Add CI for GDC builds

.github/workflows/ci.yml

@@ -23,16 +23,79 @@ jobs:
         exclude:
           - { os: windows, arch: arm64 }   # windows-arm64 runner not in free tier
           - { compiler: dmd, arch: arm64 } # DMD arm64 still WIP
+        include:
+          # GDC: host-only, native arch only (cross promotes to LDC).
+          - { os: ubuntu, compiler: gdc, arch: x86_64 }
+          - { os: ubuntu, compiler: gdc, arch: arm64 }
     env:
       MAKE_OS: ${{ matrix.os == 'ubuntu' && 'linux' || matrix.os }}
     steps:
       - uses: actions/checkout@v4
 
       - name: Setup D compiler
+        if: matrix.compiler != 'gdc'
         uses: dlang-community/setup-dlang@v2
         with:
           compiler: ${{ matrix.compiler == 'dmd' && 'dmd-master' || matrix.compiler }}
 
+      # TODO: drop the PPA dance and switch to the bare ubuntu-26.04 (and
+      # ubuntu-26.04-arm) runners once GHA ships them. Tracking issue:
+      # https://github.com/actions/runner-images/issues  (filed 2026-04-24
+      # "Add Ubuntu 26.04 LTS (Resolute Raccoon)"). Noble (24.04) ships gcc-13;
+      # numbat (26.04) will ship gcc-16+ which has the DMD frontend 2.112+
+      # that uRT requires.
+      #
+      # gdc-16 lives in ppa:ubuntu-toolchain-r/test (launchpad.net, not
+      # Azure-mirrored, ~10 min cold install). The .deb files are cached via
+      # actions/cache so subsequent runs restore from GHA's Azure cache in
+      # seconds. Bump `gdc-cache-v<N>` below to invalidate the cache when a
+      # new gdc-16 ships from the PPA.
+      # Restore now; save unconditionally at the end (build still fails on
+      # URT-side D-source issues so we need the cache to persist across red
+      # runs while we chip at them).
+      - name: Restore gdc-16 .debs
+        if: matrix.compiler == 'gdc'
+        id: gdc-cache
+        uses: actions/cache/restore@v4
+        with:
+          path: ~/gdc-debs
+          key: gdc-16-debs-${{ runner.os }}-${{ runner.arch }}-v1
+      - name: Download gdc-16 .debs (cache miss)
+        if: matrix.compiler == 'gdc' && steps.gdc-cache.outputs.cache-hit != 'true'
+        run: |
+          sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
+          sudo apt-get update
+          mkdir -p ~/gdc-debs
+          cd ~/gdc-debs
+          apt-get download $(apt-cache depends --recurse --no-recommends --no-suggests \
+            --no-conflicts --no-breaks --no-replaces --no-enhances \
+            --no-pre-depends gdc-16 | grep "^\w" | sort -u)
+      - name: Save gdc-16 .debs
+        if: matrix.compiler == 'gdc' && steps.gdc-cache.outputs.cache-hit != 'true'
+        uses: actions/cache/save@v4
+        with:
+          path: ~/gdc-debs
+          key: ${{ steps.gdc-cache.outputs.cache-primary-key }}
+      - name: Install gdc-16 from .debs
+        if: matrix.compiler == 'gdc'
+        run: |
+          sudo dpkg -i ~/gdc-debs/*.deb || sudo apt-get install -f -y --no-install-recommends
+          sudo update-alternatives --install /usr/bin/gdc gdc /usr/bin/gdc-16 100
+          # Bundled druntime/phobos stay on disk: -I src takes priority so
+          # uRT's core.* / std.* shadows win, and modules we don't shadow
+          # (e.g. __importc_builtins.d, gcc.*) resolve from the bundled tree.
+          # We don't link druntime, so unused symbols don't matter.
+          #
+          # Exception: `object` is special-cased by GDC (hardcoded druntime
+          # lookup wins over -I), so the bundled object.d must be removed
+          # to expose uRT's src/object.d. The same problem hits nested
+          # modules under std/math/ -- gdc resolves std.math.exponential to
+          # its bundled file before checking -I src/std/math/exponential.d.
+          for d in /usr/lib/gcc/*-linux-gnu/16/include/d; do
+            sudo rm -f "$d/object.d"
+            sudo rm -rf "$d/std"
+          done
+
       - name: Install mbedtls (Linux host)
         if: matrix.os == 'ubuntu'
         run: sudo apt-get update && sudo apt-get install -y libmbedtls-dev
@@ -47,8 +110,40 @@ jobs:
       - name: Build unittest
         run: make ARCH=${{ matrix.arch }} OS=${{ env.MAKE_OS }} CONFIG=unittest COMPILER=${{ matrix.compiler }}
 
