Virtual function instantiation for suppressed class template specializations [GH #431] by nv-cmeerw · Pull Request #433 · edgcpp/compiler

nv-cmeerw · 2026-10-29T15:25:18Z

Fixes #431

Previously, defining an inline member function of a class template
specialization whose instantiation was suppressed could unnecessarily
instantiate the class's virtual functions. This no longer occurs unless the
specialization is named in a GNU "inline template" directive. For example,
with --c++11 -tused:

  template<int N> struct C {
    virtual void f() {
      static_assert(N != 1, "");  // Previously a spurious error, now okay.
    }
  };
  extern template class C<1>;
  C<1> c;

Summary by CodeRabbit

Bug Fixes
- On IA-64 ABI configurations using compatibility version 410 or later, defining an inline member of an externally linked class-template specialization with instantiation suppressed no longer unnecessarily instantiates its virtual functions. Virtual functions are still instantiated when the specialization is named in a GNU “inline template” directive.
Tests
- Added regression coverage for template members, explicit instantiation, virtual-function generation, and inline-template directives across supported compiler modes, including compile-time behavior and linking.

coderabbitai · 2026-10-29T15:27:14Z

No actionable comments were generated in the recent review.

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Reviewing files that changed from the base of the PR and between f779d6c and 96d0b4c.

Files selected for processing: 21

src/func_def.c
src/pending_changes/gh-431.txt
tests/tests/regressions/.gh-431-diag.rto/default.1.1.txt
tests/tests/regressions/.gh-431-diag.rto/default.2.1.txt
tests/tests/regressions/.gh-431-diag.rto/default.3.1.txt
tests/tests/regressions/.gh-431-diag.rto/default.4.1.txt
tests/tests/regressions/.gh-431-ia64.rto/default.1.1.txt
tests/tests/regressions/.gh-431-ia64.rto/default.2.1.txt
tests/tests/regressions/.gh-431-ia64.rto/default.3.1.txt
tests/tests/regressions/.gh-431-ia64.rto/default.4.1.txt
tests/tests/regressions/.gh-431.rto/default.1.1.txt
tests/tests/regressions/.gh-431.rto/default.2.1.txt
tests/tests/regressions/.gh-431.rto/default.3.1.txt
tests/tests/regressions/.gh-431.rto/default.4.1.txt
tests/tests/regressions/gh-431-diag.sft.cpp
tests/tests/regressions/gh-431-ia64.sft.cpp
tests/tests/regressions/gh-431-link/.t1.rto/default.1.1.txt
tests/tests/regressions/gh-431-link/.t1.rto/default.1.2.txt
tests/tests/regressions/gh-431-link/t1.mft.cpp
tests/tests/regressions/gh-431-link/t2.cpp
tests/tests/regressions/gh-431.sft.cpp

Files skipped from review as they are similar to previous changes: 1

src/pending_changes/extern template class does not suppress inline virtual member instantiation #431.txt

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Walkthrough

For IA-64 ABI compatibility version 410 or later, the compiler skips virtual-function requirement checks for qualifying class-template specializations. Regression tests cover instantiation scenarios, compiler configurations, and cross-translation-unit linking.

Changes: Extern-template virtual-function suppression

Layer / File(s)	Summary
Virtual-function definition check `src/func_def.c`, `src/pending_changes/gh-431.txt`	For qualifying unspecialized template classes, the check skips virtual-function requirements under the specified instantiation and linkage conditions. The release note describes the GNU inline-template exception and includes an example.
Instantiation diagnostics and expected output `tests/tests/regressions/gh-431-diag.sft.cpp`, `tests/tests/regressions/.gh-431-diag.rto/`, `tests/tests/regressions/gh-431.sft.cpp`, `tests/tests/regressions/.gh-431.rto/`	Tests cover direct calls, explicit instantiation, derived-class vtables, internal-linkage types, nested classes, GNU inline-template cases, and constant evaluation. Commands and expected diagnostics cover four compiler configurations.
IA-64 and cross-translation-unit tests `tests/tests/regressions/gh-431-ia64.sft.cpp`, `tests/tests/regressions/.gh-431-ia64.rto/`, `tests/tests/regressions/gh-431-link/`	IA-64 tests cover class-template instantiation scenarios across four compiler configurations. A two-file test explicitly instantiates a class template and checks a virtual call across translation units.

Priority: Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Possibly related PRs

edgcpp/compiler#432: Changes inline-member emission when an extern template declaration suppresses a class specialization.

Suggested labels: bug

Suggested reviewers: wchilders-nvidia, darkarc

Merge Risk: Minimal · up to 96d0b

The change suppresses unnecessary virtual-function instantiation for eligible IA-64 templates. No merge-blocking issue remains established.

Security architecture risk: Minimal · up to 0866b

The change limits unnecessary template instantiation to cases where the corresponding vtable is already suppressed. Required instantiation paths remain intact, and the review found no introduced security concern.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

inferred — The demonstrated exposure is compiler processing of supplied C++ source under the qualifying ABI configuration. The change alters whether suppressed virtual bodies are unnecessarily instantiated; the inspected path does not expand caller privileges or introduce a new external interface.

