[mlir] Fix signed ceildiv, loop normalization.

[hgreving]

Summary

Fixes using the signed ceildiv op instead of incorrectly assuming positive loop bounds.

Adjusts the tests.

[ftynse]

Nice, we actually have an op for that.

mlir/lib/Dialect/SCF/Utils/Utils.cpp

static LoopParams normalizeLoop(OpBuilder &boundsBuilder,
 
   bool isStepOne = false;
   if (auto stepCst = step.getDefiningOp<arith::ConstantIndexOp>())
     isStepOne = stepCst.value() == 1;
 
   // Compute the number of iterations the loop executes: ceildiv(ub - lb, step)
   // assuming the step is strictly positive.  Update the bounds and the step
   // of the loop to go from 0 to the number of iterations, if necessary.
-  // TODO: introduce support for negative steps or emit dynamic asserts
-  // on step positivity, whatever gets implemented first.
   if (isZeroBased && isStepOne)
     return {/*lowerBound=*/lowerBound, /*upperBound=*/upperBound,
             /*step=*/step};
 
   Value diff = boundsBuilder.create<arith::SubIOp>(loc, upperBound, lowerBound);
-  Value newUpperBound = ceilDivPositive(boundsBuilder, loc, diff, step);
+  Value newUpperBound =
+      boundsBuilder.create<arith::CeilDivSIOp>(loc, diff, step);
 
   Value newLowerBound =
       isZeroBased ? lowerBound
                   : boundsBuilder.create<arith::ConstantIndexOp>(loc, 0);
   Value newStep =
       isStepOne ? step : boundsBuilder.create<arith::ConstantIndexOp>(loc, 1);
 
   // Insert code computing the value of the original loop induction variable

mlir/test/Dialect/Affine/loop-coalescing.mlir

func.func @unnormalized_loops() {
   %c3 = arith.constant 3 : index
   %c5 = arith.constant 5 : index
   %c7 = arith.constant 7 : index
   %c10 = arith.constant 10 : index
   %c17 = arith.constant 17 : index
 
   // Number of iterations in the outer scf.
   // CHECK: %[[diff_i:.*]] = arith.subi %[[orig_ub_i]], %[[orig_lb_i]]
-  // CHECK: %[[c1:.*]] = arith.constant 1
-  // CHECK: %[[step_minus_c1:.*]] = arith.subi %[[orig_step_i]], %[[c1]]
-  // CHECK: %[[dividend:.*]] = arith.addi %[[diff_i]], %[[step_minus_c1]]
-  // CHECK: %[[numiter_i:.*]] = arith.divui %[[dividend]], %[[orig_step_i]]
+  // CHECK: %[[numiter_i:.*]] = arith.ceildivsi %[[diff_i]], %[[orig_step_i]]
   
   // Normalized lower bound and step for the outer scf.
   // CHECK: %[[lb_i:.*]] = arith.constant 0
   // CHECK: %[[step_i:.*]] = arith.constant 1
 
   // Number of iterations in the inner loop, the pattern is the same as above,
   // only capture the final result.
-  // CHECK: %[[numiter_j:.*]] = arith.divui {{.*}}, %[[orig_step_j]]
+  // CHECK: %[[numiter_j:.*]] = arith.ceildivsi {{.*}}, %[[orig_step_j]]
 
   // New bounds of the outer scf.
   // CHECK: %[[range:.*]] = arith.muli %[[numiter_i]], %[[numiter_j]]
   // CHECK: scf.for %[[i:.*]] = %[[lb_i]] to %[[range]] step %[[step_i]]
   scf.for %i = %c5 to %c10 step %c2 {
     // The inner loop has been removed.
     // CHECK-NOT: scf.for
     scf.for %j = %c7 to %c17 step %c3 {

 // CHECK-SAME: %[[orig_lb2:[A-Za-z0-9]+]]:
 // CHECK-SAME: %[[orig_ub2:[A-Za-z0-9]+]]:
 // CHECK-SAME: %[[orig_step2:[A-Za-z0-9]+]]:
 func.func @parametric(%lb1 : index, %ub1 : index, %step1 : index,
                  %lb2 : index, %ub2 : index, %step2 : index) {
   // Compute the number of iterations for each of the loops and the total
   // number of iterations.
   // CHECK: %[[range1:.*]] = arith.subi %[[orig_ub1]], %[[orig_lb1]]
-  // CHECK: %[[orig_step1_minus_1:.*]] = arith.subi %[[orig_step1]], %c1
-  // CHECK: %[[dividend1:.*]] = arith.addi %[[range1]], %[[orig_step1_minus_1]]
-  // CHECK: %[[numiter1:.*]] = arith.divui %[[dividend1]], %[[orig_step1]]
+  // CHECK: %[[numiter1:.*]] = arith.ceildivsi %[[range1]], %[[orig_step1]]
   // CHECK: %[[range2:.*]] = arith.subi %[[orig_ub2]], %[[orig_lb2]]
-  // CHECK: %[[orig_step2_minus_1:.*]] = arith.subi %arg5, %c1
-  // CHECK: %[[dividend2:.*]] = arith.addi %[[range2]], %[[orig_step2_minus_1]]
-  // CHECK: %[[numiter2:.*]] = arith.divui %[[dividend2]], %[[orig_step2]]
+  // CHECK: %[[numiter2:.*]] = arith.ceildivsi %[[range2]], %[[orig_step2]]
   // CHECK: %[[range:.*]] = arith.muli %[[numiter1]], %[[numiter2]] : index
 
   // Check that the outer loop is updated.
   // CHECK: scf.for %[[i:.*]] = %c0{{.*}} to %[[range]] step %c1
   scf.for %i = %lb1 to %ub1 step %step1 {
     // Check that the inner loop is removed.
     // CHECK-NOT: scf.for
     scf.for %j = %lb2 to %ub2 step %step2 {