nintendoswitch: support outputting .nro files directly

By modifying the linker script a bit and adding the NRO0 header directly
in the assembly, it's possible to craft an ELF file that can be
converted straight to a binary (using objcopy or similar) that is a NRO
file. This avoids custom code for NRO files or an extra build step.

With another change, .nro files are recognized by TinyGo so that this
will create a ready-to-run NRO file:

    tinygo build -o test.nro -target=nintendoswitch examples/serial

Makefile

@@ -373,8 +373,8 @@ endif
 	@$(MD5SUM) test.hex
 	$(TINYGO) build -size short -o test.hex -target=pca10040 -opt=1     examples/blinky1
 	@$(MD5SUM) test.hex
-	$(TINYGO) build             -o test.elf -target=nintendoswitch      examples/serial
-	@$(MD5SUM) test.elf
+	$(TINYGO) build             -o test.nro -target=nintendoswitch      examples/serial
+	@$(MD5SUM) test.nro
 
 wasmtest:
 	$(GO) test ./tests/wasm

compileopts/config.go

@@ -243,7 +243,7 @@ func (c *Config) Debug() bool {
 // extension and the configured binary format in the target JSON file.
 func (c *Config) BinaryFormat(ext string) string {
 	switch ext {
-	case ".bin", ".gba":
+	case ".bin", ".gba", ".nro":
 		// The simplest format possible: dump everything in a raw binary file.
 		if c.Target.BinaryFormat != "" {
 			return c.Target.BinaryFormat

targets/nintendoswitch.ld

@@ -1,61 +1,69 @@
-OUTPUT_FORMAT(elf64-littleaarch64)
-OUTPUT_ARCH(aarch64)
-ENTRY(_start)
-
-PHDRS
-{
-  text    PT_LOAD     FLAGS(5);
-  rodata  PT_LOAD     FLAGS(4);
-  data    PT_LOAD     FLAGS(6);
-  bss     PT_LOAD     FLAGS(6);
-  dynamic PT_DYNAMIC;
-}
-
 SECTIONS
 {
   . = 0;
 
-  .text : ALIGN(0x1000) {
+  /* Code and file header */
+
+  .text : {
     HIDDEN(__text_start = .);
     KEEP(*(.text.jmp))
 
     . = 0x80;
 
     *(.text .text.*)
+
+    . = ALIGN(0x1000);
+    HIDDEN(__text_end = .);
+    HIDDEN(__text_size = . - __text_start);
   }
 
   /* Read-only sections */
 
-  . = ALIGN(0x1000);
+  .rodata : {
+    HIDDEN(__rodata_start = .);
+
+    *(.rodata .rodata.*)
+
+    *(.got)
 
-  .rodata : { *(.rodata .rodata.*) } :rodata
-  .mod0 : {
     KEEP(crt0.nso.o(.data.mod0))
     KEEP(crt0.nro.o(.data.mod0))
     KEEP(crt0.lib.nro.o(.data.mod0))
+    KEEP(*(.data.mod0))
+
+    HIDDEN(__dynamic_start = .);
+    *(.dynamic)
+
+    . = ALIGN(0x1000);
+    HIDDEN(__rodata_end = .);
+    HIDDEN(__rodata_size = . - __rodata_start);
   }
 
   /* Read-write sections */
-   . = ALIGN(0x1000);
 
   .data : {
-    *(.data .data.*)
-  } :data
+    HIDDEN(__data_start = .);
 
-  .dynamic : {
-    HIDDEN(__dynamic_start = .);
-    *(.dynamic)
+    *(.data .data.*)
+
+    HIDDEN(__data_end = .);
+    HIDDEN(__data_size = . - __data_start);
   }
 
   /* BSS section */
 
-  . = ALIGN(0x1000);
-
   .bss : {
     HIDDEN(__bss_start = .);
+
     *(.bss .bss.*)
     *(COMMON)
-    . = ALIGN(8);
+
     HIDDEN(__bss_end = .);
-  } :bss
+    HIDDEN(__bss_size = . - __bss_start);
+  }
+
+  /DISCARD/ :
+  {
+    *(.eh_frame) /* This is probably unnecessary and bloats the binary. */
+  }
 }

targets/nintendoswitch.s

@@ -1,8 +1,34 @@
+// For more information on the .nro file format, see:
+// https://switchbrew.org/wiki/NRO
+
 .section .text.jmp, "x"
 .global _start
 _start:
     b start
     .word _mod_header - _start
+    .word 0
+    .word 0
+
+    .ascii "NRO0"              // magic
+    .word 0                    // version (always 0)
+    .word __bss_start - _start // total NRO file size
+    .word 0                    // flags (unused)
+
+    // segment headers
+    .word __text_start
+    .word __text_size
+    .word __rodata_start
+    .word __rodata_size
+    .word __data_start
+    .word __data_size
+    .word __bss_size
+    .word 0
+
+    // ModuleId (not supported)
+    . = 0x50; // skip 32 bytes
+
+    .word 0 // DSO Module Offset (unused)
+    .word 0 // reserved (unused)
 
 .section .data.mod0
     .word 0, 8