[unittest] Exercise SCEV's udiv and udiv ceiling routines

The ceiling variant was recently added (due to the work towards D105216), and we're spending a lot of time trying to find optimizations for the expression. This patch brute forces the space of i8 unsigned divides and checks that we get a correct (well consistent with APInt) result for both udiv and udiv ceiling.

(This is basically what I've been doing locally in a hand rolled C++ program, and I realized there no good reason not to check it in as a unit test which directly exercises the logic on constants.)

Differential Revision: https://reviews.llvm.org/D106083

llvm/include/llvm/Analysis/ScalarEvolution.h

@@ -643,6 +643,16 @@ class ScalarEvolution {
                            SCEV::NoWrapFlags Flags = SCEV::FlagAnyWrap,
                            unsigned Depth = 0);
 
+  /// Compute ceil(N / D). N and D are treated as unsigned values.
+  ///
+  /// Since SCEV doesn't have native ceiling division, this generates a
+  /// SCEV expression of the following form:
+  ///
+  /// umin(N, 1) + floor((N - umin(N, 1)) / D)
+  ///
+  /// A denominator of zero or poison is handled the same way as getUDivExpr().
+  const SCEV *getUDivCeilSCEV(const SCEV *N, const SCEV *D);
+
   /// Return a SCEV corresponding to a conversion of the input value to the
   /// specified type.  If the type must be extended, it is zero extended.
   const SCEV *getTruncateOrZeroExtend(const SCEV *V, Type *Ty,
@@ -2029,16 +2039,6 @@ class ScalarEvolution {
   /// that the result is undefined if it does.
   const SCEV *computeBECount(const SCEV *Delta, const SCEV *Stride);
 
-  /// Compute ceil(N / D). N and D are treated as unsigned values.
-  ///
-  /// Since SCEV doesn't have native ceiling division, this generates a
-  /// SCEV expression of the following form:
-  ///
-  /// umin(N, 1) + floor((N - umin(N, 1)) / D)
-  ///
-  /// A denominator of zero or poison is handled the same way as getUDivExpr().
-  const SCEV *getUDivCeilSCEV(const SCEV *N, const SCEV *D);
-
   /// Compute the maximum backedge count based on the range of values
   /// permitted by Start, End, and Stride. This is for loops of the form
   /// {Start, +, Stride} LT End.

llvm/unittests/Analysis/ScalarEvolutionTest.cpp

@@ -1507,4 +1507,35 @@ TEST_F(ScalarEvolutionsTest, MatchURem) {
   });
 }
 
+TEST_F(ScalarEvolutionsTest, SCEVUDivFloorCeiling) {
+  LLVMContext C;
+  SMDiagnostic Err;
+  std::unique_ptr<Module> M = parseAssemblyString("define void @foo() { "
+                                                  "  ret void "
+                                                  "} ",
+                                                  Err, C);
+
+  ASSERT_TRUE(M && "Could not parse module?");
+  ASSERT_TRUE(!verifyModule(*M) && "Must have been well formed!");
+
+  runWithSE(*M, "foo", [](Function &F, LoopInfo &LI, ScalarEvolution &SE) {
+    // Check that SCEV's udiv and uceil handling produce the correct results
+    // for all 8 bit options. Div-by-zero is deliberately excluded.
+    for (unsigned N = 0; N < 256; N++)
+      for (unsigned D = 1; D < 256; D++) {
+        APInt NInt(8, N);
+        APInt DInt(8, D);
+        using namespace llvm::APIntOps;
+        APInt FloorInt = RoundingUDiv(NInt, DInt, APInt::Rounding::DOWN);
+        APInt CeilingInt = RoundingUDiv(NInt, DInt, APInt::Rounding::UP);
+        auto *NS = SE.getConstant(NInt);
+        auto *DS = SE.getConstant(DInt);
+        auto *FloorS = cast<SCEVConstant>(SE.getUDivExpr(NS, DS));
+        auto *CeilingS = cast<SCEVConstant>(SE.getUDivCeilSCEV(NS, DS));
+        ASSERT_TRUE(FloorS->getAPInt() == FloorInt);
+        ASSERT_TRUE(CeilingS->getAPInt() == CeilingInt);
+      }
+  });
+}
+
 }  // end namespace llvm