-      - name: Run unittest
-        run: ./bin/${{ matrix.arch }}_${{ env.MAKE_OS }}_unittest/urt_test${{ matrix.os == 'windows' && '.exe' || '' }}
+      - name: GDC diagnostics
+        if: matrix.compiler == 'gdc' && matrix.os == 'ubuntu'
+        run: |
+          objdump -d -M intel --disassemble=co_swap \
+            ./bin/${{ matrix.arch }}_${{ env.MAKE_OS }}_unittest/urt_test || true
+          # Also dump co_switch and the unittest fibre body for the
+          # urt.fibre crash so we can see the whole call site.
+          objdump -d -M intel ./bin/${{ matrix.arch }}_${{ env.MAKE_OS }}_unittest/urt_test \
+            | awk '/<.*co_switch>:/,/^$/' | head -60 || true
+
+      - name: Run unittest (Linux, collect core on crash)
+        if: matrix.os == 'ubuntu'
+        run: |
+          sudo apt-get install -y gdb || true
+          ulimit -c unlimited
+          sudo sysctl -w kernel.core_pattern=core.%p || true
+          mkdir -p cores; cd cores
+          ../bin/${{ matrix.arch }}_${{ env.MAKE_OS }}_unittest/urt_test || rc=$?
+          if [ -n "$rc" ]; then
+            echo "exit code: $rc"
+            ls -la
+            for c in core* /var/lib/apport/coredump/*; do
+              [ -f "$c" ] || continue
+              echo "=== $c ==="
+              gdb -batch -ex 'bt' -ex 'info registers' \
+                ../bin/${{ matrix.arch }}_${{ env.MAKE_OS }}_unittest/urt_test \
+                "$c" 2>&1 | head -80
+            done
+            exit $rc
+          fi
+
+      - name: Run unittest (Windows)
+        if: matrix.os == 'windows'
+        run: ./bin/${{ matrix.arch }}_${{ env.MAKE_OS }}_unittest/urt_test.exe
 
       - name: Build release library
         if: github.ref == 'refs/heads/release'

Makefile

@@ -1,5 +1,10 @@
 URT_SRCDIR := src
 
+# The first non-pattern target in this Makefile -- needed up-front because
+# the GDC block below defines a static rule for the preprocessed mbedtls.i
+# that would otherwise become the default goal and short-circuit the build.
+.DEFAULT_GOAL := all
+
 include platforms.mk
 
 # =======================================================================
@@ -19,6 +24,57 @@ include platforms.mk
 OBJDIR    := obj/$(BUILDNAME)_$(CONFIG)
 TARGETDIR := bin/$(BUILDNAME)_$(CONFIG)
 
