arm: enable functions in RAM for go & cgo

src/examples/ram-func/main.go

@@ -0,0 +1,93 @@
+package main
+
+// This example demonstrates how to use go:section to place code into RAM for
+// execution.  The code is present in flash in the `.data` region and copied
+// into the correct place in RAM early in startup sequence (at the same time
+// as non-zero global variables are initialized).
+//
+// This example should work on any ARM Cortex MCU.
+//
+// For Go code use the pragma "//go:section", for cgo use the "section" and
+// "noinline" attributes.  The `.ramfuncs` section is explicitly placed into
+// the `.data` region by the linker script.
+//
+// Running the example should print out the program counter from the functions
+// below.  The program counters should be in different memory regions.
+//
+// On RP2040, for example, the output is something like this:
+//
+//  Go in RAM:    0x20000DB4
+//  Go in flash:  0x10007610
+//  cgo in RAM:   0x20000DB8
+//  cgo in flash: 0x10002C26
+//
+// This can be confirmed using `objdump -t xxx.elf | grep main | sort`:
+//
+//  00000000 l    df *ABS*  00000000 main
+//  1000760d l     F .text  00000004 main.in_flash
+//  10007611 l     F .text  0000000c __Thumbv6MABSLongThunk_main.in_ram
+//  1000761d l     F .text  0000000c __Thumbv6MABSLongThunk__Cgo_static_eea7585d7291176ad3bb_main_c_in_ram
+//  1000bdb5 l     O .text  00000013 main$string
+//  1000bdc8 l     O .text  00000013 main$string.1
+//  1000bddb l     O .text  00000013 main$string.2
+//  1000bdee l     O .text  00000013 main$string.3
+//  20000db1 l     F .data  00000004 main.in_ram
+//  20000db5 l     F .data  00000004 _Cgo_static_eea7585d7291176ad3bb_main_c_in_ram
+//
+
+import (
+	"device"
+	"fmt"
+	"time"
+	_ "unsafe" // unsafe is required for "//go:section"
+)
+
+/*
+	#define ram_func __attribute__((section(".ramfuncs"),noinline))
+
+	static ram_func void* main_c_in_ram() {
+		void* p = 0;
+
+		asm(
+			"MOV %0, PC"
+			: "=r"(p)
+		);
+
+		return p;
+	}
+
+	static void* main_c_in_flash() {
+		void* p = 0;
+
+		asm(
+			"MOV %0, PC"
+			: "=r"(p)
+		);
+
+		return p;
+	}
+*/
+import "C"
+
+func main() {
+	time.Sleep(2 * time.Second)
+
+	fmt.Printf("Go in RAM:    0x%X\n", in_ram())
+	fmt.Printf("Go in flash:  0x%X\n", in_flash())
+	fmt.Printf("cgo in RAM:   0x%X\n", C.main_c_in_ram())
+	fmt.Printf("cgo in flash: 0x%X\n", C.main_c_in_flash())
+}
+
+//go:section .ramfuncs
+func in_ram() uintptr {
+	return device.AsmFull("MOV {}, PC", nil)
+}
+
+// go:noinline used here to prevent function being 'inlined' into main()
+// so it appears in objdump output.  In normal use, go:inline is not
+// required for functions running from flash (flash is the default).
+//
+//go:noinline
+func in_flash() uintptr {
+	return device.AsmFull("MOV {}, PC", nil)
+}