Add support for system BDWGC by ivmai · Pull Request #29 · uim/sigscheme

ivmai · 2026-03-13T16:56:35Z

BDWGC 8.3.0 will have the feature that is provided by libgcroots: bdwgc/bdwgc#378

If we find system BDWGC 8.3.0 or later, we use it instead of libgcroots. We'll deprecate libgcroots and drop support for libgcroots eventually.

To use bdwgc, pass `--with_libgcroots=bundled-bdwgc` to configure.

kou · 2026-03-16T21:22:10Z

Thanks!

But we don't want to bundle bdwgc. We want to use system bdwgc instead. If system bdwgc is enough recent, we'll use bdwgc. Otherwise, we'll use bundled libgcroots. We'll remove bundled libgcroots eventually.

Can I push to this branch something? I'll add a CI job that install unreleased bdwgc separately and use it as a system bdwgc.

ivmai · 2026-03-17T08:42:59Z

We want to use system bdwgc instead.

Agree.

Can I push to this branch something? I'll add a CI job that install unreleased bdwgc separately and use it as a system bdwgc.

Sure.

kou · 2026-03-26T08:53:24Z

Add CI jobs with system BDWGC master
Don't use src/gcroots/ with system BDWGC
src/storage-gc.c uses BDWGC directly

I'm not sure whether this works well but this is buildable now.

I'll merge this and test this in uim CI.

Copilot

Pull request overview

This PR adds a build-time option to prefer a system-installed BDWGC (when available) over libgcroots, with corresponding build-system and CI updates to exercise the different GC backend configurations.

Changes:

Add SCM_WITH_BDWGC code path in src/storage-gc.c to use BDWGC stack-base handling and stack/register scanning callbacks.
Update Autotools + pkg-config metadata to detect and link BDWGC (falling back to libgcroots) and to make GC deps private for pkg-config consumers.
Adjust GitHub Actions CI to build with different --with-libgcroots settings and build/install BDWGC in the “installed” matrix case.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
`src/storage-gc.c`	Adds BDWGC-based GC ready-stack handling and stack/register scanning integration.
`src/sigscheme.h`	Removes unconditional `gcroots.h` inclusion from the public header.
`src/Makefile.am`	Adds `@BDWGC_*@` substitution vars to compilation/link lines.
`sigscheme.pc.in`	Moves GC dependencies to `Requires.private`.
`configure.ac`	Adds BDWGC pkg-config detection (fallback to gcroots) and substitutes BDWGC vars.
`.github/workflows/linux.yml`	Adds a matrix over `--with-libgcroots` and builds BDWGC for the “installed” case.

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Copilot · 2026-03-26T09:00:55Z

+       AC_DEFINE(SCM_WITH_BDWGC, 1, [Define to 1 to use BDWGC.])],
+      [PKG_CHECK_MODULES(
+        GCROOTS, [gcroots >= 0.2.3],
+        [GCROOTS_REQ=gcroots],
+        [AC_MSG_ERROR([installed libgcroots is not found.])])])
    use_libgcroots="installed one"


When BDWGC is found, use_libgcroots is still set to "installed one", so the final configure summary prints libgcroots: installed one even though the build is actually using BDWGC instead of libgcroots. Consider updating the summary string (or adding a separate use_bdwgc/gc_backend summary) so ./configure output accurately reflects the selected GC backend.

Suggested change

AC_DEFINE(SCM_WITH_BDWGC, 1, [Define to 1 to use BDWGC.])],

[PKG_CHECK_MODULES(

GCROOTS, [gcroots >= 0.2.3],

[GCROOTS_REQ=gcroots],

[AC_MSG_ERROR([installed libgcroots is not found.])])])

use_libgcroots="installed one"

AC_DEFINE(SCM_WITH_BDWGC, 1, [Define to 1 to use BDWGC.])

use_libgcroots="BDWGC (installed)"],

[PKG_CHECK_MODULES(

GCROOTS, [gcroots >= 0.2.3],

[GCROOTS_REQ=gcroots

use_libgcroots="installed one"],

[AC_MSG_ERROR([installed libgcroots is not found.])])])

