[Driver] Include crt0.o in the baremetal link

[petrhosek]

The common baremetal libc implementations already provide crt0.o and GCC automatically links it so this improves parity.

[llvmbot]

@llvm/pr-subscribers-clang-driver

@llvm/pr-subscribers-clang

Author: Petr Hosek (petrhosek)

Changes

The common baremetal libc implementations already provide crt0.o and GCC automatically links it so this improves parity.

---

Full diff: https://github.com/llvm/llvm-project/pull/101258.diff

4 Files Affected:

• (modified) clang/lib/Driver/ToolChains/BareMetal.cpp (+5)
• (added) clang/test/Driver/Inputs/baremetal_arm/lib/crt0.o ()
• (added) clang/test/Driver/Inputs/basic_baremetal_tree/lib/armv6m-unknown-none-eabi/crt0.o ()
• (modified) clang/test/Driver/baremetal.cpp (+8)

diff --git a/clang/lib/Driver/ToolChains/BareMetal.cpp b/clang/lib/Driver/ToolChains/BareMetal.cpp
index 852e0442f50a2..102d12700229e 100644
--- a/clang/lib/Driver/ToolChains/BareMetal.cpp
+++ b/clang/lib/Driver/ToolChains/BareMetal.cpp
@@ -487,6 +487,11 @@ void baremetal::Linker::ConstructJob(Compilation &C, const JobAction &JA,
     CmdArgs.push_back(Arch == llvm::Triple::aarch64_be ? "-EB" : "-EL");
   }
 
+  if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles,
+                   options::OPT_r)) {
+    CmdArgs.push_back(Args.MakeArgString(TC.GetFilePath("crt0.o")));
+  }
+
   Args.addAllArgs(CmdArgs, {options::OPT_L, options::OPT_T_Group,
                             options::OPT_s, options::OPT_t, options::OPT_r});
 
diff --git a/clang/test/Driver/Inputs/baremetal_arm/lib/crt0.o b/clang/test/Driver/Inputs/baremetal_arm/lib/crt0.o
new file mode 100644
index 0000000000000..e69de29bb2d1d
diff --git a/clang/test/Driver/Inputs/basic_baremetal_tree/lib/armv6m-unknown-none-eabi/crt0.o b/clang/test/Driver/Inputs/basic_baremetal_tree/lib/armv6m-unknown-none-eabi/crt0.o
new file mode 100644
index 0000000000000..e69de29bb2d1d
diff --git a/clang/test/Driver/baremetal.cpp b/clang/test/Driver/baremetal.cpp
index de4e93434e203..9814bf7b0a395 100644
--- a/clang/test/Driver/baremetal.cpp
+++ b/clang/test/Driver/baremetal.cpp
@@ -16,6 +16,7 @@
 // CHECK-V6M-C-SAME: "-internal-isystem" "[[SYSROOT]]{{[/\\]+}}include"
 // CHECK-V6M-C-SAME: "-x" "c++" "{{.*}}baremetal.cpp"
 // CHECK-V6M-C-NEXT: ld{{(.exe)?}}" "{{.*}}.o" "-Bstatic" "-EL"
+// CHECK-V6M-C-SAME: "[[SYSROOT:[^"]+]]{{[/\\]+}}lib{{[/\\]+}}crt0.o"
 // CHECK-V6M-C-SAME: "-T" "semihosted.lds" "-Lsome{{[/\\]+}}directory{{[/\\]+}}user{{[/\\]+}}asked{{[/\\]+}}for"
 // CHECK-V6M-C-SAME: "-L[[SYSROOT:[^"]+]]{{[/\\]+}}lib"
 // CHECK-V6M-C-SAME: "-lc" "-lm" "{{[^"]*}}libclang_rt.builtins.a" "--target2=rel" "-o" "{{.*}}.tmp.out"
@@ -37,6 +38,7 @@
 // CHECK-V6M-TREE-SAME: "-internal-isystem" "[[INSTALLED_DIR]]{{[/\\]+}}..{{[/\\]+}}include{{[/\\]+}}armv6m-unknown-none-eabi"
 // CHECK-V6M-TREE-SAME: "-x" "c++" "{{.*}}baremetal.cpp"
 // CHECK-V6M-TREE-NEXT: ld{{(.exe)?}}" "{{.*}}.o" "-Bstatic" "-EL"
+// CHECK-V6M-TREE-SAME: "[[INSTALLED_DIR]]{{[/\\]+}}..{{[/\\]+}}lib{{[/\\]+}}armv6m-unknown-none-eabi{{[/\\]+}}crt0.o"
 // CHECK-V6M-TREE-SAME: "-L[[INSTALLED_DIR]]{{[/\\]+}}..{{[/\\]+}}lib{{[/\\]+}}armv6m-unknown-none-eabi"
 // CHECK-V6M-TREE-SAME: "-lc" "-lm" "{{[^"]*}}libclang_rt.builtins.a" "--target2=rel" "-o" "{{.*}}.tmp.out"
 
