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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-flang-fir-hlfir

Author: Miguel Saldivar (Saldivarcher)

<details>
<summary>Changes</summary>

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

---
Full diff: https://github.com/llvm/llvm-project/pull/156403.diff


1 Files Affected:

- (modified) flang/lib/Lower/ConvertConstant.cpp (+17) 


``````````diff
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));

``````````

</details>


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