Copilot · 2026-03-26T09:00:55Z

 AC_ARG_WITH(libgcroots,
            AS_HELP_STRING([--with-libgcroots@<:@=WHICH@:>@],
-                           [use alternative libgcroots instead of
-                            package-bundled one.
+                           [use system BDWGC or alternative libgcroots
+                            instead of package-bundled libgcroots.
                            WHICH=(installed|tiny-subst)
                            @<:@default=installed@:>@]))


The help text says --with-libgcroots defaults to installed, but the implementation still falls back to no when the option is omitted (case "${with_libgcroots:=no}"). This makes the configure help misleading and also prevents the advertised "prefer system BDWGC when available" behavior unless the user explicitly passes --with-libgcroots=installed. Align the actual default with the help text (or update the help text to match the real default).

Copilot · 2026-03-26T09:00:56Z

+static void GC_CALLBACK
+bdwgc_mark(void **start, void **end, void *data, unsigned hint)
+{
+    gc_mark_locations(*start, *end, scm_false, scm_false);


In the BDWGC marking callback, gc_mark_locations is being called with *start / *end (dereferencing the pointers) instead of the pointer range itself. gc_mark_locations expects a start/end address range; dereferencing will pass arbitrary stack contents as addresses and can lead to incorrect marking or crashes. Pass the range (with appropriate casts) rather than dereferencing.

Suggested change

gc_mark_locations(*start, *end, scm_false, scm_false);

gc_mark_locations(start, end, scm_false, scm_false);

Copilot · 2026-03-26T09:00:56Z

+      [PKG_CHECK_MODULES(
+        GCROOTS, [gcroots >= 0.2.3],
+        [GCROOTS_REQ=gcroots],
+        [AC_MSG_ERROR([installed libgcroots is not found.])])])


If bdw-gc is not found and gcroots is also missing, the configure error currently says only "installed libgcroots is not found" even though BDWGC was also attempted first. Adjust the failure message to mention that neither bdw-gc nor gcroots was found (and ideally include the required minimum versions) to make diagnosis easier.

Suggested change

[AC_MSG_ERROR([installed libgcroots is not found.])])])

[AC_MSG_ERROR([neither BDWGC (bdw-gc >= 8.3.0) nor libgcroots (gcroots >= 0.2.3) was found.])])])

Copilot

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

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Copilot · 2026-03-26T09:20:04Z

+        GCROOTS, [gcroots >= 0.2.3],
+        [GCROOTS_REQ=gcroots
+         use_libgcroots="installed one"],
+        [AC_MSG_ERROR([neighter BDWGC nor libgcroots was found.])])])


The error message has a spelling mistake: "neighter" → "neither". This string is user-visible during configure failures, so it’s worth correcting.

Suggested change

[AC_MSG_ERROR([neighter BDWGC nor libgcroots was found.])])])

[AC_MSG_ERROR([neither BDWGC nor libgcroots was found.])])])

Copilot

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

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Copilot · 2026-03-26T09:34:27Z

+            --enable-conf=uim \
+            --enable-maintainer-mode \
+            --prefix=/usr \


This job now configures with --enable-conf=uim, --prefix=/usr, and a --with-libgcroots matrix, whereas previously it only ran ./configure --enable-maintainer-mode. If the goal of this workflow is specifically to validate the GC backend selection, these extra flags change what configuration is being exercised and may reduce coverage of the default build. Consider either keeping the default configure flags and adding a separate job for the uim config, or documenting why the uim config is required here.

Suggested change

--enable-conf=uim \

--enable-maintainer-mode \

--prefix=/usr \

--enable-maintainer-mode \

Copilot · 2026-03-26T09:34:28Z

+
+static void GC_CALLBACK
+bdwgc_mark(void **start, void **end, void *data, unsigned hint)
+{


bdwgc_mark doesn’t use the data or hint parameters. With -Wall this can produce unused-parameter warnings; consider explicitly marking them unused (e.g., via (void)data; (void)hint;) or annotating the parameters if you want warning-clean builds.

Suggested change

{

{

(void)data;

(void)hint;

Copilot · 2026-03-26T09:34:28Z

+static void GC_CALLBACK
+bdwgc_find_object(void **start, void **end, void *data, unsigned hint)
+{
+    BDWGCFindObjectData *find_object_data = (BDWGCFindObjectData *)data;
+    char *p = (char *)start;


bdwgc_find_object doesn’t use the hint parameter. To avoid unused-parameter warnings under -Wall, consider marking it unused explicitly (or using an attribute/macro consistent with the project).

