[llvm] [LoopCacheAnalysis] Replace delinearization for fixed size array (PR #164798)

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj created https://github.com/llvm/llvm-project/pull/164798

This patch replaces the delinearization function used in LoopCacheAnalysis, switching from one that depends on type information in GEPs to one that does not. Once this patch and https://github.com/llvm/llvm-project/pull/161822 are landed, we can delete `tryDelinearizeFixedSize` from Delienarization, which is a optimization heuristic guided by GEP type information. After Polly eliminates its use of `getIndexExpressionsFromGEP`, we will be able to completely delete GEP-driven heuristics from Delinearization.

>From bd15ed3ab23376303fdb5e1bfefb0cc19bd9e20f Mon Sep 17 00:00:00 2001
From: Ryotaro Kasuga <kasuga.ryotaro at fujitsu.com>
Date: Thu, 16 Oct 2025 11:57:46 +0000
Subject: [PATCH] [LoopCacheAnalysis] Replace delinearization for fixed size
 array

---
 .../include/llvm/Analysis/LoopCacheAnalysis.h |  3 ++-
 llvm/lib/Analysis/LoopCacheAnalysis.cpp       | 26 ++++++++-----------
 .../interchange-refcost-overflow.ll           |  2 +-
 3 files changed, 14 insertions(+), 17 deletions(-)

diff --git a/llvm/include/llvm/Analysis/LoopCacheAnalysis.h b/llvm/include/llvm/Analysis/LoopCacheAnalysis.h
index 3e22487e5e349..70ccd8aaed20f 100644
--- a/llvm/include/llvm/Analysis/LoopCacheAnalysis.h
+++ b/llvm/include/llvm/Analysis/LoopCacheAnalysis.h
@@ -102,7 +102,8 @@ class IndexedReference {
 
   /// Attempt to delinearize \p AccessFn for fixed-size arrays.
   bool tryDelinearizeFixedSize(const SCEV *AccessFn,
-                               SmallVectorImpl<const SCEV *> &Subscripts);
+                               SmallVectorImpl<const SCEV *> &Subscripts,
+                               const SCEV *ElementSize);
 
   /// Return true if the index reference is invariant with respect to loop \p L.
   bool isLoopInvariant(const Loop &L) const;
diff --git a/llvm/lib/Analysis/LoopCacheAnalysis.cpp b/llvm/lib/Analysis/LoopCacheAnalysis.cpp
index 050c32707596a..c1ce736bb51b0 100644
--- a/llvm/lib/Analysis/LoopCacheAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopCacheAnalysis.cpp
@@ -355,22 +355,18 @@ CacheCostTy IndexedReference::computeRefCost(const Loop &L,
 }
 
 bool IndexedReference::tryDelinearizeFixedSize(
-    const SCEV *AccessFn, SmallVectorImpl<const SCEV *> &Subscripts) {
-  SmallVector<int, 4> ArraySizes;
-  if (!tryDelinearizeFixedSizeImpl(&SE, &StoreOrLoadInst, AccessFn, Subscripts,
-                                   ArraySizes))
+    const SCEV *AccessFn, SmallVectorImpl<const SCEV *> &Subscripts,
+    const SCEV *ElementSize) {
+  const SCEV *Offset = SE.removePointerBase(AccessFn);
+  if (!delinearizeFixedSizeArray(SE, Offset, Subscripts, Sizes, ElementSize)) {
+    Sizes.clear();
     return false;
+  }
 
-  // Populate Sizes with scev expressions to be used in calculations later.
-  for (auto Idx : seq<unsigned>(1, Subscripts.size()))
-    Sizes.push_back(
-        SE.getConstant(Subscripts[Idx]->getType(), ArraySizes[Idx - 1]));
-
-  LLVM_DEBUG({
-    dbgs() << "Delinearized subscripts of fixed-size array\n"
-           << "GEP:" << *getLoadStorePointerOperand(&StoreOrLoadInst)
-           << "\n";
-  });
+  // Drop the last element of Sizes which is the same as ElementSize.
+  assert(!Sizes.empty() && Sizes.back() == ElementSize &&
+         "Expecting the last one to be the element size");
+  Sizes.pop_back();
   return true;
 }
 
@@ -397,7 +393,7 @@ bool IndexedReference::delinearize(const LoopInfo &LI) {
 
     bool IsFixedSize = false;
     // Try to delinearize fixed-size arrays.
-    if (tryDelinearizeFixedSize(AccessFn, Subscripts)) {
+    if (tryDelinearizeFixedSize(AccessFn, Subscripts, ElemSize)) {
       IsFixedSize = true;
       // The last element of Sizes is the element size.
       Sizes.push_back(ElemSize);
diff --git a/llvm/test/Analysis/LoopCacheAnalysis/interchange-refcost-overflow.ll b/llvm/test/Analysis/LoopCacheAnalysis/interchange-refcost-overflow.ll
index 7b6529601da32..30554d08a6181 100644
--- a/llvm/test/Analysis/LoopCacheAnalysis/interchange-refcost-overflow.ll
+++ b/llvm/test/Analysis/LoopCacheAnalysis/interchange-refcost-overflow.ll
@@ -9,8 +9,8 @@
 ;       A[c][d][d] = 0;
 ; }
 
-; CHECK: Loop 'outer.loop' has cost = 9223372036854775807
 ; CHECK: Loop 'inner.loop' has cost = 9223372036854775807
+; CHECK: Loop 'outer.loop' has cost = 10000
 
 @A = local_unnamed_addr global [11 x [11 x [11 x i32]]] zeroinitializer, align 16