How can I prevent functions from being aligned to 16 bytes boundary when compiling for X86?

Question

I'm working in an embedded-like environment where each byte is extremely precious, much more so than additional cycles for unaligned accesses. I have some simple Rust code from an OS development example:

#![feature(lang_items)]
#![no_std]
extern crate rlibc;
#[no_mangle]
pub extern fn rust_main() {

    // ATTENTION: we have a very small stack and no guard page

    let hello = b"Hello World!";
    let color_byte = 0x1f; // white foreground, blue background

    let mut hello_colored = [color_byte; 24];
    for (i, char_byte) in hello.into_iter().enumerate() {
        hello_colored[i*2] = *char_byte;
    }

    // write `Hello World!` to the center of the VGA text buffer
    let buffer_ptr = (0xb8000 + 1988) as *mut _;
    unsafe { *buffer_ptr = hello_colored };

    loop{}

}

#[lang = "eh_personality"] extern fn eh_personality() {}
#[lang = "panic_fmt"] #[no_mangle] pub extern fn panic_fmt() -> ! {loop{}}

I also use this linker script:

OUTPUT_FORMAT("binary")
ENTRY(rust_main)
phys = 0x0000;
SECTIONS
{
  .text phys : AT(phys) {
    code = .;
    *(.text.start);
    *(.text*)
    *(.rodata)
    . = ALIGN(4);
  }
  __text_end=.;
  .data : AT(phys + (data - code))
  {
    data = .;
    *(.data)
    . = ALIGN(4);
  }
  __data_end=.;
  .bss : AT(phys + (bss - code))
  {
    bss = .;
    *(.bss)
    . = ALIGN(4);
  }
  __binary_end = .;
}

I optimize it with opt-level: 3 and LTO using an i586 targeted compiler and the GNU ld linker, including -O3 in the linker command. I've also tried opt-level: z and a coupled -Os at the linker, but this resulted in code that was bigger (it didn't unroll the loop). As it stands, the size seems pretty reasonable with opt-level: 3.

There are quite a few bytes that seem wasted on aligning functions to some boundary. After the unrolled loop, 7 nop instructions are inserted and then there is an infinite loop as expected. After this, there appears to be another infinite loop that is preceded by 7 16-bit override nop instructions (ie, xchg ax,ax rather than xchg eax,eax). This adds up to about 26 bytes wasted in a 196 byte flat binary.

• What exactly is the optimizer doing here?
• What options do I have to disable it?
• Why is unreachable code being included in the binary?

The full assembly listing below:

   0:   c6 05 c4 87 0b 00 48    movb   $0x48,0xb87c4
   7:   c6 05 c5 87 0b 00 1f    movb   $0x1f,0xb87c5
   e:   c6 05 c6 87 0b 00 65    movb   $0x65,0xb87c6
  15:   c6 05 c7 87 0b 00 1f    movb   $0x1f,0xb87c7
  1c:   c6 05 c8 87 0b 00 6c    movb   $0x6c,0xb87c8
  23:   c6 05 c9 87 0b 00 1f    movb   $0x1f,0xb87c9
  2a:   c6 05 ca 87 0b 00 6c    movb   $0x6c,0xb87ca
  31:   c6 05 cb 87 0b 00 1f    movb   $0x1f,0xb87cb
  38:   c6 05 cc 87 0b 00 6f    movb   $0x6f,0xb87cc
  3f:   c6 05 cd 87 0b 00 1f    movb   $0x1f,0xb87cd
  46:   c6 05 ce 87 0b 00 20    movb   $0x20,0xb87ce
  4d:   c6 05 cf 87 0b 00 1f    movb   $0x1f,0xb87cf
  54:   c6 05 d0 87 0b 00 57    movb   $0x57,0xb87d0
  5b:   c6 05 d1 87 0b 00 1f    movb   $0x1f,0xb87d1
  62:   c6 05 d2 87 0b 00 6f    movb   $0x6f,0xb87d2
  69:   c6 05 d3 87 0b 00 1f    movb   $0x1f,0xb87d3
  70:   c6 05 d4 87 0b 00 72    movb   $0x72,0xb87d4
  77:   c6 05 d5 87 0b 00 1f    movb   $0x1f,0xb87d5
  7e:   c6 05 d6 87 0b 00 6c    movb   $0x6c,0xb87d6
  85:   c6 05 d7 87 0b 00 1f    movb   $0x1f,0xb87d7
  8c:   c6 05 d8 87 0b 00 64    movb   $0x64,0xb87d8
  93:   c6 05 d9 87 0b 00 1f    movb   $0x1f,0xb87d9
  9a:   c6 05 da 87 0b 00 21    movb   $0x21,0xb87da
  a1:   c6 05 db 87 0b 00 1f    movb   $0x1f,0xb87db
  a8:   90                      nop
  a9:   90                      nop
  aa:   90                      nop
  ab:   90                      nop
  ac:   90                      nop
  ad:   90                      nop
  ae:   90                      nop
  af:   90                      nop
  b0:   eb fe                   jmp    0xb0
  b2:   66 90                   xchg   %ax,%ax
  b4:   66 90                   xchg   %ax,%ax
  b6:   66 90                   xchg   %ax,%ax
  b8:   66 90                   xchg   %ax,%ax
  ba:   66 90                   xchg   %ax,%ax
  bc:   66 90                   xchg   %ax,%ax
  be:   66 90                   xchg   %ax,%ax
  c0:   eb fe                   jmp    0xc0
  c2:   66 90                   xchg   %ax,%ax

Answer 1

As Ross states, aligning functions and branch points to 16 bytes is a common x86 optimization recommended by Intel, although it can occasionally be less efficient, such as in your case. For a compiler to optimally decide whether or not to align is a hard problem, and I believe LLVM simply opts to always align. See more info on Performance optimisations of x86-64 assembly - Alignment and branch prediction.

As red75prime's comment hints (but doesn't explain), LLVM uses the value of the align-all-blocks as the byte alignment for branch points, so setting it to 1 will disable alignment. Note that this applies globally, and that comparison benchmarks are recommended.