+# =======================================================================
+# GDC ImportC: GDC's ImportC does not preprocess #include directives, so
+# any *.c the D side imports (urt.internal.os, urt.internal.mbedtls, ...)
+# must be preprocessed to *.i first via the host C compiler. We stage
+# them under $(OBJDIR)/imports/ and prepend that to the import path so
+# GDC resolves urt.internal.os to os.i instead of os.c.
+#
+# Two layouts of preprocessed .i:
+#  * `urt/internal/foo.c` → `imports/urt/internal/foo.i` for files
+#    consumed via path-based `import urt.internal.foo` (e.g. os.c).
+#  * `urt/internal/bar.c` → `imports/bar.i` (flat) for files passed as
+#    sources in URT_SOURCES (e.g. mbedtls.c) -- DMD/LDC's ImportC names
+#    those by basename, so urt.internal.bar.d is a thin shim that does
+#    `public import bar;` and stays consistent across all compilers.
+# =======================================================================
+ifeq ($(COMPILER),gdc)
+GDC_I_DIR := $(OBJDIR)/imports
+URT_C_SOURCES   := $(filter %.c,$(URT_SOURCES))
+URT_C_HEADERS   := $(filter-out $(URT_C_SOURCES),$(shell find "$(URT_SRCDIR)" -type f -name '*.c' -not -path '$(URT_SRCDIR)/urt/driver/*'))
+URT_I_HEADERS   := $(patsubst $(URT_SRCDIR)/%.c,$(GDC_I_DIR)/%.i,$(URT_C_HEADERS))
+URT_I_SOURCES   := $(patsubst %.c,$(GDC_I_DIR)/%.i,$(notdir $(URT_C_SOURCES)))
+URT_I_FILES     := $(URT_I_HEADERS) $(URT_I_SOURCES)
+URT_SOURCES     := $(filter-out %.c,$(URT_SOURCES)) $(URT_I_SOURCES)
+DFLAGS          := -I $(GDC_I_DIR) $(DFLAGS)
+
+# AArch64: GCC's __attribute__((naked)) is silently dropped on aarch64,
+# so co_swap can't be inline asm there. Compile src/urt/co_swap.S via host
+# gcc and link the .o; fibre.d's aarch64 GNU branch is just an extern decl.
+ifeq ($(ARCH),arm64)
+URT_S_OBJECTS   := $(OBJDIR)/host/co_swap.o
+URT_SOURCES     := $(URT_SOURCES) $(URT_S_OBJECTS)
+endif
+
+$(GDC_I_DIR)/%.i: $(URT_SRCDIR)/%.c
+	@mkdir -p $(@D)
+	gcc -E -P -dD $< | \
+	  perl -0777 -pe 's/\b(register|__restrict|__restrict__|__inline__|__inline|__extension__|__signed__|__signed)\b//g; s/__attribute__\s*(\((?:[^()]++|(?1))*\))//g; s/__asm__\s*\(\s*(?:"[^"]*"\s*)+\)//g' \
+	         > $@
+
+# Flat-path rule for URT_SOURCES C files (basename in $(GDC_I_DIR)).
+$(GDC_I_DIR)/mbedtls.i: $(URT_SRCDIR)/urt/internal/mbedtls.c
+	@mkdir -p $(@D)
+	gcc -E -P -dD $< | \
+	  perl -0777 -pe 's/\b(register|__restrict|__restrict__|__inline__|__inline|__extension__|__signed__|__signed)\b//g; s/__attribute__\s*(\((?:[^()]++|(?1))*\))//g; s/__asm__\s*\(\s*(?:"[^"]*"\s*)+\)//g' \
+	         > $@
+
+$(OBJDIR)/host/%.o: $(URT_SRCDIR)/%.S
+	@mkdir -p $(@D)
+	gcc -c -o $@ $<
+endif
+
 # Linker script selection for cross-target unittest builds (ESP excluded --
 # no ESP-IDF on build slave). platforms.mk falls back to compile-only when
 # BAREMETAL_LD isn't set.
@@ -111,7 +167,10 @@ endif
 # Build rule
 # =======================================================================
 
-$(TARGET): $(BAREMETAL_OBJS)
+.PHONY: all
+all: $(TARGET)
+
+$(TARGET): $(BAREMETAL_OBJS) $(URT_I_FILES) $(URT_S_OBJECTS)
 	mkdir -p $(OBJDIR) $(TARGETDIR)
 ifeq ($(COMPILER),ldc)
 	"$(DC)" $(DFLAGS) $(BUILD_CMD_FLAGS) -of$(TARGET) -od$(OBJDIR) -deps=$(DEPFILE) $(BAREMETAL_OBJS) $(URT_SOURCES)
@@ -122,6 +181,13 @@ ifeq ($(BUILD_MODE),lib)
 else
 	"$(DC)" $(DFLAGS) $(BUILD_CMD_FLAGS) -of$(TARGET) -od$(OBJDIR) -makedeps $(URT_SOURCES) > $(DEPFILE)
 endif
+else ifeq ($(COMPILER),gdc)
+ifeq ($(BUILD_MODE),lib)
+	"$(DC)" $(DFLAGS) -c -o $(OBJDIR)/urt.o $(URT_SOURCES)
+	$(AR) rcs $(TARGET) $(OBJDIR)/urt.o
+else
+	"$(DC)" $(DFLAGS) -o $(TARGET) $(URT_SOURCES)
+endif
 endif
 ifeq ($(BUILD_MODE),embedded-exe)
 	$(BAREMETAL_OBJCOPY) -O binary $(OBJCOPY_FLAGS) $(TARGET) $(TARGETDIR)/$(TARGETNAME).bin

