Document ODR violations in KOKKOS_IF_ON_HOST/DEVICE constexpr usage by Copilot · Pull Request #813 · kokkos/kokkos-core-wiki

Copilot · 2026-02-17T18:47:52Z

Extend constexpr Context Documentation for KOKKOS_IF_ON_HOST/DEVICE ✅

Summary of Changes:

Extended the "constexpr Context" section in the documentation of KOKKOS_IF_ON_HOST and KOKKOS_IF_ON_DEVICE macros with comprehensive explanation of ODR violations based on developer discussions.

Completed Work:

Documentation Enhancements:

Strengthened warning: Changed from "cannot" to "must not" with emphasis on ODR violations
Added "Why This Is Problematic" section: Explains the difference between constexpr and runtime contexts
Added analogy: Comparison to #ifdef __AVX2__ preprocessor usage across translation units
Added "Examples of ODR Violations" section: Two concrete problematic patterns:
- Using in constexpr function affecting struct size
- Lambda capture dependency issues
Added "Correct Alternatives" section: Three practical solutions:
- Remove constexpr for runtime functions
- Move KOKKOS_IF_ON_* to calling context
- Use template specialization on execution spaces (Preferred)

Build Verification:

Documentation builds successfully with Sphinx
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… examples Co-authored-by: crtrott <9490481+crtrott@users.noreply.github.com>

… memory space Co-authored-by: crtrott <9490481+crtrott@users.noreply.github.com>

Co-authored-by: crtrott <9490481+crtrott@users.noreply.github.com>

crtrott · 2026-02-17T19:06:52Z

+In this example, the ``Functor`` struct would have different sizes on the host
+and device, causing serious memory corruption issues when passed between them.
+
+**Problematic: Lambda captures with** ``constexpr`` **usage**


Suggested change

**Problematic: Lambda captures with** ``constexpr`` **usage**

**Problematic: Lambda capture dependency

Updated to "Lambda capture dependency" in commit c160bf9.

… specialization preference Co-authored-by: crtrott <9490481+crtrott@users.noreply.github.com>

JBludau · 2026-02-17T19:13:09Z

+``#ifdef __AVX2__`` in different translation units.
+
+This is analogous to the following problematic pattern:
+
+.. code-block:: cpp
+
+    // DO NOT DO THIS - ODR violation
+    static constexpr int foo() {
+      #ifdef __AVX2__
+        return 4;
+      #else
+        return 2;
+      #endif
+    }
+
+If you compile this code in two different translation units with different
+compiler flags (one with AVX2 and one without) and then link them together, you
+have an ODR violation because the same function has different definitions.


do we need this example? I think the next example makes it clear enough?

I kinda do like to connect this to something not involving host/device compilation.

but the explanation is about different compilation units with different flags...which if you know how some compilers do device code is helpful, but if you are oblivious to that, the question is: why shouldn't I do that?

I also find the avx example very confusing.

Per review let's not get into that compile flag mess here.

ldh4 · 2026-02-17T20:34:11Z

+and template instantiations. When ``KOKKOS_IF_ON_HOST`` and
+``KOKKOS_IF_ON_DEVICE`` are used in ``constexpr`` contexts, they cause the same
+function or variable to have different compile-time values on the host versus
+the device—similar to using architecture-specific preprocessor macros like


Suggested change

the device—similar to using architecture-specific preprocessor macros like

the device to using architecture-specific preprocessor macros like

I get what it's getting to, but I feel like just the device is sufficient.

I think a comma or a parenthesis should be added. It looks like the device-similar to ... was a kind of device (similar to ....

ldh4 · 2026-02-17T20:39:21Z

+      ))
+    }
+
+**Alternative 3 (Preferred): Use template specialization**


I think the Preferred method should be placed first as the Alternative 1.

dalg24 · 2026-03-09T21:13:18Z

-(constant expression).
+These macros **must not** be used in a context that requires a ``constexpr``
+(constant expression). Using ``KOKKOS_IF_ON_HOST`` or ``KOKKOS_IF_ON_DEVICE``
+within ``constexpr`` functions or to initialize ``constexpr`` variables leads to


Suggested change

within ``constexpr`` functions or to initialize ``constexpr`` variables leads to

within ``constexpr`` functions or to initialize ``constexpr`` variables can lead to

dalg24 · 2026-03-09T21:13:59Z

+These macros **must not** be used in a context that requires a ``constexpr``
+(constant expression). Using ``KOKKOS_IF_ON_HOST`` or ``KOKKOS_IF_ON_DEVICE``
+within ``constexpr`` functions or to initialize ``constexpr`` variables leads to
+**One Definition Rule (ODR) violations** and undefined behavior.


