Make spacer sections allocatable (#2515)

* Default to allocating spacer sections (stack & heap), with PICO_CRT0_ALLOCATE_SPACERS config option to disable the behaviour

* Add pico_check_linker_script function to check for compatibility of custom linker scripts

* tweak error message

---------

Co-authored-by: Graham Sanderson <graham.sanderson@raspberrypi.com>

src/rp2_common/pico_crt0/crt0.S

@@ -582,10 +582,16 @@ runtime_init:
 //
 // Strictly the most correct thing to do (as .stack and .heap are unreferenced) is to mark them as "a", and also KEEP, which
 // works correctly for both GCC and Clang, however doing so may break anyone who already has custom linker scripts without
-// the KEEP. Therefore we will only add the "a" on Clang, but will also use KEEP to our own linker scripts.
+// the KEEP. Therefore we add a define of PICO_CRT0_ALLOCATE_SPACERS to switch between the old and new behaviour, so anyone
+// with custom linker scripts without the KEEP can set it to 0 (or update their linker script).
+
+// PICO_CONFIG: PICO_CRT0_ALLOCATE_SPACERS, Set spacer sections as allocatable. This makes them appear in print-memory-usage but is incompatible with linker scripts that do not KEEP the sections, type=bool, default=1, advanced=true, group=pico_crt0
+#ifndef PICO_CRT0_ALLOCATE_SPACERS
+#define PICO_CRT0_ALLOCATE_SPACERS 1
+#endif
 
 .macro spacer_section name
-#if PICO_ASSEMBLER_IS_CLANG
+#if PICO_CRT0_ALLOCATE_SPACERS
 .section \name, "a"
 #else
 .section \name

src/rp2_common/pico_standard_link/CMakeLists.txt

@@ -25,11 +25,48 @@ if (NOT TARGET pico_standard_link)
         set_target_properties(${TARGET} PROPERTIES ${PROP} "${_LINK_DEPENDS}")
     endfunction()
 
+    # pico_check_linker_script(LDSCRIPT)
+    # \brief_nodesc\ Check the linker script for compatibility
+    #
+    # Checks the linker script for compatibility with the current SDK version,
+    # and if not, raises warnings and enables workarounds to maintain
+    # compatibility where possible.
+    #
+    # \param\ LDSCRIPT Full path to the linker script to check
+    function(pico_check_linker_script TARGET LDSCRIPT)
+        if (EXISTS ${LDSCRIPT})
+            file(READ ${LDSCRIPT} LDSCRIPT_CONTENTS)
+        else()
+            return()
+        endif()
+
+        # Check if the linker script uses KEEP to keep the .stack and .heap sections
+        # and if not, set PICO_CRT0_ALLOCATE_SPACERS to 0 to maintain compatibility
+        string(FIND "${LDSCRIPT_CONTENTS}" "KEEP(*(.stack*))" KEEP_STACK_FOUND)
+        string(FIND "${LDSCRIPT_CONTENTS}" "KEEP(*(.heap*))" KEEP_HEAP_FOUND)
+        string(FIND "${LDSCRIPT_CONTENTS}" "*(.stack*)" STACK_FOUND)
+        string(FIND "${LDSCRIPT_CONTENTS}" "*(.heap*)" HEAP_FOUND)
+        set(PICO_CRT0_ALLOCATE_SPACERS TRUE)
+        if ((${STACK_FOUND} GREATER -1) AND NOT (${KEEP_STACK_FOUND} GREATER -1))
+            message(WARNING "Linker script ${LDSCRIPT} does not KEEP the .stack section - replace `*(.stack*)` with `KEEP(*(.stack*))`")
+            set(PICO_CRT0_ALLOCATE_SPACERS FALSE)
+        endif()
+        if ((${HEAP_FOUND} GREATER -1) AND NOT (${KEEP_HEAP_FOUND} GREATER -1))
+            message(WARNING "Linker script ${LDSCRIPT} does not KEEP the .heap section - replace `*(.heap*)` with `KEEP(*(.heap*))`")
+            set(PICO_CRT0_ALLOCATE_SPACERS FALSE)
+        endif()
+        if (NOT ${PICO_CRT0_ALLOCATE_SPACERS})
+            message(WARNING "Linker script ${LDSCRIPT} is incompatible with certain Pico SDK >2.1.1 features; setting PICO_CRT0_ALLOCATE_SPACERS=0 as a workaround")
+            target_compile_definitions(${TARGET} PRIVATE PICO_CRT0_ALLOCATE_SPACERS=0)
+        endif()
+    endfunction()
+
     # pico_set_linker_script(TARGET LDSCRIPT)
     # \brief\ Set the linker script for the target
     #
     # \param\ LDSCRIPT Full path to the linker script to set
     function(pico_set_linker_script TARGET LDSCRIPT)
+        pico_check_linker_script(${TARGET} ${LDSCRIPT})
         set_target_properties(${TARGET} PROPERTIES PICO_TARGET_LINKER_SCRIPT ${LDSCRIPT})
     endfunction()