@@ -48,6 +50,7 @@
 // CHECK-ARMV7M-PER-TARGET: "-isysroot" "[[SYSROOT:[^"]*]]"
 // CHECK-ARMV7M-PER-TARGET: "-x" "c++" "{{.*}}baremetal.cpp"
 // CHECK-ARMV7M-PER-TARGET: ld{{(.exe)?}}" "{{.*}}.o" "-Bstatic" "-EL"
+// CHECK-ARMV7M-PER_TARGET: "[[SYSROOT:[^"]+]]{{[/\\]+}}lib{{[/\\]+}}crt0.o"
 // CHECK-ARMV7M-PER-TARGET: "-L[[SYSROOT:[^"]+]]{{[/\\]+}}lib"
 // CHECK-ARMV7M-PER-TARGET: "-L[[RESOURCE_DIR:[^"]+]]{{[/\\]+}}lib{{[/\\]+}}armv7m-vendor-none-eabi
 // CHECK-ARMV7M-PER-TARGET: "-lc" "-lm" "{{[^"]*}}libclang_rt.builtins.a"
@@ -56,6 +59,7 @@
 // RUN:     --sysroot=%S/Inputs/baremetal_arm | FileCheck --check-prefix=CHECK-V6M-DEFAULTCXX %s
 // CHECK-V6M-DEFAULTCXX: "-resource-dir" "[[RESOURCE_DIR:[^"]+]]"
 // CHECK-V6M-DEFAULTCXX: ld{{(.exe)?}}" "{{.*}}.o" "-Bstatic" "-EL"
+// CHECK-V6M-DEFAULTCXX-SAME: "[[SYSROOT:[^"]+]]{{[/\\]+}}lib{{[/\\]+}}crt0.o"
 // CHECK-V6M-DEFAULTCXX-SAME: "-L{{[^"]*}}{{[/\\]+}}Inputs{{[/\\]+}}baremetal_arm{{[/\\]+}}lib"
 // CHECK-V6M-DEFAULTCXX-SAME: "-lc++" "-lc++abi" "-lunwind"
 // CHECK-V6M-DEFAULTCXX-SAME: "-lc" "-lm" "{{[^"]*}}libclang_rt.builtins.a" "--target2=rel" "-o" "a.out"
@@ -105,6 +109,10 @@
 // CHECK-V6M-LIBCXX-USR-SAME: "-lc++" "-lc++abi" "-lunwind"
 // CHECK-V6M-LIBCXX-USR-SAME: "-lc" "-lm" "{{[^"]*}}libclang_rt.builtins.a"
 
+// RUN: %clangxx --target=arm-none-eabi -nostartfiles -v 2>&1 \
+// RUN:   | FileCheck %s --check-prefix=CHECK-NOSTARTFILES
+// CHECK-NOSTARTFILES-NOT: "crt0.o"
+
 // RUN: %clangxx --target=arm-none-eabi -v 2>&1 \
 // RUN:   | FileCheck %s --check-prefix=CHECK-THREAD-MODEL
 // CHECK-THREAD-MODEL: Thread model: posix

[MaskRay]

-v without an input file doesn't print commands. -### ?

[petrhosek]

Done.

[smithp35]

One thing the GNU toolchain does with specs files is to rename the crt0.o when semihosting, or other variants are used, for example the --specs=rdimon.specs will add rdimon-crt0.o rather than crt0.o
The part in the specs file is:

*startfile:
crti%O%s crtbegin%O%s %{!pg:rdimon-crt0%O%s} %{pg:rdimon-crt0%O%s}

In picolibc we've kind of worked around this by using a crt library rather than an object file for example libcrt0.a libcrt0-semihosting.a that way we've been able to mimic the specs file by adding -lcrt0-semihosting this uses the library path added by multilib to find the libcrt0.a library without the user having to know the path to the crt0-semihosting.o object file.

I don't think this is fundamentally incompatible with unconditionally adding crt0.o as -nostartfiles can be used to suppress it. I'm wondering if in the future we might be able to offer an option to add a prefix to the crt0.o.

Picolibc reference:
https://github.com/picolibc/picolibc/blob/61560a1a4bfd78d0be3a36d0507cbeb795eebddb/picocrt/meson.build#L72