Add LLVM libc sample (#538)

This will be built into the LLVM libc overlay package. Sample installs
to samples/src/llvmlibc/baremetal-semihosting.

* Add llvmlibc-samples directory to contain LLVM libc specific samples.
* Add single baremetal-semihosting sample using LLVM libc.

Sample contains a number of calls to LLVM libc functionality:
* heap.
* printf.
* string library.
* math library.

A special linker script is used to set the bounds of the heap. This uses
different linker defined symbols to picolibc.

The crt0.S from the existing llvmlibc-support can't be used directly as
the RW and ZI from the linker script need initializing.

Author: smithp35

CMakeLists.txt

@@ -378,6 +378,12 @@ if(LLVM_TOOLCHAIN_C_LIBRARY STREQUAL llvmlibc)
         COMPONENT llvm-toolchain-llvmlibc-configs
     )
 
+    install(
+      DIRECTORY
+      ${CMAKE_CURRENT_SOURCE_DIR}/llvmlibc-samples/src
+      DESTINATION samples
+      COMPONENT llvm-toolchain-llvmlibc-configs
+    )
     # We aren't yet able to build C++ libraries to go with llvm-libc
     set(CXX_LIBS OFF)
 

docs/llvmlibc.md

@@ -54,12 +54,21 @@ following command line options, in addition to `--target`, `-march` or
   `__llvm_libc_heap_limit` in addition to whatever other memory layout
   you want.
 
+* LLVM libc does not define errno. If you are using a function that
+  sets errno then you must implement the function `int *__llvm_libc_errno()`
+  that returns the address of your definition of errno.
+
 For example:
 
 ```
 clang --config=llvmlibc.cfg --target=arm-none-eabi -march=armv7m -o hello hello.c -lsemihost -lcrt0 -Wl,--defsym=__stack=0x200000
 ```
 
+## Samples
+
+The overlay package installs a llvmlibc directory in the samples/src
+directory containing sample programs that use LLVM libc.
+
 ## Limitations of LLVM libc in LLVM Embedded Toolchain for Arm
 
 At present, this toolchain does not build any C++ libraries to go with

llvmlibc-samples/src/llvmlibc/baremetal-semihosting/Makefile

@@ -0,0 +1,37 @@
+#
+# Copyright (c) 2020-2024, Arm Limited and affiliates.
+# SPDX-License-Identifier: Apache-2.0
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+include ../../../Makefile.conf
+
+build: hello.elf
+
+hello.elf: *.c
+	../$(BIN_PATH)/clang --config=llvmlibc.cfg $(MICROBIT_TARGET) -nostartfiles -lsemihost -g -fno-exceptions -fno-rtti -T microbit-llvmlibc.ld -o hello.elf $^
+
+%.hex: %.elf
+	../$(BIN_PATH)/llvm-objcopy -O ihex $< $@
+
+run: hello.hex
+	qemu-system-arm -M microbit -semihosting -nographic -device loader,file=$<
+
+debug: hello.hex
+	qemu-system-arm -M microbit -semihosting -nographic -device loader,file=$< -s -S
+
+clean:
+	rm -f *.elf *.hex
+
+.PHONY: clean run debug

llvmlibc-samples/src/llvmlibc/baremetal-semihosting/crt0llvmlibc.c

@@ -0,0 +1,41 @@
+// Copyright (c) 2024, Arm Limited and affiliates.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+
+//    http://www.apache.org/licenses/LICENSE-2.0
+
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <stddef.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdint.h>
+
+extern int main(int argc, char** argv);
+
+extern void _platform_init();
+
+extern char __data_source[];
+extern char __data_start[];
+extern char __data_end[];
+extern char __data_size[];
+extern char __bss_start[];
+extern char __bss_end[];
+extern char __bss_size[];
+extern char __tls_base[];
+extern char __tdata_end[];
+extern char __tls_end[];
+
+void _start(void) {
+  memcpy(__data_start, __data_source, (size_t) __data_size);
+  memset(__bss_start, 0, (size_t) __bss_size);
+  _platform_init();
+  _Exit(main(0, NULL));
+}

llvmlibc-samples/src/llvmlibc/baremetal-semihosting/hello.c

@@ -0,0 +1,46 @@
+// Copyright (c) 2024, Arm Limited and affiliates.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+
+//    http://www.apache.org/licenses/LICENSE-2.0
+
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <math.h>
+
+// Implementation of errno
+int *__llvm_libc_errno() {
+  static int internal_err;
+  return &internal_err;
+}
+
+// Example that uses heap, string and math library.
+
+int main(void) {
+  const char *hello_s = "hello ";
+  const char *world_s = "world";
+  const size_t hello_s_len = strlen(hello_s);
+  const size_t world_s_len = strlen(world_s);
+  const size_t out_s_len = hello_s_len + world_s_len + 1;
+  char *out_s = (char*) malloc(out_s_len);
+  assert(out_s_len >= hello_s_len + 1);
+  strncpy(out_s, hello_s, hello_s_len + 1);
+  assert(out_s_len >= strlen(out_s) + world_s_len + 1);
+  strncat(out_s, world_s, world_s_len + 1);
+  // 2024-10-17 Embedded printf implementation does not currently
+  // support printing floating point numbers.
+  printf("%s %li\n", out_s, lround(400000 * atanf(1.0f)));
+  free(out_s);
+  return 0;
+}