[flang-commits] [flang] be1e50f - [flang] Avoid unnecessary looping for constants (#156403)

[via flang-commits]

Author: Miguel Saldivar
Date: 2025-09-03T10:24:21+01:00
New Revision: be1e50f56af8e270a0396eef8f62626fbbb84996

URL: https://github.com/llvm/llvm-project/commit/be1e50f56af8e270a0396eef8f62626fbbb84996
DIFF: https://github.com/llvm/llvm-project/commit/be1e50f56af8e270a0396eef8f62626fbbb84996.diff

LOG: [flang] Avoid unnecessary looping for constants (#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

Added: 
    

Modified: 
    flang/lib/Lower/ConvertConstant.cpp

Removed: 
    


################################################################################
diff  --git a/flang/lib/Lower/ConvertConstant.cpp b/flang/lib/Lower/ConvertConstant.cpp
index 768a237c92396..376ec12150c71 100644
--- a/flang/lib/Lower/ConvertConstant.cpp
+++ b/flang/lib/Lower/ConvertConstant.cpp
@@ -145,6 +145,9 @@ class DenseGlobalBuilder {
       fir::FirOpBuilder &builder,
       const Fortran::evaluate::Constant<Fortran::evaluate::Type<TC, KIND>>
           &constant) {
+    using Element =
+        Fortran::evaluate::Scalar<Fortran::evaluate::Type<TC, KIND>>;
+
     static_assert(TC != Fortran::common::TypeCategory::Character,
                   "must be numerical or logical");
     auto attrTc = TC == Fortran::common::TypeCategory::Logical
@@ -152,7 +155,24 @@ class DenseGlobalBuilder {
                       : TC;
     attributeElementType =
         Fortran::lower::getFIRType(builder.getContext(), attrTc, KIND, {});
-    for (auto element : constant.values())
+
+    const std::vector<Element> &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 : values)
       attributes.push_back(
           convertToAttribute<TC, KIND>(builder, element, attributeElementType));
   }