Add PICO_CRT0_NEAR_CALLS to indicate that crt0 can call runtimine_init/main/exit etc via near calls (#2452)

---------

Co-authored-by: will-v-pi <108662275+will-v-pi@users.noreply.github.com>

Author: kilograham

src/rp2_common/pico_crt0/crt0.S

@@ -15,6 +15,11 @@
 #include "boot/picobin.h"
 #include "pico/bootrom.h"
 
+// PICO_CONFIG: PICO_CRT0_NEAR_CALLS, Whether calls from crt0 into the binary are near (<16M away) - ignored for PICO_COPY_TO_RAM, default=0, type=bool, group=pico_crt0
+#ifndef PICO_CRT0_NEAR_CALLS
+#define PICO_CRT0_NEAR_CALLS 0
+#endif
+
 #ifdef NDEBUG
 #ifndef COLLAPSE_IRQS
 #define COLLAPSE_IRQS
@@ -377,6 +382,12 @@ _entry_point:
     sev
 1:
 #endif
+#if !__ARM_ARCH_6M__
+    // Make sure stack limit is 0 if we came in thru the debugger; we do not know what it should be
+    movs r0, #0
+    msr msplim, r0
+#endif
+
     ldr r0, =__vectors
     // Vector through our own table (SP, VTOR will not have been set up at
     // this point). Same path for debugger entry and bootloader entry.
@@ -403,6 +414,9 @@ _enter_vtable_in_r0:
 .type _reset_handler,%function
 .thumb_func
 _reset_handler:
+    // Note if we entered thru here on core 0, then we should have gone thru bootrom, so SP (and MSPLIM) on Armv8-M
+    // should already be set
+
     // Only core 0 should run the C runtime startup code; core 1 is normally
     // sleeping in the bootrom at this point but check to be sure (e.g. if
     // debugger put core 1 at the ELF entry point for some reason)
@@ -465,20 +479,20 @@ bss_fill_test:
     bne bss_fill_loop
 
 platform_entry: // symbol for stack traces
+#if PICO_CRT0_NEAR_CALLS && !PICO_COPY_TO_RAM
+    bl runtime_init
+    bl main
+    bl exit
+#else
     // Use 32-bit jumps, in case these symbols are moved out of branch range
     // (e.g. if main is in SRAM and crt0 in flash)
-#if !__ARM_ARCH_6M__
-    // Make sure stack limit is 0 - the user can set it themselves
-    //  todo probably worth adding to the EXE_DEF in the future
-    movs r0, #0
-    msr msplim, r0
-#endif
     ldr r1, =runtime_init
     blx r1
     ldr r1, =main
     blx r1
     ldr r1, =exit
     blx r1
+#endif
     // exit should not return.  If it does, hang the core.
 1: // separate label because _exit can be moved out of branch range
     bkpt #0

src/rp2_common/pico_runtime/CMakeLists.txt

@@ -68,6 +68,7 @@ endif()
 # FPGA_CHECK            - checks for FPGA which allows Raspberry Pi to run your binary on FPGA;
 # PANIC                 - default panic impl which brings in stdio;
 # AUTO_INIT_MUTEX       - auto init mutexes, without this you get no printf mutex either;
+# CRT0_FAR_CALLS        - use blx not bl for calls from crt0 to user overridable functions;
 #
 # \param\ INCLUDE The items to include
 # \param\ EXCLUDE The items to exclude
@@ -148,4 +149,7 @@ function(pico_minimize_runtime TARGET)
     if (NOT RUNTIME_INCLUDE_FPGA_CHECK)
         target_compile_definitions(${TARGET} PRIVATE PICO_NO_FPGA_CHECK=1)
     endif()
-endfunction()
+    if (NOT RUNTIME_CRT0_FAR_CALLS)
+        target_compile_definitions(${TARGET} PRIVATE PICO_CRT0_NEAR_CALLS=1)
+    endif()
+endfunction()