Merge #10

10: support linking with lld r=dvc94ch a=danc86

Rust now ships an embedded copy of lld, so if we can link crates with lld it will make the getting started experience way smoother (don't need to find a separate GNU ld).

Co-authored-by: Dan Callaghan <djc@djc.id.au>

riscv-rt/link.x

@@ -7,7 +7,7 @@ SECTIONS
 {
   PROVIDE(_stext = ORIGIN(FLASH));
 
-  .text _stext : ALIGN(4)
+  .text ALIGN(_stext,4) :
   {
     /* Put reset handler first in .text section so it ends up as the entry */
     /* point of the program. */
@@ -19,21 +19,20 @@ SECTIONS
     *(.text .text.*);
   } > FLASH
 
-  .rodata : ALIGN(4)
+  .rodata ALIGN(4) :
   {
     *(.rodata .rodata.*);
-    . = ALIGN(4);
   } > FLASH
 
-  PROVIDE(_sbss = ORIGIN(RAM));
-  .bss _sbss : ALIGN(4)
+  .bss :
   {
+    _sbss = .;
     *(.bss .bss.*);
     . = ALIGN(4);
     _ebss = .;
   } > RAM
 
-  .data _ebss : ALIGN(4)
+  .data : AT(LOADADDR(.rodata) + SIZEOF(.rodata))
   {
     _sidata = LOADADDR(.data);
     _sdata = .;
@@ -42,57 +41,45 @@ SECTIONS
     *(.data .data.*);
     . = ALIGN(4);
     _edata = .;
-  } > RAM AT > FLASH /* LLD fails on AT > FLASH */
+  } > RAM
 
   PROVIDE(_heap_size = 0);
 
   /* fictitious region that represents the memory available for the heap */
-  .heap _edata (INFO) : ALIGN(4)
+  .heap (INFO) :
   {
     _sheap = .;
     . += _heap_size;
     . = ALIGN(4);
     _eheap = .;
-  }
+  } > RAM
 
   /* fictitious region that represents the memory available for the stack */
-  .stack _eheap (INFO) : ALIGN(4)
+  .stack (INFO) :
   {
     _estack = .;
     . = _stack_start;
     _sstack = .;
-  }
+  } > RAM
 
   /* fake output .got section */
   /* Dynamic relocations are unsupported. This section is only used to detect
      relocatable code in the input files and raise an error if relocatable code
      is found */
-  .got :
+  .got (INFO) :
   {
-    _sgot = .;
     KEEP(*(.got .got.*));
-    _egot = .;
-  } > RAM AT > FLASH /* LLD fails on AT > FLASH */
-
+  }
 
-  /* Due to an unfortunate combination of legacy concerns,
-     toolchain drawbacks, and insufficient attention to detail,
-     rustc has no choice but to mark .debug_gdb_scripts as allocatable.
-     We really do not want to upload it to our target, so we
-     remove the allocatable bit. Unfortunately, it appears
-     that the only way to do this in a linker script is
-     the extremely obscure "INFO" output section type specifier. */
-  /* a rustc hack will force the program to read the first byte of this section,
-     so we'll set the (fake) start address of this section to something we're
-     sure can be read at runtime: the start of the .text section */
-  /* LLD fails to parse _stext (INFO) */
-  .debug_gdb_scripts _stext (INFO) : {
-    KEEP(*(.debug_gdb_scripts))
+  /* Discard .eh_frame, we are not doing unwind on panic so it is not needed */
+  /DISCARD/ :
+  {
+    *(.eh_frame);
   }
 }
 
 /* Do not exceed this mark in the error messages below                | */
-ASSERT(_sgot == _egot, "
+ASSERT(SIZEOF(.got) == 0, "
 .got section detected in the input files. Dynamic relocations are not
 supported. If you are linking to C code compiled using the `gcc` crate
 then modify your build script to compile the C code _without_ the

riscv-rt/src/lib.rs

@@ -205,7 +205,7 @@ extern "C" {
 /// pointer. Then it calls _start_rust.
 #[cfg(any(target_arch = "riscv32", target_arch = "riscv64"))]
 global_asm!(r#"
-.section .init
+.section .init, "ax"
 .globl _start
 _start:
   .cfi_startproc
@@ -295,7 +295,7 @@ macro_rules! entry {
 /// restores caller saved registers and then returns.
 #[cfg(any(target_arch = "riscv32", target_arch = "riscv64"))]
 global_asm!(r#"
-  .section .trap
+  .section .trap, "ax"
   .align 4
   .global _start_trap