refactor: move BSS initializing from Assembly to Swift

Author: kkebo

Sources/AsmSupport/aarch64/asmfunc.S

@@ -29,4 +29,16 @@ get_kernel_end:
     add x0, x0, :lo12:__kernel_end
     ret
 
+.global get_bss_start
+get_bss_start:
+    adrp x0, __bss_start
+    add x0, x0, :lo12:__bss_start
+    ret
+
+.global get_bss_end
+get_bss_end:
+    adrp x0, __bss_end
+    add x0, x0, :lo12:__bss_end
+    ret
+
 #endif

Sources/AsmSupport/aarch64/boot.S

@@ -65,15 +65,6 @@ el1_entry:
     ldr x1, =_start
     mov sp, x1
 
-    // clear bss
-    ldr x1, =__bss_start
-    ldr w2, =__bss_size
-clear_bss_loop:
-    cbz w2, jump_to_main
-    str xzr, [x1], #8  // stores a 64-bit doubleword so __bss_end must be 8-byte aligned
-    sub w2, w2, #1
-    cbnz w2, clear_bss_loop
-
 jump_to_main:
     // jump to Swift code, should not return
     bl Kernel_main

Sources/AsmSupport/include/Support.h

@@ -6,6 +6,8 @@ void delay(uint64_t);
 void halt();
 uintptr_t get_kernel_start();
 uintptr_t get_kernel_end();
+uintptr_t get_bss_start();
+uintptr_t get_bss_end();
 #ifdef __aarch64__
 uint32_t get_el();
 #endif

Sources/Kernel/Kernel.swift

@@ -4,9 +4,25 @@ private import AsmSupport
     private import RaspberryPi
 #endif
 
+func zeroBSS() {
+    let start = get_bss_start()
+    let end = get_bss_end()
+    let size = end - start
+
+    precondition(start % 8 == 0)
+    precondition(size % 8 == 0)
+
+    // swift-format-ignore: NeverForceUnwrap
+    let ptr = unsafe UnsafeMutableRawPointer(bitPattern: start)!
+
+    unsafe ptr.initializeMemory(as: UInt64.self, repeating: 0, count: Int(size >> 3))
+}
+
 @main
 struct Kernel {
     static func main() {
+        zeroBSS()
+
         #if RASPI
             initUART()
 

linker.ld

@@ -24,4 +24,3 @@ SECTIONS {
         *(.swift_modhash*)
     }
 }
-__bss_size = (__bss_end - __bss_start) >> 3;