[IRCE] Fix miscompile with range checks against negative values

In the patch rL329547, we have lifted the over-restrictive limitation on collected range
checks, allowing to work with range checks with the end of their range not being
provably non-negative. However it appeared that the non-negativity of this value was
assumed in the utility function `ClampedSubtract`. In particular, its reasoning is based
on the fact that `0 <= SINT_MAX - X`, which is not true if `X` is negative.

The function `ClampedSubtract` is only called twice, once with `X = 0` (which is OK)
and the second time with `X = IRC.getEnd()`, where we may now see the problem if
the end is actually a negative value. In this case, we may sometimes miscompile.

This patch is the conservative fix of the miscompile problem. Rather than rejecting
non-provably non-negative `getEnd()` values, we will check it for non-negativity in
runtime. For this, we use function `smax(smin(X, 0), -1) + 1` that is equal to `1` if `X`
is non-negative and is equal to 0 if `X` is negative. If we multiply `Begin, End` of safe
iteration space by this function calculated for `X = IRC.getEnd()`, we will get the original
`[Begin, End)` if `IRC.getEnd()` was non-negative (and, thus, `ClampedSubtract` worked
correctly) and the empty range `[0, 0)` in case if ` IRC.getEnd()` was negative.

So we in fact prohibit execution of the main loop if at least one of range checks was
made against a negative value (and we figured it out in runtime). It is still better than
what we have before (non-negativity had to be proved in compile time) and prevents
us from miscompile, however it is sometiles too restrictive for unsigned range checks
against a negative value (which in fact can be eliminated).

Once we re-implement `ClampedSubtract` in a way that it handles negative `X` correctly,
this limitation can be lifted, too.

Differential Revision: https://reviews.llvm.org/D46860
Reviewed By: samparker

llvm-svn: 332809

Author: Max Kazantsev

llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp

@@ -813,6 +813,13 @@ static bool isKnownNonNegativeInLoop(const SCEV *BoundSCEV, const Loop *L,
          SE.isLoopEntryGuardedByCond(L, ICmpInst::ICMP_SGE, BoundSCEV, Zero);
 }
 
+static bool isKnownNegativeInLoop(const SCEV *BoundSCEV, const Loop *L,
+                                  ScalarEvolution &SE) {
+  const SCEV *Zero = SE.getZero(BoundSCEV->getType());
+  return SE.isAvailableAtLoopEntry(BoundSCEV, L) &&
+         SE.isLoopEntryGuardedByCond(L, ICmpInst::ICMP_SLT, BoundSCEV, Zero);
+}
+
 Optional<LoopStructure>
 LoopStructure::parseLoopStructure(ScalarEvolution &SE,
                                   BranchProbabilityInfo *BPI, Loop &L,
@@ -1700,6 +1707,10 @@ InductiveRangeCheck::computeSafeIterationSpace(
   // values, depending on type of latch condition that defines IV iteration
   // space.
   auto ClampedSubtract = [&](const SCEV *X, const SCEV *Y) {
+    // FIXME: The current implementation assumes that X is in [0, SINT_MAX].
+    // This is required to ensure that SINT_MAX - X does not overflow signed and
+    // that X - Y does not overflow unsigned if Y is negative. Can we lift this
+    // restriction and make it work for negative X either?
     if (IsLatchSigned) {
       // X is a number from signed range, Y is interpreted as signed.
       // Even if Y is SINT_MAX, (X - Y) does not reach SINT_MIN. So the only
@@ -1729,8 +1740,34 @@ InductiveRangeCheck::computeSafeIterationSpace(
   };
   const SCEV *M = SE.getMinusSCEV(C, A);
   const SCEV *Zero = SE.getZero(M->getType());
-  const SCEV *Begin = ClampedSubtract(Zero, M);
-  const SCEV *End = ClampedSubtract(getEnd(), M);
+
+  // This function returns SCEV equal to 1 if X is non-negative 0 otherwise.
+  auto SCEVCheckNonNegative = [&](const SCEV *X) {
+    const Loop *L = IndVar->getLoop();
+    const SCEV *One = SE.getOne(X->getType());
+    // Can we trivially prove that X is a non-negative or negative value?
+    if (isKnownNonNegativeInLoop(X, L, SE))
+      return One;
+    else if (isKnownNegativeInLoop(X, L, SE))
+      return Zero;
+    // If not, we will have to figure it out during the execution.
+    // Function smax(smin(X, 0), -1) + 1 equals to 1 if X >= 0 and 0 if X < 0.
+    const SCEV *NegOne = SE.getNegativeSCEV(One);
+    return SE.getAddExpr(SE.getSMaxExpr(SE.getSMinExpr(X, Zero), NegOne), One);
+  };
+  // FIXME: Current implementation of ClampedSubtract implicitly assumes that
+  // X is non-negative (in sense of a signed value). We need to re-implement
+  // this function in a way that it will correctly handle negative X as well.
+  // We use it twice: for X = 0 everything is fine, but for X = getEnd() we can
+  // end up with a negative X and produce wrong results. So currently we ensure
+  // that if getEnd() is negative then both ends of the safe range are zero.
+  // Note that this may pessimize elimination of unsigned range checks against
+  // negative values.
+  const SCEV *REnd = getEnd();
+  const SCEV *EndIsNonNegative = SCEVCheckNonNegative(REnd);
+
+  const SCEV *Begin = SE.getMulExpr(ClampedSubtract(Zero, M), EndIsNonNegative);
+  const SCEV *End = SE.getMulExpr(ClampedSubtract(REnd, M), EndIsNonNegative);
   return InductiveRangeCheck::Range(Begin, End);
 }