Trust Boundaries and Controls

inferred — The skipped check is an instantiation-demand decision, not an authorization control. Its scope matches existing vtable suppression, while explicit-instantiation and GNU inline-template paths independently preserve required definitions. No newly reachable security-control bypass was established.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 25.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documenta…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly identifies the main change: controlling virtual function instantiation for suppressed class template specializations. It also includes the relevant issue number.
Linked Issues check		Issue `#431` requires `extern template class C<1>;` to suppress unnecessary virtual-function instantiation for `C<1> c;` under `--c++11 -tused`. In `src/func_def.c`, the new guard skips the virtual-func…
Out of Scope Changes check		The source change directly addresses issue `#431`. The release note documents the behavior. The regression and link tests verify suppression, required instantiation, linkage behavior, GNU `inline templa…

Full details: Linked Issues check

Explanation

Issue #431 requires extern template class C<1>; to suppress unnecessary virtual-function instantiation for C<1> c; under --c++11 -tused. In src/func_def.c, the new guard skips the virtual-function requirement for a non-specialized template class marked do_not_instantiate, when it is not explicitly instantiated, has C++ external linkage, and is not named in a GNU inline template directive. The regression fixtures cover the C++11 case, inline and out-of-class virtual members, explicit instantiation, linkage cases, and the preserved GNU inline template behavior.

Full details: Out of Scope Changes check

Explanation

The source change directly addresses issue #431. The release note documents the behavior. The regression and link tests verify suppression, required instantiation, linkage behavior, GNU inline template, and cross-translation-unit vtable use. These changes support the linked issue and show no demonstrated unrelated change.

Full details: Docstring Coverage

Explanation

Docstring coverage is 25.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 4 functions across 6 files. (15 skipped: 15 unsupported.)

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

Finishing Touches: 💡 1

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coderabbitai

Actionable comments posted: 1

🪄 Fix comments: Fix CodeRabbit comments on this PR

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only still-valid issues, skip the rest with a brief reason, keep changes
minimal, and validate.

Inline comments:
Review comments at @src/func_def.c:
- Around line 374-375: Update the do_not_instantiate guard in
require_definitions_of_virtual_functions_in_class so it stays consistent with
virtual_function_table_should_be_defined_here and lower_il.c: either suppress
non-IA-64 vtable selection for the specialization or retain virtual-member
definition requirements whenever lowering emits its vtable. Do not skip the
requirement while the table is still emitted.

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Commits

Reviewing files that changed from the base of the PR and between 6c38e1f and 07d255a.

Files selected for processing: 12

src/func_def.c
src/pending_changes/gh-431.txt
tests/tests/regressions/.gh-431-diag.rto/default.1.1.txt
tests/tests/regressions/.gh-431-diag.rto/default.2.1.txt
tests/tests/regressions/.gh-431-diag.rto/default.3.1.txt
tests/tests/regressions/.gh-431-diag.rto/default.4.1.txt
tests/tests/regressions/.gh-431.rto/default.1.1.txt
tests/tests/regressions/.gh-431.rto/default.2.1.txt
tests/tests/regressions/.gh-431.rto/default.3.1.txt
tests/tests/regressions/.gh-431.rto/default.4.1.txt
tests/tests/regressions/gh-431-diag.sft.cpp
tests/tests/regressions/gh-431.sft.cpp

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daveedvdv-nvidia

THe CodeRabbit note seems legit:

// c.h
template<int N>
struct C {
  virtual int f() { return N + 41; }
};
extern template struct C<1>;
// use.cpp
#include "c.h"
C<1> c;
int main() { C<1> *p = &c; return p->f() == 42 ? 0 : 1; }
// inst.cpp
#include "c.h"
template struct C<1>;

now gets a linker error about 'f__17C__tm__9_XCiL_1_1Fv_i' being undefined with the Cfront ABI.

edgcpp-bot · 2026-10-29T17:25:40Z

AcknowlEDG review results updated for 07d255a32ff0:

Test - Changes (new)

edgcpp-bot · 2026-10-29T20:36:38Z

AcknowlEDG review results updated for 0866b8e5cbd8:

Test - Changes (updated)

edgcpp-bot · 2026-10-29T22:33:52Z

AcknowlEDG review results updated for 0866b8e5cbd8:

Bench - C (new)
Bench - C++ (new)
Test - Changes (unchanged)

CI success.

codecov · 2026-10-29T22:34:12Z

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/func_def.c	60.00%	0 Missing and 2 partials ⚠️

📢 Thoughts on this report? Let us know!

…zations [GH #431]

edgcpp-bot · 2026-10-30T13:12:04Z

AcknowlEDG review results updated for 96d0b4c2cea5:

Bench - C (unchanged)
Bench - C++ (unchanged)
Test - Changes (updated)

edgcpp-bot · 2026-10-30T13:30:45Z

AcknowlEDG review results updated for 96d0b4c2cea5:

Bench - C (updated)
Bench - C++ (updated)
Test - Changes (unchanged)

CI success.

daveedvdv-nvidia

Looks good to me.

nv-cmeerw requested a review from a team October 29, 2026 15:25

nv-cmeerw self-assigned this Oct 29, 2026

coderabbitai Bot reviewed Oct 29, 2026

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Comment thread src/func_def.c Resolved

coderabbitai Bot mentioned this pull request Oct 29, 2026

Fix members of an extern template class from being in every TU [GH #252] #432

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daveedvdv-nvidia reviewed Oct 29, 2026

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nv-cmeerw force-pushed the fix/431 branch from 0866b8e to f779d6c Compare October 30, 2026 09:42

Virtual function instantiation for suppressed class template speciali…

96d0b4c

…zations [GH #431]

nv-cmeerw force-pushed the fix/431 branch from f779d6c to 96d0b4c Compare October 30, 2026 11:11

nv-cmeerw requested a review from daveedvdv-nvidia October 30, 2026 13:42

nv-cmeerw enabled auto-merge October 30, 2026 13:42

daveedvdv-nvidia approved these changes Oct 30, 2026

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nv-cmeerw disabled auto-merge October 30, 2026 20:03

nv-cmeerw added this pull request to the merge queue Oct 30, 2026

Merged via the queue into main with commit fd3fc54 Oct 30, 2026
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Virtual function instantiation for suppressed class template specializations [GH #431] - #433

Virtual function instantiation for suppressed class template specializations [GH #431]#433
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