[llvm] [LoopInterchange] Ignore the cost-model, force interchange if legal (PR #148858)

[Sjoerd Meijer via llvm-commits]

https://github.com/sjoerdmeijer created https://github.com/llvm/llvm-project/pull/148858

This is and has been proven useful for testing purposes, to get more test coverage.

>From db9c37e08f5fbb4458ea87f539eae41c275a8a2e Mon Sep 17 00:00:00 2001
From: Sjoerd Meijer <smeijer at nvidia.com>
Date: Tue, 15 Jul 2025 04:04:01 -0700
Subject: [PATCH] [LoopInterchange] Ignore the cost-model, force interchange if
 legal

This is useful for testing purposes, to get more test coverage.
---
 llvm/lib/Transforms/Scalar/LoopInterchange.cpp             | 7 ++++++-
 .../LoopInterchange/profitability-vectorization.ll         | 4 ++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
index a5008907b9014..b62c25d52861b 100644
--- a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
@@ -66,6 +66,10 @@ static cl::opt<unsigned int> MaxMemInstrCount(
         "in the dependency matrix. Higher value may lead to more interchanges "
         "at the cost of compile-time"));
 
+static cl::opt<bool> ForceLoopInterchange(
+    "loop-interchange-force", cl::init(false), cl::Hidden,
+    cl::desc("Ignore the cost model, and force interchange if it is legal"));
+
 namespace {
 
 using LoopVector = SmallVector<Loop *, 8>;
@@ -599,7 +603,8 @@ struct LoopInterchange {
     }
     LLVM_DEBUG(dbgs() << "Loops are legal to interchange\n");
     LoopInterchangeProfitability LIP(OuterLoop, InnerLoop, SE, ORE);
-    if (!LIP.isProfitable(InnerLoop, OuterLoop, InnerLoopId, OuterLoopId,
+    if (!ForceLoopInterchange &&
+        !LIP.isProfitable(InnerLoop, OuterLoop, InnerLoopId, OuterLoopId,
                           DependencyMatrix, CCM)) {
       LLVM_DEBUG(dbgs() << "Interchanging loops not profitable.\n");
       return false;
diff --git a/llvm/test/Transforms/LoopInterchange/profitability-vectorization.ll b/llvm/test/Transforms/LoopInterchange/profitability-vectorization.ll
index 16952a66aa78e..59196bbbb9c00 100644
--- a/llvm/test/Transforms/LoopInterchange/profitability-vectorization.ll
+++ b/llvm/test/Transforms/LoopInterchange/profitability-vectorization.ll
@@ -2,6 +2,10 @@
 ; RUN:     -pass-remarks-output=%t -disable-output
 ; RUN: FileCheck -input-file %t --check-prefix=PROFIT-CACHE %s
 
+; RUN: opt < %s -passes=loop-interchange -cache-line-size=64 \
+; RUN:     -pass-remarks-output=%t -disable-output -loop-interchange-force=true
+; RUN: FileCheck -input-file %t --check-prefix=PROFIT-VEC %s
+
 ; RUN: opt < %s -passes=loop-interchange -cache-line-size=64 \
 ; RUN:     -pass-remarks-output=%t -disable-output -loop-interchange-profitabilities=vectorize,cache,instorder
 ; RUN: FileCheck -input-file %t --check-prefix=PROFIT-VEC %s