SCEV should use NSW to get trip count for positive nonunit stride loops.

SCEV currently fails to compute loop counts for nonunit stride
loops. This comes up frequently. It prevents loop optimization and
forces vectorization to insert extra loop checks.

For example:
void foo(int n, int *x) {
 for (int i = 0; i < n; i += 3) {
   x[i] = i;
   x[i+1] = i+1;
   x[i+2] = i+2;
 }
}

We need to properly handle the case in which limit > INT_MAX-stride. In
the above case: n > INT_MAX-3. In this case the loop counter will step
beyond the limit and overflow at the same time. However, knowing that
signed integer overlow in undefined, we can assume the loop test
behavior is arbitrary after overflow. This obeys both C undefined
behavior rules, and the more strict LLVM poison value rules.

I'm finally fixing this in response to Hal Finkel's persistence.
The most probable reason that we never optimized this before is that
we were being careful to handle case where the developer expected a
side-effect free infinite loop relying on overflow:

for (int i = 0; i < n; i += s) {
  ++j;
}
return j;

If INT_MAX+1 is a multiple of s and n > INT_MAX-s, then we might
expect an infinite loop. However there are plenty of ways to achieve
this effect without relying on undefined behavior of signed overflow.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193015 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 21 insertions, 18 deletions

diff --git a/lib/Analysis/ScalarEvolution.cpp b/lib/Analysis/ScalarEvolution.cpp
index 069c3fc..5047c66 100644
--- a/lib/Analysis/ScalarEvolution.cpp
+++ b/lib/Analysis/ScalarEvolution.cpp
@@ -6398,13 +6398,6 @@ ScalarEvolution::HowManyLessThans(const SCEV *LHS, const SCEV *RHS,
   if (!AddRec || AddRec->getLoop() != L)
     return getCouldNotCompute();
 
-  // Check to see if we have a flag which makes analysis easy.
-  bool NoWrap = false;
-  if (!IsSubExpr) {
-    NoWrap = AddRec->getNoWrapFlags(
-      (SCEV::NoWrapFlags)(((isSigned ? SCEV::FlagNSW : SCEV::FlagNUW))
-                          | SCEV::FlagNW));
-  }
   if (AddRec->isAffine()) {
     unsigned BitWidth = getTypeSizeInBits(AddRec->getType());
     const SCEV *Step = AddRec->getStepRecurrence(*this);
@@ -6414,20 +6407,21 @@ ScalarEvolution::HowManyLessThans(const SCEV *LHS, const SCEV *RHS,
     if (Step->isOne()) {
       // With unit stride, the iteration never steps past the limit value.
     } else if (isKnownPositive(Step)) {
-      // Test whether a positive iteration can step past the limit
-      // value and past the maximum value for its type in a single step.
-      // Note that it's not sufficient to check NoWrap here, because even
-      // though the value after a wrap is undefined, it's not undefined
-      // behavior, so if wrap does occur, the loop could either terminate or
-      // loop infinitely, but in either case, the loop is guaranteed to
-      // iterate at least until the iteration where the wrapping occurs.
+      // Test whether a positive iteration can step past the limit value and
+      // past the maximum value for its type in a single step. The NSW/NUW flags
+      // can imply that stepping past RHS would immediately result in undefined
+      // behavior. No self-wrap is not useful here because the loop counter may
+      // signed or unsigned wrap but continue iterating and terminate with
+      // defined behavior without ever self-wrapping.
       const SCEV *One = getConstant(Step->getType(), 1);
       if (isSigned) {
-        APInt Max = APInt::getSignedMaxValue(BitWidth);
-        if ((Max - getSignedRange(getMinusSCEV(Step, One)).getSignedMax())
+        if (!AddRec->getNoWrapFlags(SCEV::FlagNSW)) {
+          APInt Max = APInt::getSignedMaxValue(BitWidth);
+          if ((Max - getSignedRange(getMinusSCEV(Step, One)).getSignedMax())
               .slt(getSignedRange(RHS).getSignedMax()))
-          return getCouldNotCompute();
-      } else {
+            return getCouldNotCompute();
+        }
+      } else if (!AddRec->getNoWrapFlags(SCEV::FlagNUW)){
         APInt Max = APInt::getMaxValue(BitWidth);
         if ((Max - getUnsignedRange(getMinusSCEV(Step, One)).getUnsignedMax())
               .ult(getUnsignedRange(RHS).getUnsignedMax()))
@@ -6481,6 +6475,15 @@ ScalarEvolution::HowManyLessThans(const SCEV *LHS, const SCEV *RHS,
                   getMinusSCEV(getConstant(APInt::getMaxValue(BitWidth)),
                                StepMinusOne));
 
+    // If the loop counter does not self-wrap, then the trip count may be
+    // computed by dividing the distance by the step. This is independent of
+    // signed or unsigned wrap.
+    bool NoWrap = false;
+    if (!IsSubExpr) {
+      NoWrap = AddRec->getNoWrapFlags(
+        (SCEV::NoWrapFlags)(((isSigned ? SCEV::FlagNSW : SCEV::FlagNUW))
+                            | SCEV::FlagNW));
+    }
     // Finally, we subtract these two values and divide, rounding up, to get
     // the number of times the backedge is executed.
     const SCEV *BECount = getBECount(Start, End, Step, NoWrap);