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[mlir][scf] Add value bound for computed upper bound of forall loop #171158

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Merged
Groverkss merged 3 commits into from
Dec 11, 2025
Merged

[mlir][scf] Add value bound for computed upper bound of forall loop #171158

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cc45adb
[mlir][scf] Add value bound for computed upper bound of forall loop
Groverkss Dec 8, 2025
3268e1f
Address comments
Groverkss Dec 11, 2025
537b7d6
Merge branch 'main' into forall-upper-bound
Groverkss Dec 11, 2025
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63 changes: 34 additions & 29 deletions mlir/lib/Dialect/SCF/IR/ValueBoundsOpInterfaceImpl.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -17,19 +17,36 @@ namespace mlir {
namespace scf {
namespace {

static AffineExpr getTripCountExpr(OpFoldResult lb, OpFoldResult ub,
OpFoldResult step,
ValueBoundsConstraintSet &cstr) {
AffineExpr lbExpr = cstr.getExpr(lb);
AffineExpr ubExpr = cstr.getExpr(ub);
AffineExpr stepExpr = cstr.getExpr(step);
AffineExpr tripCountExpr =
AffineExpr(ubExpr - lbExpr).ceilDiv(stepExpr); // (ub - lb) / step
return tripCountExpr;
}

static void populateIVBounds(OpFoldResult lb, OpFoldResult ub,
OpFoldResult step, Value iv,
ValueBoundsConstraintSet &cstr) {
cstr.bound(iv) >= cstr.getExpr(lb);
cstr.bound(iv) < cstr.getExpr(ub);
// 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(lb, ub, step, cstr) - cstr.getExpr(1);
AffineExpr computedUpperBound =
cstr.getExpr(lb) + AffineExpr(tripCountMinusOne * cstr.getExpr(step));
cstr.bound(iv) <= computedUpperBound;
}

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 @@ -87,7 +104,8 @@ 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 tripCountExpr = getTripCountExpr(forOp, cstr);
AffineExpr tripCountExpr = getTripCountExpr(
forOp.getLowerBound(), forOp.getUpperBound(), forOp.getStep(), cstr);
AffineExpr oneIterAdvanceExpr =
cstr.getExpr(yieldedValue) - cstr.getExpr(iterArg);
cstr.bound(value) ==
Expand All @@ -99,19 +117,8 @@ struct ForOpInterface
auto forOp = cast<ForOp>(op);

if (value == forOp.getInductionVar()) {
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;
return populateIVBounds(forOp.getLowerBound(), forOp.getUpperBound(),
forOp.getStep(), value, cstr);
}

// Handle iter_args and OpResults.
Expand Down Expand Up @@ -141,11 +148,9 @@ struct ForallOpInterface
assert(blockArg.getArgNumber() < forallOp.getInductionVars().size() &&
"expected index value to be an induction var");
int64_t idx = blockArg.getArgNumber();
// TODO: Take into account step size.
AffineExpr lb = cstr.getExpr(forallOp.getMixedLowerBound()[idx]);
AffineExpr ub = cstr.getExpr(forallOp.getMixedUpperBound()[idx]);
cstr.bound(value) >= lb;
cstr.bound(value) < ub;
return populateIVBounds(forallOp.getMixedLowerBound()[idx],
forallOp.getMixedUpperBound()[idx],
forallOp.getMixedStep()[idx], value, cstr);
}

void populateBoundsForShapedValueDim(Operation *op, Value value, int64_t dim,
Expand Down
9 changes: 9 additions & 0 deletions mlir/test/Dialect/SCF/value-bounds-op-interface-impl.mlir
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Expand Up @@ -379,3 +379,12 @@ func.func @scf_for_result_infer_dynamic_init_big_step(%i : index) {
"test.compare"(%0, %7) {cmp = "LE"} : (index, index) -> ()
return
}

func.func @scf_forall_computed_upper_bound(%x: index) {
%c6 = arith.constant 6 : index
scf.forall (%iv) = (0) to (8) step (3) {
// expected-remark @below{{true}}
"test.compare"(%iv, %c6) {cmp = "LE"} : (index, index) -> ()
}
return
}