Suggested change

**One Definition Rule (ODR) violations** and undefined behavior.

**One Definition Rule (ODR) violations** and cause undefined behavior.

dalg24 · 2026-03-09T21:18:42Z

+``#ifdef __AVX2__`` in different translation units.
+
+This is analogous to the following problematic pattern:
+
+.. code-block:: cpp
+
+    // DO NOT DO THIS - ODR violation
+    static constexpr int foo() {
+      #ifdef __AVX2__
+        return 4;
+      #else
+        return 2;
+      #endif
+    }
+
+If you compile this code in two different translation units with different
+compiler flags (one with AVX2 and one without) and then link them together, you
+have an ODR violation because the same function has different definitions.


Per review let's not get into that compile flag mess here.

Signed-off-by: Christian Trott <crtrott@sandia.gov>

tcclevenger · 2026-04-06T12:47:27Z

+**Problematic: Lambda capture dependency**
+
+.. code-block:: cpp
+
+    // DO NOT DO THIS - causes ODR violation
+    void foo() {
+      int a = 0;
+      double b = 1.0;
+      auto lambda = KOKKOS_LAMBDA(int) {
+        KOKKOS_IF_ON_HOST((printf("%i\n", a);))     // Captures 'a'
+        KOKKOS_IF_ON_DEVICE((printf("%lf\n", b);))  // Captures 'b'
+      };
+      // Lambda has different size on host vs device due to different captures
+    }
+
+The lambda object has different sizes on host and device because of the
+different captures, violating the ODR.


So this problematic example never gets resolved, unlike the one above? If so, I think this example should be first, then the one that gets resolved should be second and it should be clear that the "Correct Alternatives" section is a subsection of that example.

Initial plan

2b534ce

Copilot AI assigned Copilot and crtrott Feb 17, 2026

Copilot started work on behalf of crtrott February 17, 2026 18:48 View session

Copilot AI and others added 3 commits February 17, 2026 18:51

Extend constexpr Context documentation with ODR violation details and…

1c2ec9e

… examples Co-authored-by: crtrott <9490481+crtrott@users.noreply.github.com>

Fix template specialization example to use execution space instead of…

d5dd700

… memory space Co-authored-by: crtrott <9490481+crtrott@users.noreply.github.com>

Remove platform-specific size assumptions from lambda capture example

9ddc87e

Co-authored-by: crtrott <9490481+crtrott@users.noreply.github.com>

Copilot AI changed the title ~~[WIP] Update documentation for constexpr Context in KOKKOS_IF_ON_HOST~~ Document ODR violations in KOKKOS_IF_ON_HOST/DEVICE constexpr usage Feb 17, 2026

Copilot AI requested a review from crtrott February 17, 2026 18:55

Copilot finished work on behalf of crtrott February 17, 2026 18:55

crtrott reviewed Feb 17, 2026

View reviewed changes

crtrott requested changes Feb 17, 2026

View reviewed changes

Comment thread docs/source/API/core/macros-special/if_on_host_or_device.rst Outdated

Comment thread docs/source/API/core/macros-special/if_on_host_or_device.rst Outdated

Copilot started work on behalf of crtrott February 17, 2026 19:08 View session

Address review feedback: update section titles and emphasize template…

c160bf9

… specialization preference Co-authored-by: crtrott <9490481+crtrott@users.noreply.github.com>

Copilot AI requested a review from crtrott February 17, 2026 19:12

Copilot finished work on behalf of crtrott February 17, 2026 19:12

JBludau reviewed Feb 17, 2026

View reviewed changes

ldh4 reviewed Feb 17, 2026

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crtrott marked this pull request as ready for review February 25, 2026 17:34

dalg24 reviewed Mar 9, 2026

View reviewed changes

crtrott added 2 commits March 9, 2026 16:52

Update based on reviews

e90a4fe

Signed-off-by: Christian Trott <crtrott@sandia.gov>

Add another one

3dd7d9a

Signed-off-by: Christian Trott <crtrott@sandia.gov>

tcclevenger reviewed Apr 6, 2026

View reviewed changes

	Problematic: Lambda captures with ``constexpr`` usage
	**Problematic: Lambda capture dependency

	the device—similar to using architecture-specific preprocessor macros like
	the device to using architecture-specific preprocessor macros like

	within ``constexpr`` functions or to initialize ``constexpr`` variables leads to
	within ``constexpr`` functions or to initialize ``constexpr`` variables can lead to

	One Definition Rule (ODR) violations and undefined behavior.
	One Definition Rule (ODR) violations and cause undefined behavior.

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Extend constexpr Context Documentation for KOKKOS_IF_ON_HOST/DEVICE ✅

Summary of Changes:

Completed Work:

Documentation Enhancements:

Build Verification:

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Copilot AI commented Feb 17, 2026 •

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