Use picolibc instead of newlib-nano

 1. Support picolibc stdio. There was already picolibc support code
    present in the library for the ARM LLVM toolchain. The
    conditionals which selected it have been changed to use
    __PICOLIBC__ instead of __clang_major__.

 2. Add _exit stub. Code using assert or abort end up calling _exit
    through the picolibc signal handling code.

 3. Switch to picolibc.specs. This is needed for toolchains which
    don't use picolibc by default, and can also be used without
    trouble in toolchains where picolibc is the default.

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

Author: keith-packard

platform/CMakeLists.txt

@@ -90,6 +90,7 @@ target_sources(platform_s
 target_sources(tfm_s
     PRIVATE
         ext/common/faults.c
+        ext/common/picolibc.c
 )
 
 target_link_libraries(platform_s

platform/ext/common/picolibc.c

@@ -0,0 +1,48 @@
+/*
+ * Copyright © 2025, Keith Packard <[email protected]>
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ */
+
+#include <stdint.h>
+#include <string.h>
+#include "config_impl.h"
+
+/*
+ * Picolibc's startup code only initializes a single data and bss
+ * segment.  Augment that by initializing the remaining segments using
+ * the lists provided by the linker script in a constructor which will
+ * be called from _start
+ */
+
+#if defined(__PICOLIBC__) && TFM_ISOLATION_LEVEL > 0
+
+static void __attribute__((constructor))
+picolibc_startup(void)
+{
+  typedef struct __copy_table {
+    uint32_t const* src;
+    uint32_t* dest;
+    uint32_t  wlen;
+  } __copy_table_t;
+
+  typedef struct __zero_table {
+    uint32_t* dest;
+    uint32_t  wlen;
+  } __zero_table_t;
+
+  extern const __copy_table_t __copy_table_start__;
+  extern const __copy_table_t __copy_table_end__;
+  extern const __zero_table_t __zero_table_start__;
+  extern const __zero_table_t __zero_table_end__;
+
+  for (__copy_table_t const* pTable = &__copy_table_start__; pTable < &__copy_table_end__; ++pTable) {
+    memcpy(pTable->dest, pTable->src, pTable->wlen << 2);
+  }
+
+  for (__zero_table_t const* pTable = &__zero_table_start__; pTable < &__zero_table_end__; ++pTable) {
+    memset(pTable->dest, 0, pTable->wlen << 2);
+  }
+}
+#endif

platform/ext/common/syscalls_stub.c

@@ -14,6 +14,14 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#ifdef __PICOLIBC__
+__attribute__((weak))
+void _exit(int status)
+{
+    (void) status;
+    for(;;);
+}
+#else
 __attribute__((weak))
 void _close(void)
 {
@@ -53,3 +61,5 @@ __attribute__((weak))
 void _write(void)
 {
 }
+
+#endif /* !__PICOLIBC__ */

platform/ext/common/uart_stdout.c

@@ -85,7 +85,7 @@ int fputc(int ch, FILE *f)
 /* Redirect sdtio for PicoLib in LLVM toolchain
    as per https://github.com/picolibc/picolibc/blob/main/doc/os.md
    'fputch()' named intentionally different from 'fputc()' from picolib */
-#elif defined(__clang_major__)
+#elif defined(__PICOLIBC__)
 
 int fputch(char ch, struct __file *f)
 {

platform/ns/toolchain_ns_GNUARM.cmake

@@ -177,8 +177,7 @@ string(APPEND CMAKE_ASM_LINK_FLAGS " " ${LINKER_CP_OPTION})
 add_compile_definitions($<$<STREQUAL:${TFM_SYSTEM_ARCHITECTURE},armv8.1-m.main>:__ARM_ARCH_8_1M_MAIN__=1>)
 
 add_compile_options(
-    -specs=nano.specs
-    -specs=nosys.specs
+    -specs=picolibc.specs
     -Wall
     -Wno-format
     -Wno-array-parameter
@@ -200,8 +199,8 @@ add_compile_options(
 
 add_link_options(
     --entry=Reset_Handler
-    -specs=nano.specs
-    -specs=nosys.specs
+    -specs=picolibc.specs
+    -mthumb
     LINKER:-check-sections
     LINKER:-fatal-warnings
     LINKER:--gc-sections

toolchain_GNUARM.cmake

@@ -107,8 +107,7 @@ if(GCC_VERSION VERSION_GREATER_EQUAL "8.0.0")
 endif()
 
 add_compile_options(
-    -specs=nano.specs
-    -specs=nosys.specs
+    -specs=picolibc.specs
     -Wall
     -Wno-format
     -Wno-array-parameter
@@ -147,8 +146,8 @@ endif()
 
 add_link_options(
     --entry=Reset_Handler
-    -specs=nano.specs
-    -specs=nosys.specs
+    -specs=picolibc.specs
+    -mthumb
     LINKER:-check-sections
     LINKER:-fatal-warnings
     LINKER:--gc-sections