitLimit HowFarToZero(const SCEV *V, const Loop *L, bool IsSubExpr);
 
     /// HowFarToNonZero - Return the number of times an exit condition checking
     /// the specified value for nonzero will execute.  If not computable, return
@@ -495,7 +497,7 @@ namespace llvm {
     /// computable, return CouldNotCompute. isSigned specifies whether the
     /// less-than is signed.
     ExitLimit HowManyLessThans(const SCEV *LHS, const SCEV *RHS,
-                               const Loop *L, bool isSigned);
+                               const Loop *L, bool isSigned, bool IsSubExpr);
 
     /// getPredecessorWithUniqueSuccessorForBB - Return a predecessor of BB
     /// (which may not be an immediate predecessor) which has exactly one