[Flang-RT] Change alignment for allocatable and pointer arrays to 64 … by Jason-Van-Beusekom · Pull Request #206525 · llvm/llvm-project

Jason-Van-Beusekom · 2026-06-29T16:53:40Z

…bytes

in #194969 the default array alignment was changed to 64bytes for Fortran globals. In this PR the malloc wrapper in the flang runtime is modified to accept an alignment argument (0 default) and for cases above 16 byte (std::max_align_t) uses aligned_alloc instead of malloc.

The issue of alignment was discussed in this RFC: https://discourse.llvm.org/t/rfc-alignment-of-global-arrays/90397/13

Assisted-by: Opus 4.6

…bytes

github-actions · 2026-06-29T16:55:32Z

✅ With the latest revision this PR passed the C/C++ code formatter.

clementval · 2026-06-29T17:25:09Z

 static constexpr unsigned kDeviceAllocatorPos = 2;
 static constexpr unsigned kManagedAllocatorPos = 3;
 static constexpr unsigned kUnifiedAllocatorPos = 4;
+static constexpr unsigned kDefaultArrayAlignment = 64;


Maybe add a blank line of a comment to separate this from the CUF allocator positions.

Done in 24826e2

eugeneepshteyn

LGTM.

Note: for this flang-rt change, please ensure that both static and shared library builds of flang work.

Should the new alignment for global, allocatable, and pointer arrays be documented somewhere? Should definitely be mentioned in release notes, but maybe also in another more permanent doc?

eugeneepshteyn · 2026-06-29T20:02:10Z

@Jason-Van-Beusekom , when building with gcc 9.3.0:

.../llvm-project/flang-rt/include/flang-rt/runtime/allocator-registry.h:35:27: error: no member named 'max_align_t' in namespace 'std'; did you mean simply 'max_align_t'?
   35 |   if (alignment > alignof(std::max_align_t)) {
      |                           ^    ~~~~~~~~~~~
.../build/release/llvm/lib/clang/23/include/__stddef_max_align_t.h:24:3: note: 'max_align_t' declared here
   24 | } max_align_t;
      |   ^

I think you need to explicitly include <cstddef> for max_align_t.

eugeneepshteyn · 2026-06-29T23:34:52Z

Assisted with Opus 4.6

Nit: please use Assisted-by: ... tag, per process (and also some people search for this particular string).

Jason-Van-Beusekom · 2026-06-30T19:37:28Z

Assisted with Opus 4.6

Nit: please use Assisted-by: ... tag, per process (and also some people search for this particular string).

Updated Pr message and will make sure to have the commit with correct tag.

Jason-Van-Beusekom · 2026-06-30T19:38:13Z

@Jason-Van-Beusekom , when building with gcc 9.3.0:

.../llvm-project/flang-rt/include/flang-rt/runtime/allocator-registry.h:35:27: error: no member named 'max_align_t' in namespace 'std'; did you mean simply 'max_align_t'?
   35 |   if (alignment > alignof(std::max_align_t)) {
      |                           ^    ~~~~~~~~~~~
.../build/release/llvm/lib/clang/23/include/__stddef_max_align_t.h:24:3: note: 'max_align_t' declared here
   24 | } max_align_t;
      |   ^

I think you need to explicitly include <cstddef> for max_align_t.

Updated in 3c7d7fb

Jason-Van-Beusekom · 2026-06-30T20:53:20Z

Will hold off on merging till tomorrow for any last minute feedback, also this does not cover all cases as simple allocatable arrays are not handled in the runtime and instead in llvm-project/flang/lib/Lower/Allocatable.cpp via inlineAllocation. Will have that change be in a seperate PR

ceseo · 2026-07-02T19:28:20Z

This broke recent versions of macOS. I created a PR to fix it: #207248

vzakhari · 2026-07-10T17:31:48Z


 ## Non-comprehensive list of changes in this release

+- Global, allocatable, and pointer arrays are now aligned to 64 bytes. The


Is there a special wording to indicate that binaries compiled with this release are not compatible with binaries built by the previous one? We should probably stress that in the release notes.

I cannot find notes on it but in the discussion for the RFC for this change: https://discourse.llvm.org/t/rfc-alignment-of-global-arrays/90397/14 if I remember correctly this was mentioned in the meeting, and if I recall correctly flang makes no guarantees that new releases work with binaries compiled with older versions.

I can add a mention of it in the release notes if desired however.

Since Flang is part of LLVM, I believe there is some comptatibility guarantees within the same major version. @tstellar @tru do you have an advice about documenting compatibility breaking change within one major release?

…ys (#208781) In #206525 the runtime was set to default alloctable and pointer arrays to 64byte alignment. In this commit the inlineallocation case is handled. The same approach was taken and an alignment attribute was added, with logic to promote to aligned_alloc or posixMemalign for apple targets, when requested alignment is greater than 16. Of note alignof(std::max_align_t)) was not used as the runtime target may be different than the compile target, so 16 was hardcoded as when to use aligned_alloc Assisted-by: Opus 4.8

…on for arrays (#208781) In llvm/llvm-project#206525 the runtime was set to default alloctable and pointer arrays to 64byte alignment. In this commit the inlineallocation case is handled. The same approach was taken and an alignment attribute was added, with logic to promote to aligned_alloc or posixMemalign for apple targets, when requested alignment is greater than 16. Of note alignof(std::max_align_t)) was not used as the runtime target may be different than the compile target, so 16 was hardcoded as when to use aligned_alloc Assisted-by: Opus 4.8

[Flang-RT] Change alignment for allocatable and pointer arrays to 64 …

e174b89

…bytes

Jason-Van-Beusekom requested review from clementval, eugeneepshteyn, klausler and tarunprabhu June 29, 2026 16:53

llvmorg-github-actions Bot added flang Flang issues not falling into any other category flang-rt labels Jun 29, 2026

eugeneepshteyn removed the request for review from klausler June 29, 2026 16:55

clementval reviewed Jun 29, 2026

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formatting

24826e2

eugeneepshteyn approved these changes Jun 29, 2026

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eugeneepshteyn reviewed Jun 29, 2026

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Comment thread flang-rt/include/flang-rt/runtime/allocator-registry.h

Comment thread flang-rt/include/flang-rt/runtime/allocator-registry.h Outdated

feedback update

3c7d7fb

Jason-Van-Beusekom merged commit a675a66 into llvm:main Jul 1, 2026
12 checks passed

Jason-Van-Beusekom mentioned this pull request Jul 10, 2026

[Flang][Fir] Set default alignment to 64 on inlineAllocation for arrays #208781

Merged

vzakhari reviewed Jul 10, 2026

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		## Non-comprehensive list of changes in this release

		- Global, allocatable, and pointer arrays are now aligned to 64 bytes. The

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