[Mlir-commits] [mlir] [mlir][Analysis][Presburger] Remove redundant local vars after composition (PR #211023)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-mlir

Author: asraa

<details>
<summary>Changes</summary>

Call `removeRedundantLocalVars()` at the end of `IntegerRelation::compose` to simplify existential constraints generated when projecting out intermediate dimensions.

Removing redundant local vars also happens in `IntegerRelation::mergeAndCompose` and `FlatAffineRelation::compose`.

When composing A->B with B->C, intermediate vars B are converted to local existential variables. When performing multiple compositions, this causes bloat of existential variables that impact later performance, bounds checks, and emptiness checks.


---
Full diff: https://github.com/llvm/llvm-project/pull/211023.diff


2 Files Affected:

- (modified) mlir/lib/Analysis/Presburger/IntegerRelation.cpp (+3) 
- (modified) mlir/unittests/Analysis/Presburger/IntegerRelationTest.cpp (+19) 


``````````diff
diff --git a/mlir/lib/Analysis/Presburger/IntegerRelation.cpp b/mlir/lib/Analysis/Presburger/IntegerRelation.cpp
index 93a725571078e..719aa5962c8fa 100644
--- a/mlir/lib/Analysis/Presburger/IntegerRelation.cpp
+++ b/mlir/lib/Analysis/Presburger/IntegerRelation.cpp
@@ -2573,6 +2573,9 @@ void IntegerRelation::compose(const IntegerRelation &rel) {
 
   // Project out B in R1.
   convertVarKind(VarKind::Range, 0, numBVars, VarKind::Local);
+
+  // Eliminate local variables  
+  removeRedundantLocalVars();
 }
 
 void IntegerRelation::applyDomain(const IntegerRelation &rel) {
diff --git a/mlir/unittests/Analysis/Presburger/IntegerRelationTest.cpp b/mlir/unittests/Analysis/Presburger/IntegerRelationTest.cpp
index 90753d502d12c..c3da1c1556d26 100644
--- a/mlir/unittests/Analysis/Presburger/IntegerRelationTest.cpp
+++ b/mlir/unittests/Analysis/Presburger/IntegerRelationTest.cpp
@@ -119,6 +119,25 @@ TEST(IntegerRelationTest, applyDomainAndRange) {
   }
 }
 
+TEST(IntegerRelationTest, composeRemoveRedundantLocalVars) {
+  // map x to  y = x + 10.
+  IntegerRelation map1 =
+      parseRelationFromSet("(x, y) : (y - x - 10 == 0)", /*numDomain=*/1);
+
+  // map y to z = y + 20.
+  IntegerRelation map2 =
+      parseRelationFromSet("(y, z) : (z - y - 20 == 0)", /*numDomain=*/1);
+
+  // composing projects out y (converting it to a local variable)
+  map1.compose(map2);
+
+  // y is fully determined by x so should be eliminated after composition
+  EXPECT_EQ(map1.getNumLocalVars(), 0u);
+  IntegerRelation expectedMap =
+      parseRelationFromSet("(x, z) : (z - x - 30 == 0)", /*numDomain=*/1);
+  EXPECT_TRUE(map1.isEqual(expectedMap));
+}
+
 TEST(IntegerRelationTest, symbolicLexmin) {
   SymbolicLexOpt lexmin =
       parseRelationFromSet("(a, x)[b] : (x - a >= 0, x - b >= 0)", 1)

``````````

</details>


https://github.com/llvm/llvm-project/pull/211023