[SCEV] Fix incorrect trip count computation

The way we elide max expressions when computing trip counts is incorrect
-- it breaks cases like this:

```
static int wrapping_add(int a, int b) {
  return (int)((unsigned)a + (unsigned)b);
}

void test() {
  volatile int end_buf = 2147483548; // INT_MIN - 100
  int end = end_buf;

  unsigned counter = 0;
  for (int start = wrapping_add(end,  200); start < end; start++)
    counter++;

  print(counter);
}
```

Note: the `NoWrap` variable that was being tested has little to do with
the values flowing into the max expression; it is a property of the
induction variable.

test/Transforms/LoopUnroll/nsw-tripcount.ll was added to solely test
functionality I'm reverting in this change, so I've deleted the test
fully.

llvm-svn: 273079

1 files changed, 4 insertions, 24 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 2509031..c2e1825 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -8668,18 +8668,8 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
                                       : ICmpInst::ICMP_ULT;
   const SCEV *Start = IV->getStart();
   const SCEV *End = RHS;
-  if (!isLoopEntryGuardedByCond(L, Cond, getMinusSCEV(Start, Stride), RHS)) {
-    const SCEV *Diff = getMinusSCEV(RHS, Start);
-    // If we have NoWrap set, then we can assume that the increment won't
-    // overflow, in which case if RHS - Start is a constant, we don't need to
-    // do a max operation since we can just figure it out statically
-    if (NoWrap && isa<SCEVConstant>(Diff)) {
-      if (cast<SCEVConstant>(Diff)->getAPInt().isNegative())
-        End = Start;
-    } else
-      End = IsSigned ? getSMaxExpr(RHS, Start)
-                     : getUMaxExpr(RHS, Start);
-  }
+  if (!isLoopEntryGuardedByCond(L, Cond, getMinusSCEV(Start, Stride), RHS))
+    End = IsSigned ? getSMaxExpr(RHS, Start) : getUMaxExpr(RHS, Start);
 
   const SCEV *BECount = computeBECount(getMinusSCEV(End, Start), Stride, false);
 
@@ -8754,18 +8744,8 @@ ScalarEvolution::howManyGreaterThans(const SCEV *LHS, const SCEV *RHS,
 
   const SCEV *Start = IV->getStart();
   const SCEV *End = RHS;
-  if (!isLoopEntryGuardedByCond(L, Cond, getAddExpr(Start, Stride), RHS)) {
-    const SCEV *Diff = getMinusSCEV(RHS, Start);
-    // If we have NoWrap set, then we can assume that the increment won't
-    // overflow, in which case if RHS - Start is a constant, we don't need to
-    // do a max operation since we can just figure it out statically
-    if (NoWrap && isa<SCEVConstant>(Diff)) {
-      if (!cast<SCEVConstant>(Diff)->getAPInt().isNegative())
-        End = Start;
-    } else
-      End = IsSigned ? getSMinExpr(RHS, Start)
-                     : getUMinExpr(RHS, Start);
-  }
+  if (!isLoopEntryGuardedByCond(L, Cond, getAddExpr(Start, Stride), RHS))
+    End = IsSigned ? getSMinExpr(RHS, Start) : getUMinExpr(RHS, Start);
 
   const SCEV *BECount = computeBECount(getMinusSCEV(Start, End), Stride, false);