picocrt: Add a 'none' variant

This builds an empty object file so that you can pass '--crt0=none' while
linking and skip the picolibc-specific startup code.

Signed-off-by: Keith Packard <[email protected]>

Author: keith-packard

picocrt/crt0-none.c

@@ -0,0 +1,39 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright © 2024 Keith Packard
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above
+ *    copyright notice, this list of conditions and the following
+ *    disclaimer in the documentation and/or other materials provided
+ *    with the distribution.
+ *
+ * 3. Neither the name of the copyright holder nor the names of its
+ *    contributors may be used to endorse or promote products derived
+ *    from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
+ * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+/*
+ * This file is intentionally empty so that it generates no code at
+ * all for the 'none' crt variant
+ */

picocrt/meson.build

@@ -34,6 +34,7 @@
 #
 
 src_picocrt = []
+src_picocrt_none = files('crt0-none.c')
 
 machine_dir = 'machine' / host_cpu_family
 picocrt_march_add=''
@@ -64,25 +65,30 @@ foreach target : targets
     crt_hosted_name = 'crt0-hosted.o'
     crt_minimal_name = 'crt0-minimal.o'
     crt_semihost_name = 'crt0-semihost.o'
+    crt_none_name = 'crt0-none.o'
     libcrt_name = 'crt0'
     libcrt_hosted_name = 'crt0-hosted'
     libcrt_minimal_name = 'crt0-minimal'
     libcrt_semihost_name = 'crt0-semihost'
+    libcrt_none_name = 'crt0-none'
   else
     crt_name = join_paths(target, 'crt0.o')
     crt_hosted_name = join_paths(target, 'crt0-hosted.o')
     crt_minimal_name = join_paths(target, 'crt0-minimal.o')
     crt_semihost_name = join_paths(target, 'crt0-semihost.o')
+    crt_none_name = join_paths(target, 'crt0-none.o')
     libcrt_name = join_paths(target, 'libcrt0')
     libcrt_hosted_name = join_paths(target, 'libcrt0-hosted')
     libcrt_minimal_name = join_paths(target, 'libcrt0-minimal')
     libcrt_semihost_name = join_paths(target, 'libcrt0-semihost')
+    libcrt_none_name = join_paths(target, 'libcrt0-none')
   endif
 
   crt0_name = 'crt0' + target
   crt0_hosted_name = 'crt0_hosted' + target
   crt0_minimal_name = 'crt0_minimal' + target
   crt0_semihost_name = 'crt0_semihost' + target
+  crt0_none_name = 'crt0_none' + target
 
   _c_args = value[1] + arg_fnobuiltin + ['-ffreestanding']
   _link_args = value[1] + ['-r', '-ffreestanding']
@@ -185,4 +191,19 @@ foreach target : targets
 		     c_args : value[1] + ['-DCRT0_EXIT', '-DCRT0_SEMIHOST'])
     endif
   endif
+
+  # The 'none' variant is completely empty
+  _crt = executable(crt_none_name,
+		    src_picocrt_none,
+		    include_directories : inc,
+		    install : true,
+		    install_dir : instdir,
+		    c_args : _c_args,
+		    link_args : _link_args)
+
+  set_variable(crt0_none_name,
+	       _crt.extract_objects(src_picocrt_none)
+	      )
+
+  
 endforeach