[flang-commits] [flang] [flang] Avoid unnecessary looping for constants (PR #156403)

[Miguel Saldivar via flang-commits]

https://github.com/Saldivarcher created https://github.com/llvm/llvm-project/pull/156403

Going through and doing `convertToAttribute` for all elements, if they are the same can be costly. If the elements are the same, we can just call `convertToAttribute` once.

This does give us a significant speed-up:
```console
$ hyperfine --warmup 1 --runs 5 ./slow.sh ./fast.sh
Benchmark 1: ./slow.sh
  Time (mean ± σ):      1.606 s ±  0.014 s    [User: 1.393 s, System: 0.087 s]
  Range (min … max):    1.591 s …  1.628 s    5 runs

Benchmark 2: ./fast.sh
  Time (mean ± σ):     452.9 ms ±   7.6 ms    [User: 249.9 ms, System: 83.3 ms]
  Range (min … max):   443.9 ms … 461.7 ms    5 runs

Summary
  ./fast.sh ran
    3.55 ± 0.07 times faster than ./slow.sh
```

Fixes #125444

>From f0638bce8bd8a4da6011cf7fb7ac6a6688bc09d8 Mon Sep 17 00:00:00 2001
From: Miguel Saldivar <miguel.saldivar at hpe.com>
Date: Mon, 1 Sep 2025 23:48:37 -0500
Subject: [PATCH] [flang] Avoid unnecessary looping for constants
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Going through and doing `convertToAttribute` for all elements, if they
are the same can be costly. If the elements are the same, we can just
call `convertToAttribute` once.

This does give us a significant speed-up:
```console
$ hyperfine --warmup 1 --runs 5 ./slow.sh ./fast.sh
Benchmark 1: ./slow.sh
  Time (mean ± σ):      1.606 s ±  0.014 s    [User: 1.393 s, System: 0.087 s]
  Range (min … max):    1.591 s …  1.628 s    5 runs

Benchmark 2: ./fast.sh
  Time (mean ± σ):     452.9 ms ±   7.6 ms    [User: 249.9 ms, System: 83.3 ms]
  Range (min … max):   443.9 ms … 461.7 ms    5 runs

Summary
  ./fast.sh ran
    3.55 ± 0.07 times faster than ./slow.sh
```

Fixes #125444
---
 flang/lib/Lower/ConvertConstant.cpp | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/flang/lib/Lower/ConvertConstant.cpp b/flang/lib/Lower/ConvertConstant.cpp
index 768a237c92396..068c81be7c45d 100644
--- a/flang/lib/Lower/ConvertConstant.cpp
+++ b/flang/lib/Lower/ConvertConstant.cpp
@@ -152,6 +152,23 @@ class DenseGlobalBuilder {
                       : TC;
     attributeElementType =
         Fortran::lower::getFIRType(builder.getContext(), attrTc, KIND, {});
+
+    auto values = constant.values();
+    auto sameElements = [&]() -> bool {
+      if (values.empty())
+        return false;
+
+      return std::all_of(values.begin(), values.end(),
+                         [&](const auto &v) { return v == values.front(); });
+    };
+
+    if (sameElements()) {
+      auto attr = convertToAttribute<TC, KIND>(builder, values.front(),
+                                               attributeElementType);
+      attributes.assign(values.size(), attr);
+      return;
+    }
+
     for (auto element : constant.values())
       attributes.push_back(
           convertToAttribute<TC, KIND>(builder, element, attributeElementType));