platforms.mk

@@ -304,6 +304,11 @@ endif
 
 # =======================================================================
 # Compiler auto-selection: cross-compilation targets use LDC
+#
+# DMD: only x86/x86_64 host, anything else -> LDC.
+# GDC: host-only (any arch with a host toolchain); freertos/baremetal -> LDC.
+#      Cross GCC toolchains do ship gdc, but wiring those in is a separate
+#      effort; for now we promote to LDC.
 # =======================================================================
 
 ifeq ($(COMPILER),dmd)
@@ -316,6 +321,12 @@ endif
 endif
 endif
 
+ifeq ($(COMPILER),gdc)
+ifneq ($(filter freertos baremetal,$(OS)),)
+    COMPILER := ldc
+endif
+endif
+
 # =======================================================================
 # Toolchain discovery (Espressif)
 # =======================================================================
@@ -336,7 +347,8 @@ URT_SOURCES := $(shell find "$(URT_SRCDIR)" -type f -name '*.d' -not -path '$(UR
 URT_SOURCES := $(URT_SOURCES) $(shell find "$(URT_SRCDIR)/urt/driver" -maxdepth 1 -type f -name '*.d')
 URT_SOURCES := $(URT_SOURCES) $(shell find "$(URT_SRCDIR)/urt/driver/baremetal" -type f -name '*.d')
 
-# mbedtls C glue needs host mbedtls headers -- exclude for embedded targets
+# mbedtls C glue needs host mbedtls headers -- exclude for embedded targets.
+# urt/internal/os.c already enters via the posix driver dir below.
 ifeq ($(filter freertos baremetal,$(OS)),)
     URT_SOURCES := $(URT_SOURCES) $(URT_SRCDIR)/urt/internal/mbedtls.c
 endif
@@ -382,7 +394,9 @@ endif
 # imports stay in the consumer Makefile.
 # =======================================================================
 
-DFLAGS := $(DFLAGS) -preview=bitfields -preview=rvaluerefparam -preview=in #-preview=nosharedaccess <- TODO
+# Preview flags translated per-compiler below: dmd/ldc accept -preview=X,
+# gdc takes -fpreview=X.
+D_PREVIEWS := bitfields rvaluerefparam in #nosharedaccess <- TODO
 
 # OS-level versions
 ifeq ($(OS),freertos)
@@ -467,15 +481,15 @@ else ifeq ($(PLATFORM),esp32-p4)
     DFLAGS := $(DFLAGS) -d-version=ESP32_P4
 endif
 
-ifeq ($(CONFIG),unittest)
-    DFLAGS := $(DFLAGS) -unittest
-endif
-
 # =======================================================================
 # Compiler configuration -- triple, mattr, link flags
 # =======================================================================
 
 ifeq ($(COMPILER),ldc)
+    DFLAGS := $(DFLAGS) $(addprefix -preview=,$(D_PREVIEWS))
+    ifeq ($(CONFIG),unittest)
+        DFLAGS := $(DFLAGS) -unittest
+    endif
     # Prefer dlang-installer LDC (avoids system package conflicts with cross-compile)
     DC := $(lastword $(sort $(wildcard $(HOME)/dlang/ldc-*/bin/ldc2)))
     DC := $(if $(DC),$(DC),ldc2)
@@ -652,6 +666,11 @@ ifeq ($(COMPILER),ldc)
 else ifeq ($(COMPILER),dmd)
     DC ?= dmd
 
