Ship the target's libgcc runtime with the toolchain

cc-as-linker downstream (e.g. the OCaml unikernel toolchain) pulls in the
compiler's runtime intrinsics with -lgcc. A GNU cross gcc, and clang with
compiler-rt, ship that runtime on their own search path, so -lgcc resolves with
nothing to do. Clang ships no compiler-rt builtins for the cross target on some
hosts (e.g. macOS), where -print-libgcc-file-name then names a file that does
not exist; there, fall back to a bare-metal GNU cross gcc's libgcc and install
it as the toolchain's libgcc.a. When the compiler provides its own runtime,
install nothing.

The macOS CI installs lld (for ld.lld) and aarch64-elf-gcc (for libgcc).

Author: samoht

.github/workflows/macos.yml

@@ -8,7 +8,7 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v4
       - name: Install dependencies
-        run: brew install llvm
+        run: brew install llvm lld aarch64-elf-gcc
       - name: Build
         run: |
           export PATH="$(brew --prefix llvm)/bin:$PATH"

GNUmakefile

@@ -144,6 +144,11 @@ install-toolchain: build
 	    "{}" "$(TOOLCHAIN_INCDIR)/{}" \;
 	$(INSTALL) -m 0644 bindings/solo5_stub.o $(TOOLCHAIN_LIBDIR)
 	$(INSTALL) -m 0644 bindings/solo5_stub.lds $(TOOLCHAIN_LIBDIR)
+# Only when the target compiler ships no runtime of its own (see configure.sh);
+# otherwise -lgcc already resolves against the compiler's own libgcc.
+ifdef CONFIG_TARGET_LIBGCC
+	$(INSTALL) -m 0644 $(CONFIG_TARGET_LIBGCC) $(TOOLCHAIN_LIBDIR)/libgcc.a
+endif
 ifdef CONFIG_HVT
 	$(INSTALL) -m 0644 bindings/solo5_hvt.o $(TOOLCHAIN_LIBDIR)
 	$(INSTALL) -m 0644 bindings/solo5_hvt.lds $(TOOLCHAIN_LIBDIR)

configure.sh

@@ -540,6 +540,31 @@ fi
 echo "${prog_NAME}: Using ${TARGET_LD} for target linker"
 echo "${prog_NAME}: Using ${TARGET_OBJCOPY} for target objcopy"
 
+# "cc as linker" downstream (e.g. the OCaml unikernel toolchain) pulls in the
+# compiler's runtime intrinsics with -lgcc, so the toolchain has to ship that
+# archive. Ask the target compiler for it rather than shipping a prebuilt copy:
+# for Clang on an ELF target this is the GCC cross toolchain's libgcc Clang
+# defaults to, for GCC its own. Fail loudly if the compiler provides none.
+echo -n "${prog_NAME}: Checking for the target runtime (libgcc): "
+CC_LIBGCC="$(${TARGET_CC} -print-libgcc-file-name 2>/dev/null)"
+if [ -f "${CC_LIBGCC}" ]; then
+    # The compiler ships its own runtime (GNU libgcc, or clang's compiler-rt
+    # builtins) on its search path, so -lgcc already resolves; nothing to ship.
+    # TARGET_LIBGCC stays empty and the toolchain installs no libgcc.a.
+    echo "${CC_LIBGCC} (from the compiler)"
+    TARGET_LIBGCC=
+elif TARGET_LIBGCC="$("${TARGET_ARCH}-elf-gcc" -print-libgcc-file-name 2>/dev/null)" &&
+     [ -f "${TARGET_LIBGCC}" ]; then
+    # Clang ships no compiler-rt builtins for this cross target on some hosts
+    # (e.g. macOS); borrow a bare-metal GNU cross gcc's libgcc and bundle it.
+    echo "${TARGET_LIBGCC} (bundled)"
+else
+    echo "not found"
+    die "${TARGET_CC} provides no target runtime library (libgcc)." \
+        "Install one that ships libgcc/compiler-rt for ${TARGET_CC_MACHINE}" \
+        "(e.g. \`brew install ${TARGET_ARCH}-elf-gcc\`), or set TARGET_CC."
+fi
+
 TARGET_TRIPLE="${TARGET_ARCH}-solo5-none-static"
 echo "${prog_NAME}: Target toolchain triple is ${TARGET_TRIPLE}"
 
@@ -586,6 +611,7 @@ CONFIG_TARGET_LD=${TARGET_LD}
 CONFIG_TARGET_LD_LDFLAGS=${TARGET_LD_LDFLAGS}
 CONFIG_TARGET_LD_MAX_PAGE_SIZE=${TARGET_LD_MAX_PAGE_SIZE}
 CONFIG_TARGET_OBJCOPY=${TARGET_OBJCOPY}
+CONFIG_TARGET_LIBGCC=${TARGET_LIBGCC}
 EOM
 
 #