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[mlir][scf]: Add value bound for the computed upper bound of for loop #126426

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Feb 10, 2025
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[mlir][scf]: Add value bound for the computed upper bound of for loop #126426

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27 changes: 21 additions & 6 deletions mlir/lib/Dialect/SCF/IR/ValueBoundsOpInterfaceImpl.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -20,6 +20,16 @@ namespace {
struct ForOpInterface
: public ValueBoundsOpInterface::ExternalModel<ForOpInterface, ForOp> {

static AffineExpr getTripCountExpr(scf::ForOp forOp,
ValueBoundsConstraintSet &cstr) {
AffineExpr lbExpr = cstr.getExpr(forOp.getLowerBound());
AffineExpr ubExpr = cstr.getExpr(forOp.getUpperBound());
AffineExpr stepExpr = cstr.getExpr(forOp.getStep());
AffineExpr tripCountExpr =
AffineExpr(ubExpr - lbExpr).ceilDiv(stepExpr); // (ub - lb) / step
return tripCountExpr;
}

/// Populate bounds of values/dimensions for iter_args/OpResults. If the
/// value/dimension size does not change in an iteration, we can deduce that
/// it the same as the initial value/dimension.
Expand Down Expand Up @@ -77,11 +87,7 @@ struct ForOpInterface
// `value` is result of `forOp`, we can prove that:
// %result == %init_arg + trip_count * (%yielded_value - %iter_arg).
// Where trip_count is (ub - lb) / step.
AffineExpr lbExpr = cstr.getExpr(forOp.getLowerBound());
AffineExpr ubExpr = cstr.getExpr(forOp.getUpperBound());
AffineExpr stepExpr = cstr.getExpr(forOp.getStep());
AffineExpr tripCountExpr =
AffineExpr(ubExpr - lbExpr).ceilDiv(stepExpr); // (ub - lb) / step
AffineExpr tripCountExpr = getTripCountExpr(forOp, cstr);
AffineExpr oneIterAdvanceExpr =
cstr.getExpr(yieldedValue) - cstr.getExpr(iterArg);
cstr.bound(value) ==
Expand All @@ -93,9 +99,18 @@ struct ForOpInterface
auto forOp = cast<ForOp>(op);

if (value == forOp.getInductionVar()) {
// TODO: Take into account step size.
cstr.bound(value) >= forOp.getLowerBound();
cstr.bound(value) < forOp.getUpperBound();
// iv <= lb + ((ub-lb)/step - 1) * step
// This bound does not replace the `iv < ub` constraint mentioned above,
// since constraints involving the multiplication of two constraint set
// dimensions are not supported.
AffineExpr tripCountMinusOne =
getTripCountExpr(forOp, cstr) - cstr.getExpr(1);
AffineExpr computedUpperBound =
cstr.getExpr(forOp.getLowerBound()) +
AffineExpr(tripCountMinusOne * cstr.getExpr(forOp.getStep()));
cstr.bound(value) <= computedUpperBound;
return;
}

Expand Down
41 changes: 41 additions & 0 deletions mlir/test/Dialect/SCF/value-bounds-op-interface-impl.mlir
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Expand Up @@ -270,6 +270,47 @@ func.func @compare_scf_for(%a: index, %b: index, %c: index) {

// -----

func.func @scf_for_induction_var_upper_bound() {
%c0 = arith.constant 0 : index
%c1 = arith.constant 1 : index
%c2 = arith.constant 2 : index
%c3 = arith.constant 3 : index
%c4 = arith.constant 4 : index
%c5 = arith.constant 5 : index
%c8 = arith.constant 8 : index
%c10 = arith.constant 10 : index
scf.for %iv = %c0 to %c10 step %c4 {
// expected-remark @below{{true}}
"test.compare"(%iv, %c8) {cmp = "LE"} : (index, index) -> ()
}
scf.for %iv = %c2 to %c8 step %c3 {
// expected-remark @below{{true}}
"test.compare"(%iv, %c5) {cmp = "LE"} : (index, index) -> ()
}
return
}

// -----

#map_ceildiv_dynamic_divisor = affine_map<(i)[s] -> (i ceildiv s)>
func.func @scf_for_induction_var_computed_upper_bound(%upperBound: index, %step: index) {
%c0 = arith.constant 0 : index
%c1 = arith.constant 1 : index
%tripCount = affine.apply #map_ceildiv_dynamic_divisor (%upperBound)[%step]
%tripCountMinusOne = arith.subi %tripCount, %c1 : index
%computedUpperBound = arith.muli %tripCountMinusOne, %step : index
scf.for %iv = %c0 to %upperBound step %step {
// TODO: Value bounds analysis will fail to compute upper bound
// because multiplication/division of unknown block arguments is
// not supported.
// expected-error @below{{unknown}}
"test.compare"(%iv, %computedUpperBound) {cmp = "LE"} : (index, index) -> ()
}
return
}

// -----

func.func @scf_for_result_infer() {
%c0 = arith.constant 0 : index
%c1 = arith.constant 1 : index
Expand Down