+    DFLAGS := $(DFLAGS) $(addprefix -preview=,$(D_PREVIEWS))
+    ifeq ($(CONFIG),unittest)
+        DFLAGS := $(DFLAGS) -unittest
+    endif
+
     # Strip druntime/phobos, use URT's own object.d.
     # Consumers may need to prepend their own -I to shadow druntime's
     # __importc_builtins.di (e.g. OpenWatt's third_party/dmd/ for MSVC va_list).
@@ -670,6 +689,52 @@ else ifeq ($(COMPILER),dmd)
     else
         DFLAGS := $(DFLAGS) -g -debug
     endif
+
+else ifeq ($(COMPILER),gdc)
+    DC ?= gdc
+
+    DFLAGS := $(DFLAGS) $(addprefix -fpreview=,$(D_PREVIEWS))
+    ifeq ($(CONFIG),unittest)
+        # URT defines its own extern(C) main in src/urt/package.d, so don't
+        # pass -fmain (which would emit a duplicate _Dmain).
+        DFLAGS := $(DFLAGS) -funittest
+    endif
+
+    # Strip druntime/phobos, use URT's own object.d.
+    # -fno-druntime is too aggressive: it implies -fno-rtti -fno-exceptions
+    # -fno-moduleinfo, but uRT defines its own TypeInfo (needs RTTI), uses
+    # try-catch (needs exceptions), and walks ModuleInfo to enumerate
+    # unittest functions. Pick the subset we actually want:
+    #   -nophoboslib       skip linking libgphobos
+    # GDC's bundled druntime path is shadowed per module under src/core/
+    # (newaa, array.construction, cast_, ...).
+    # -fno-omit-frame-pointer: URT's exception unwinder walks the frame chain
+    # (matches LDC's -frame-pointer=all).
+    DFLAGS := $(DFLAGS) -nophoboslib -fno-omit-frame-pointer -I $(URT_SRCDIR)
+
+    # GDC ignores pragma(lib, "mbedtls") on most binutils setups (.deplibs
+    # is not honored by ld). Pass explicit -l flags on host posix builds.
+    # Use -Wl,--no-as-needed so the libs aren't dropped despite appearing
+    # before the object files that reference them on the link line.
+    ifneq ($(filter linux ubuntu freebsd,$(OS)),)
+        DFLAGS := $(DFLAGS) -Wl,--no-as-needed -lmbedtls -lmbedx509 -lmbedcrypto -Wl,--as-needed
+    endif
+
+    ifeq ($(ARCH),x86_64)
+        DFLAGS := $(DFLAGS) -m64
+    else ifeq ($(ARCH),x86)
+        DFLAGS := $(DFLAGS) -m32
+    else ifeq ($(ARCH),arm64)
+        # Native arm64 host build -- nothing to add; gdc's default target matches.
+    else
+        $(error "GDC: unsupported ARCH=$(ARCH) for PLATFORM=$(PLATFORM) (use COMPILER=ldc)")
+    endif
+
+    ifeq ($(CONFIG),release)
+        DFLAGS := $(DFLAGS) -O3 -frelease -finline-functions
+    else
+        DFLAGS := $(DFLAGS) -g -fdebug
+    endif
 else
     $(error "Unknown D compiler: $(COMPILER)")
 endif

src/core/internal/array/construction.d

@@ -0,0 +1,8 @@
+// GDC-side shadow for druntime's core.internal.array.construction.
+//
+// uRT's src/object.d defines all the array-construction hooks GDC's lowering
+// invokes. Re-export the whole `object` module to avoid selective-import
+// quirks with extern(C) declarations under GDC.
+module core.internal.array.construction;
+
+public import object;

src/core/internal/cast_.d

@@ -0,0 +1,8 @@
+// GDC-side shadow for druntime's core.internal.cast_.
+//
+// uRT's src/object.d defines _d_dynamic_cast / _d_interface_cast as extern(C);
+// selective named imports of extern(C) symbols across modules don't always
+// resolve cleanly under GDC, so re-export the whole `object` module.
+module core.internal.cast_;
+
+public import object;

src/core/internal/newaa.d

@@ -0,0 +1,23 @@
+// GDC-side shadow for druntime's core.internal.newaa.
+//
+// GDC's bundled object.d always gets compiled (it's load-bearing for
+// semantic analysis even with -fno-druntime), and that object.d imports
+// AA hooks from core.internal.newaa. With `-I src` ahead of GDC's bundled
+// D include path, this shadow wins import resolution and forwards the
+// hooks to uRT's template-based AA implementation in urt.internal.aa.
+//
+// Without this shadow, GDC pulls its bundled core.internal.newaa which
+// tries to GC-allocate `Bucket[]` arrays -- incompatible with -fno-druntime.
+module core.internal.newaa;
+
+public import urt.internal.aa :
+    _d_assocarrayliteralTX,
+    _d_aaNew,
+    _d_aaLen,
+    _d_aaGetY,
+    _d_aaGetRvalueX,
+    _d_aaIn,
+    _d_aaDel,
+    _d_aaApply,
+    _d_aaApply2,
+    _d_aaEqual;