[mlir] Fold ceil/floordiv with negative RHS.

[jreiffers]

Currently, we only fold if the RHS is a positive constant. There doesn't seem to be a good reason to do that. The comment claims that division by negative values is undefined, but I suspect that was just copied over from the mod simplifier.

[llvmbot]

@llvm/pr-subscribers-mlir

@llvm/pr-subscribers-mlir-core

Author: Johannes Reifferscheid (jreiffers)

Changes

Currently, we only fold if the RHS is a positive constant. There doesn't seem to be a good reason to do that. The comment claims that division by negative values is undefined, but I suspect that was just copied over from the mod simplifier.

---

Full diff: https://github.com/llvm/llvm-project/pull/97031.diff

2 Files Affected:

• (modified) mlir/lib/IR/AffineExpr.cpp (+9-10)
• (modified) mlir/unittests/IR/AffineExprTest.cpp (+22)

diff --git a/mlir/lib/IR/AffineExpr.cpp b/mlir/lib/IR/AffineExpr.cpp
index cf8157cf7bb8c..798398464da8d 100644
--- a/mlir/lib/IR/AffineExpr.cpp
+++ b/mlir/lib/IR/AffineExpr.cpp
@@ -855,8 +855,7 @@ static AffineExpr simplifyFloorDiv(AffineExpr lhs, AffineExpr rhs) {
   auto lhsConst = dyn_cast<AffineConstantExpr>(lhs);
   auto rhsConst = dyn_cast<AffineConstantExpr>(rhs);
 
-  // mlir floordiv by zero or negative numbers is undefined and preserved as is.
-  if (!rhsConst || rhsConst.getValue() < 1)
+  if (!rhsConst || rhsConst.getValue() == 0)
     return nullptr;
 
   if (lhsConst) {
@@ -875,12 +874,12 @@ static AffineExpr simplifyFloorDiv(AffineExpr lhs, AffineExpr rhs) {
   if (rhsConst == 1)
     return lhs;
 
-  // Simplify (expr * const) floordiv divConst when expr is known to be a
-  // multiple of divConst.
+  // Simplify `(expr * lrhs) floordiv rhsConst` when `lrhs` is known to be a
+  // multiple of `rhsConst`.
   auto lBin = dyn_cast<AffineBinaryOpExpr>(lhs);
   if (lBin && lBin.getKind() == AffineExprKind::Mul) {
     if (auto lrhs = dyn_cast<AffineConstantExpr>(lBin.getRHS())) {
-      // rhsConst is known to be a positive constant.
+      // `rhsConst` is known to be a nonzero constant.
       if (lrhs.getValue() % rhsConst.getValue() == 0)
         return lBin.getLHS() * (lrhs.getValue() / rhsConst.getValue());
     }
@@ -891,7 +890,7 @@ static AffineExpr simplifyFloorDiv(AffineExpr lhs, AffineExpr rhs) {
   if (lBin && lBin.getKind() == AffineExprKind::Add) {
     int64_t llhsDiv = lBin.getLHS().getLargestKnownDivisor();
     int64_t lrhsDiv = lBin.getRHS().getLargestKnownDivisor();
-    // rhsConst is known to be a positive constant.
+    // rhsConst is known to be a nonzero constant.
     if (llhsDiv % rhsConst.getValue() == 0 ||
         lrhsDiv % rhsConst.getValue() == 0)
       return lBin.getLHS().floorDiv(rhsConst.getValue()) +
@@ -918,7 +917,7 @@ static AffineExpr simplifyCeilDiv(AffineExpr lhs, AffineExpr rhs) {
   auto lhsConst = dyn_cast<AffineConstantExpr>(lhs);
   auto rhsConst = dyn_cast<AffineConstantExpr>(rhs);
 
-  if (!rhsConst || rhsConst.getValue() < 1)
+  if (!rhsConst || rhsConst.getValue() == 0)
     return nullptr;
 
   if (lhsConst) {
@@ -937,12 +936,12 @@ static AffineExpr simplifyCeilDiv(AffineExpr lhs, AffineExpr rhs) {
   if (rhsConst.getValue() == 1)
     return lhs;
 
-  // Simplify (expr * const) ceildiv divConst when const is known to be a
-  // multiple of divConst.
+  // Simplify `(expr * lrhs) ceildiv rhsConst` when `lrhs` is known to be a
+  // multiple of `rhsConst`.
   auto lBin = dyn_cast<AffineBinaryOpExpr>(lhs);
   if (lBin && lBin.getKind() == AffineExprKind::Mul) {
     if (auto lrhs = dyn_cast<AffineConstantExpr>(lBin.getRHS())) {
-      // rhsConst is known to be a positive constant.
+      // `rhsConst` is known to be a nonzero constant.
       if (lrhs.getValue() % rhsConst.getValue() == 0)
         return lBin.getLHS() * (lrhs.getValue() / rhsConst.getValue());
     }
diff --git a/mlir/unittests/IR/AffineExprTest.cpp b/mlir/unittests/IR/AffineExprTest.cpp
index 9740165c6b324..a0affc4341b0b 100644
--- a/mlir/unittests/IR/AffineExprTest.cpp
+++ b/mlir/unittests/IR/AffineExprTest.cpp
@@ -76,3 +76,25 @@ TEST(AffineExprTest, constantFolding) {
       getAffineBinaryOpExpr(AffineExprKind::FloorDiv, cmin, cn1);
   ASSERT_EQ(cminfloordivcn1.getKind(), AffineExprKind::FloorDiv);
 }
+
+TEST(AffineExprTest, divisionSimplification) {
+  MLIRContext ctx;
+  OpBuilder b(&ctx);
+  auto cn6 = b.getAffineConstantExpr(-6);
+  auto c6 = b.getAffineConstantExpr(6);
+  auto d0 = b.getAffineDimExpr(0);
+  auto d1 = b.getAffineDimExpr(1);
+
+  ASSERT_EQ(c6.floorDiv(-1), cn6);
+  ASSERT_EQ((d0 * 6).floorDiv(2), d0 * 3);
+  ASSERT_EQ((d0 * 6).floorDiv(4).getKind(), AffineExprKind::FloorDiv);
+  ASSERT_EQ((d0 * 6).floorDiv(-2), d0 * -3);
+  ASSERT_EQ((d0 * 6 + d1).floorDiv(2), d0 * 3 + d1.floorDiv(2));
+  ASSERT_EQ((d0 * 6 + d1).floorDiv(-2), d0 * -3 + d1.floorDiv(-2));
+  ASSERT_EQ((d0 * 6 + d1).floorDiv(4).getKind(), AffineExprKind::FloorDiv);
+
+  ASSERT_EQ(c6.ceilDiv(-1), cn6);
+  ASSERT_EQ((d0 * 6).ceilDiv(2), d0 * 3);
+  ASSERT_EQ((d0 * 6).ceilDiv(4).getKind(), AffineExprKind::CeilDiv);
+  ASSERT_EQ((d0 * 6).ceilDiv(-2), d0 * -3);
+}