[mlir][affine] ValueBoundsConstraintSet: Fully compose affine.apply

[matthias-springer]

Fully compose affine.apply ops before adding them to the underlying FlatLinearConstraints. This works around a limitation of FlatLinearConstraints, which cannot deduce a constant bound if it involves two identical local variables.

Details for future improvements of FlatLinearConstraints: The constraint set infrastructure fails to compute a constant bound of -8 for the first variable.

Domain: 0, Range: 1, Symbols: 4, Locals: 2
8 constraints
(None    None    None    None    None    Local    Local    const)
 1    -1    0    0    0    0    0    0    = 0
 0    1    -1    1    0    0    0    0    = 0
 0    0    1    0    0    0    -16    0    = 0
 0    0    0    1    0    -16    0    -8    = 0
 0    0    0    0    -1    0    32    31    >= 0
 0    0    0    0    1    0    -32    0    >= 0
 0    0    0    0    -1    32    0    31    >= 0
 0    0    0    0    1    -32    0    0    >= 0

[llvmbot]

@llvm/pr-subscribers-mlir-affine

@llvm/pr-subscribers-mlir

Author: Matthias Springer (matthias-springer)

Changes

Fully compose affine.apply ops before adding them to the underlying FlatLinearConstraints. This works around a limitation of FlatLinearConstraints, which cannot deduce a constant bound if it involves two identical local variables.

Details for future improvements of FlatLinearConstraints: The constraint set infrastructure fails to compute a constant bound of -8 for the first variable.

Domain: 0, Range: 1, Symbols: 4, Locals: 2
8 constraints
(None    None    None    None    None    Local    Local    const)
 1    -1    0    0    0    0    0    0    = 0
 0    1    -1    1    0    0    0    0    = 0
 0    0    1    0    0    0    -16    0    = 0
 0    0    0    1    0    -16    0    -8    = 0
 0    0    0    0    -1    0    32    31    >= 0
 0    0    0    0    1    0    -32    0    >= 0
 0    0    0    0    -1    32    0    31    >= 0
 0    0    0    0    1    -32    0    0    >= 0

---

Full diff: https://github.com/llvm/llvm-project/pull/68899.diff

2 Files Affected:

• (modified) mlir/lib/Dialect/Affine/IR/ValueBoundsOpInterfaceImpl.cpp (+15-5)
• (modified) mlir/test/Dialect/Affine/value-bounds-op-interface-impl.mlir (+19)

diff --git a/mlir/lib/Dialect/Affine/IR/ValueBoundsOpInterfaceImpl.cpp b/mlir/lib/Dialect/Affine/IR/ValueBoundsOpInterfaceImpl.cpp
index 97dd70e4f1d2b7e..f564a26dfae6f76 100644
--- a/mlir/lib/Dialect/Affine/IR/ValueBoundsOpInterfaceImpl.cpp
+++ b/mlir/lib/Dialect/Affine/IR/ValueBoundsOpInterfaceImpl.cpp
@@ -27,12 +27,22 @@ struct AffineApplyOpInterface
     assert(applyOp.getAffineMap().getNumResults() == 1 &&
            "expected single result");
 
+    // Fully compose this affine.apply with other ops because the folding logic
+    // can see opportunities for simplifying the affine map that
+    // `FlatLinearConstraints` can currently not see.
+    AffineMap map = applyOp.getAffineMap();
+    SmallVector<Value> operands = llvm::to_vector(applyOp.getOperands());
+    fullyComposeAffineMapAndOperands(&map, &operands);
+
     // Align affine map result with dims/symbols in the constraint set.
-    AffineExpr expr = applyOp.getAffineMap().getResult(0);
-    SmallVector<AffineExpr> dimReplacements = llvm::to_vector(llvm::map_range(
-        applyOp.getDimOperands(), [&](Value v) { return cstr.getExpr(v); }));
-    SmallVector<AffineExpr> symReplacements = llvm::to_vector(llvm::map_range(
-        applyOp.getSymbolOperands(), [&](Value v) { return cstr.getExpr(v); }));
+    AffineExpr expr = map.getResult(0);
+    SmallVector<AffineExpr> dimReplacements, symReplacements;
+    for (int64_t i = 0, e = map.getNumDims(); i < e; ++i)
+      dimReplacements.push_back(cstr.getExpr(operands[i]));
+    for (int64_t i = map.getNumDims(),
+                 e = map.getNumDims() + map.getNumSymbols();
+         i < e; ++i)
+      symReplacements.push_back(cstr.getExpr(operands[i]));
     AffineExpr bound =
         expr.replaceDimsAndSymbols(dimReplacements, symReplacements);
     cstr.bound(value) == bound;
diff --git a/mlir/test/Dialect/Affine/value-bounds-op-interface-impl.mlir b/mlir/test/Dialect/Affine/value-bounds-op-interface-impl.mlir
index 338c48c5b210bc1..03d38475d7f4e0c 100644
--- a/mlir/test/Dialect/Affine/value-bounds-op-interface-impl.mlir
+++ b/mlir/test/Dialect/Affine/value-bounds-op-interface-impl.mlir
@@ -58,3 +58,22 @@ func.func @affine_min_lb(%a: index) -> (index) {
   %2 = "test.reify_bound"(%1) {type = "LB"}: (index) -> (index)
   return %2 : index
 }
+
+// -----
+
+// CHECK-LABEL: func @composed_affine_apply(
+//       CHECK:   %[[cst:.*]] = arith.constant -8 : index
+//       CHECK:   return %[[cst]]
+func.func @composed_affine_apply(%i1 : index) -> (index) {
+  // The ValueBoundsOpInterface implementation of affine.apply fully composes
+  // the affine map (and its operands) with other affine.apply ops drawn from
+  // its operands before adding it to the constraint set. This is to work
+  // around a limitation in `FlatLinearConstraints`, which can currently not
+  // compute a constant bound for %s. (The affine map simplification logic can
+  // simplify %s to -8.)
+  %i2 = affine.apply affine_map<(d0) -> ((d0 floordiv 32) * 16)>(%i1)
+  %i3 = affine.apply affine_map<(d0) -> ((d0 floordiv 32) * 16 + 8)>(%i1)
+  %s = affine.apply affine_map<()[s0, s1] -> (s0 - s1)>()[%i2, %i3]
+  %reified = "test.reify_constant_bound"(%s) {type = "EQ"} : (index) -> (index)
+  return %reified : index
+}