arm: enable functions in RAM for go & cgo

compiler/symbol.go

@@ -279,8 +279,12 @@ func (info *functionInfo) parsePragmas(f *ssa.Function) {
 					info.linkName = parts[2]
 				}
 			case "//go:section":
+				// Only enable go:section when the package imports "unsafe".
+				// go:section also implies go:noinline since inlining could
+				// move the code to a different section than that requested.
 				if len(parts) == 2 && hasUnsafeImport(f.Pkg.Pkg) {
 					info.section = parts[1]
+					info.inline = inlineNone
 				}
 			case "//go:nobounds":
 				// Skip bounds checking in this function. Useful for some

compiler/testdata/pragma.ll

@@ -47,13 +47,13 @@ entry:
   ret void
 }
 
-; Function Attrs: nounwind
+; Function Attrs: noinline nounwind
-define hidden void @main.functionInSection(ptr %context) unnamed_addr #1 section ".special_function_section" {
+define hidden void @main.functionInSection(ptr %context) unnamed_addr #4 section ".special_function_section" {
 entry:
   ret void
 }
 
-; Function Attrs: nounwind
+; Function Attrs: noinline nounwind
 define void @exportedFunctionInSection() #5 section ".special_function_section" {
 entry:
   ret void
@@ -66,4 +66,4 @@ attributes #1 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,
 attributes #2 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" "wasm-export-name"="extern_func" "wasm-import-module"="env" "wasm-import-name"="extern_func" }
 attributes #3 = { inlinehint nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" }
 attributes #4 = { noinline nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" }
-attributes #5 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" "wasm-export-name"="exportedFunctionInSection" "wasm-import-module"="env" "wasm-import-name"="exportedFunctionInSection" }
+attributes #5 = { noinline nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" "wasm-export-name"="exportedFunctionInSection" "wasm-import-module"="env" "wasm-import-name"="exportedFunctionInSection" }

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)
+}

targets/arm.ld

@@ -42,6 +42,8 @@ SECTIONS
         *(.data)
         *(.data.*)
         . = ALIGN(4);
+        *(.ramfuncs*)      /* Functions that must execute from RAM */
+        . = ALIGN(4);
         _edata = .;        /* used by startup code */
     } >RAM AT>FLASH_TEXT