Copilot · 2026-03-26T09:34:28Z

@@ -287,11 +287,16 @@ case "${with_libgcroots:=no}" in
    GCROOTS_REQ=gcroots
  ;;
  yes|installed)
-    PKG_CHECK_MODULES(GCROOTS, [gcroots >= 0.2.3],
-                      [],
-                      [AC_MSG_ERROR([installed libgcroots is not found.])])
-    use_libgcroots="installed one"
-    GCROOTS_REQ=gcroots
+    PKG_CHECK_MODULES(
+      BDWGC, [bdw-gc >= 8.3.0],
+      [BDWGC_REQ=bdw-gc
+       AC_DEFINE(SCM_WITH_BDWGC, 1, [Define to 1 to use BDWGC.])
+       use_libgcroots="installed BDWGC"],
+      [PKG_CHECK_MODULES(
+        GCROOTS, [gcroots >= 0.2.3],
+        [GCROOTS_REQ=gcroots
+         use_libgcroots="installed one"],
+        [AC_MSG_ERROR([neither BDWGC nor libgcroots was found.])])])


The PR description says the build should use system BDWGC when it is found, but configure currently only probes for BDWGC in the --with-libgcroots=yes|installed branch. With the current logic, the default (with_libgcroots=no) will always build/use the bundled libgcroots even if a suitable system BDWGC is available. If the intent is auto-preference for system BDWGC, consider probing BDWGC in the default path (and only falling back to bundled libgcroots if not found), or clarify the intended behavior in the option help/PR description.

Copilot · 2026-03-26T09:34:29Z

+        uses: actions/checkout@v6
+        with:
+          path: bdwgc
+          repository: bdwgc/bdwgc


The workflow checks out and builds BDWGC from the repository default branch without pinning a tag/commit (repository: bdwgc/bdwgc with no ref). This can make CI non-reproducible (upstream changes can break the build or change the reported pkg-config version relative to the bdw-gc >= 8.3.0 requirement). Consider pinning to a known-good release tag/commit (>= 8.3.0) or installing a packaged BDWGC version to keep CI deterministic.

Suggested change

repository: bdwgc/bdwgc

repository: bdwgc/bdwgc

ref: v8.3.0

There is no such tag (v8.3.0), use commit sha instead, e.g. 946696b9b16580fcece3bc4d640f35afc139426d

If BDWGC 8.3.0 or later exists, we use it directly.

ivmai added 2 commits March 13, 2026 16:02

Support optional use of bundled bdwgc instead of libgcroots

76f64d4

To use bdwgc, pass `--with_libgcroots=bundled-bdwgc` to configure.

Add bdwgc as a git submodule

ef02502

ivmai mentioned this pull request Mar 13, 2026

Provide gcroots API bdwgc/bdwgc#378

Open

kou force-pushed the add-bundled-bdwgc-option branch 9 times, most recently from b315a2c to a7ad69e Compare March 26, 2026 08:33

kou changed the title ~~Add bundled bdwgc option~~ Add support for system BDWGC Mar 26, 2026

kou requested a review from Copilot March 26, 2026 08:52

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kou force-pushed the add-bundled-bdwgc-option branch from a7ad69e to 01bd5aa Compare March 26, 2026 09:07

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kou force-pushed the add-bundled-bdwgc-option branch from 01bd5aa to 8864e47 Compare March 26, 2026 09:24

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ivmai commented Mar 26, 2026

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Comment thread src/storage-gc.c

Comment thread configure.ac

Use system BDWGC 8.3.0 or later if possible

1dab17a

If BDWGC 8.3.0 or later exists, we use it directly.

kou force-pushed the add-bundled-bdwgc-option branch from 8864e47 to 1dab17a Compare March 26, 2026 21:30

kou merged commit 2072b23 into uim:main Mar 28, 2026
11 of 12 checks passed

	gc_mark_locations(start, end, scm_false, scm_false);
	gc_mark_locations(start, end, scm_false, scm_false);

	[AC_MSG_ERROR([installed libgcroots is not found.])])])
	[AC_MSG_ERROR([neither BDWGC (bdw-gc >= 8.3.0) nor libgcroots (gcroots >= 0.2.3) was found.])])])

	[AC_MSG_ERROR([neighter BDWGC nor libgcroots was found.])])])
	[AC_MSG_ERROR([neither BDWGC nor libgcroots was found.])])])

-{
+{
+    (void)data;
+    (void)hint;

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ivmai commented Mar 13, 2026 •

edited by kou

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