[GitHub] [tvm] wrongtest-intellif commented on a diff in pull request #13724: [Arith] Use ConstIntBound to remove negative numerator when lowering

[GitBox <gi...@apache.org>]

wrongtest-intellif commented on code in PR #13724:
URL: https://github.com/apache/tvm/pull/13724#discussion_r1065295308


##########
src/tir/transforms/lower_intrin.cc:
##########
@@ -112,20 +113,33 @@ class IntrinInjecter : public tvm::arith::IRMutatorWithAnalyzer {
       // Common path, positive divisor
       if (analyzer_->CanProveGreaterEqual(op->a, 0) || analyzer_->CanProveGreaterEqual(e, 0)) {
         return truncdiv(op->a, op->b);
+      }
+
+      // If the numerator's lower bound is known, express the floordiv
+      // in terms of truncdiv using only positive operands.
+      arith::ConstIntBound const_int_bound = analyzer_->const_int_bound(op->a);
+      if (const_int_bound->min_value != arith::ConstIntBound::kNegInf &&
+          const_int_bound->min_value < 0 &&
+          const_int_bound->min_value > -(1LL << (op->a->dtype.bits() - 1))) {

Review Comment:
   The meaning of bound `-(1LL << (op->a->dtype.bits() - 1)))` is sign free or not? Could we use `min_value` interface of op.h here?



##########
src/tir/transforms/lower_intrin.cc:
##########
@@ -112,20 +113,33 @@ class IntrinInjecter : public tvm::arith::IRMutatorWithAnalyzer {
       // Common path, positive divisor
       if (analyzer_->CanProveGreaterEqual(op->a, 0) || analyzer_->CanProveGreaterEqual(e, 0)) {
         return truncdiv(op->a, op->b);
+      }
+
+      // If the numerator's lower bound is known, express the floordiv
+      // in terms of truncdiv using only positive operands.
+      arith::ConstIntBound const_int_bound = analyzer_->const_int_bound(op->a);
+      if (const_int_bound->min_value != arith::ConstIntBound::kNegInf &&
+          const_int_bound->min_value < 0 &&
+          const_int_bound->min_value > -(1LL << (op->a->dtype.bits() - 1))) {
+        IntImm min(op->a->dtype, const_int_bound->min_value);
+        PrimExpr ceildiv = truncdiv((op->b - 1) - min, op->b);
+        PrimExpr offset_numerator = analyzer_->Simplify(op->a + op->b * ceildiv);
+        return truncdiv(offset_numerator, op->b) - ceildiv;

Review Comment:
   IIUC, the rationale is:
   ```
   a // b =>
   (a + -min - -min) // b => 
   (a + (x*b-y) - (x*b-y)) // b =>  ( where x is ceildiv(-min, b), x >= 0, y >= 0 )
   (a + x*b) // b - x =>
   (a + x*b) / b - x ( since a + x*b >= a + -min >= 0 )
   ```
   Right?



##########
src/tir/transforms/lower_intrin.cc:
##########
@@ -165,21 +179,34 @@ class IntrinInjecter : public tvm::arith::IRMutatorWithAnalyzer {
       // Common pass, positive divisor
       if (analyzer_->CanProveGreaterEqual(op->a, 0)) {
         return truncmod(op->a, op->b);
+      }
+
+      // If the numerator's lower bound is known, express the floormod
+      // in terms of truncmod using only positive operands.
+      arith::ConstIntBound const_int_bound = analyzer_->const_int_bound(op->a);
+      if (const_int_bound->min_value != arith::ConstIntBound::kNegInf &&
+          const_int_bound->min_value < 0 &&
+          const_int_bound->min_value > -(1LL << (op->a->dtype.bits() - 1))) {
+        IntImm min(op->a->dtype, const_int_bound->min_value);
+        PrimExpr ceildiv = truncdiv(-min + (op->b - 1), op->b);
+        PrimExpr offset_numerator = analyzer_->Simplify(op->a + op->b * ceildiv);
+        return truncmod(offset_numerator, op->b);

Review Comment:
   ```
   floormod(a, b) =>
   floormod(a + -min - -min, b) => 
   floormod(a + (x*b-y) - (x*b-y)), b) =>  ( where x is ceildiv(-min, b), x >= 0, y >= 0 )
   floormod(a + x*b, b) =>
   (a + x*b) % b  ( since a + x*b >= a + -min >= 0 )
   ```
   Right?



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