Work around issue with unreferenced sections marked allocated in the asm being discarded by GNU linker (#1391)

src/rp2_common/pico_platform/include/pico/platform.h

@@ -523,6 +523,18 @@ __force_inline static uint get_core_num(void) {
 
 #else // __ASSEMBLER__
 
+#if defined __GNUC__
+// note LLVM defines __GNUC__
+#ifdef __clang__
+#define PICO_ASSEMBLER_IS_CLANG 1
+#else
+#define PICO_ASSEMBLER_IS_GNU 1
+#endif
+#elif defined __ICCARM__
+#else
+#error Unsupported toolchain
+#endif
+
 #define WRAPPER_FUNC_NAME(x) __wrap_##x
 #define SECTION_NAME(x) .text.##x
 #define RAM_SECTION_NAME(x) .time_critical.##x

src/rp2_common/pico_standard_link/crt0.S

@@ -324,13 +324,29 @@ hold_non_core0_in_bootrom:
 // ----------------------------------------------------------------------------
 // Stack/heap dummies to set size
 
-.section .stack, "a"
+// Prior to SDK 1.5.1 these were `.section .stack` without the `, "a"`... Clang linker gives a warning about this,
+// however setting it explicitly to `, "a"` makes GCC *now* discard the section unless it is also KEEP. This
+// seems like very surprising behavior!
+//
+// 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.
+
+.macro spacer_section name
+#if PICO_ASSEMBLER_IS_CLANG
+.section \name, "a"
+#else
+.section \name
+#endif
+.endm
+
+spacer_section .stack
 // align to allow for memory protection (although this alignment is pretty much ignored by linker script)
-.align 5
+.p2align 5
     .equ StackSize, PICO_STACK_SIZE
 .space StackSize
 
-.section .heap, "a"
-.align 2
+spacer_section .heap
+.p2align 2
     .equ HeapSize, PICO_HEAP_SIZE
 .space HeapSize

src/rp2_common/pico_standard_link/memmap_blocked_ram.ld

@@ -208,7 +208,7 @@ SECTIONS
     {
         __end__ = .;
         end = __end__;
-        *(.heap*)
+        KEEP(*(.heap*))
         __HeapLimit = .;
     } > RAM
 
@@ -227,7 +227,7 @@ SECTIONS
     } > SCRATCH_X
     .stack_dummy (NOLOAD):
     {
-        *(.stack*)
+        KEEP(*(.stack*))
     } > SCRATCH_Y
 
     .flash_end : {

src/rp2_common/pico_standard_link/memmap_copy_to_ram.ld

@@ -210,7 +210,7 @@ SECTIONS
     {
         __end__ = .;
         end = __end__;
-        *(.heap*)
+        KEEP(*(.heap*))
         __HeapLimit = .;
     } > RAM
 
@@ -229,7 +229,7 @@ SECTIONS
     } > SCRATCH_X
     .stack_dummy (NOLOAD):
     {
-        *(.stack*)
+        KEEP(*(.stack*))
     } > SCRATCH_Y
 
     .flash_end : {

src/rp2_common/pico_standard_link/memmap_default.ld

@@ -208,7 +208,7 @@ SECTIONS
     {
         __end__ = .;
         end = __end__;
-        *(.heap*)
+        KEEP(*(.heap*))
         __HeapLimit = .;
     } > RAM
 
@@ -227,7 +227,7 @@ SECTIONS
     } > SCRATCH_X
     .stack_dummy (NOLOAD):
     {
-        *(.stack*)
+        KEEP(*(.stack*))
     } > SCRATCH_Y
 
     .flash_end : {

src/rp2_common/pico_standard_link/memmap_no_flash.ld

@@ -178,7 +178,7 @@ SECTIONS
     {
         __end__ = .;
         end = __end__;
-        *(.heap*)
+        KEEP(*(.heap*))
         __HeapLimit = .;
     } > RAM
 
@@ -197,7 +197,7 @@ SECTIONS
     } > SCRATCH_X
     .stack_dummy (NOLOAD):
     {
-        *(.stack*)
+        KEEP(*(.stack*))
     } > SCRATCH_Y
 
     /* stack limit is poorly named, but historically is maximum heap ptr */