[flang] Avoid unnecessary looping for constants

[Saldivarcher]

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:

$ 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

[llvmbot]

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

Author: Miguel Saldivar (Saldivarcher)

Changes

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:

$ 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 --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));

[tblah]

Thanks for the fix

[tblah]

Thanks for the update

[Saldivarcher]

@tblah would you be able to merge this for me, please. Thanks again for the review, I appreciate it 👍