Commits on Feb 9, 2024

1. optimize from_page_table_indices …

   The new code behaves exactly the same as the old code, but can be much
   better optimized by LLVM. In particular the bounds checks previously
   implied through `set_bits` and `VirtAddr::new` can now be elided. This
   also increases the chances of these functions being inlined.
   
   The generated assembly for the old implementation looks like this:
   ```asm
   x86_64::structures::paging::page::Page::from_page_table_indices:
    push    rax
    movzx   edi, di
    cmp     edi, 512
    jae     .LBB48_8
    movzx   esi, si
    cmp     esi, 512
    jae     .LBB48_8
    movzx   edx, dx
    cmp     edx, 512
    jae     .LBB48_8
    movzx   eax, cx
    cmp     eax, 512
    jae     .LBB48_8
    mov     rcx, rdi
    shl     rcx, 39
    shl     rsi, 30
    or      rsi, rcx
    shl     rdx, 21
    shl     rax, 12
    or      rax, rdx
    or      rax, rsi
    shr     edi, 8
    je      .LBB48_7
    cmp     edi, 1
    jne     .LBB48_9
    shl     rax, 16
    sar     rax, 16
   .LBB48_7:
    shl     rax, 16
    sar     rax, 16
    and     rax, -4096
    pop     rcx
    ret
   .LBB48_8:
    lea     rdi, [rip, +, .L__unnamed_46]
    lea     rdx, [rip, +, .L__unnamed_15]
    mov     esi, 33
    call    qword, ptr, [rip, +, _ZN4core9panicking5panic17h87fd92496103e3b8E@GOTPCREL]
   .LBB48_9:
    mov     qword, ptr, [rsp], rax
    lea     rdi, [rip, +, .L__unnamed_9]
    lea     rcx, [rip, +, .L__unnamed_10]
    lea     r8, [rip, +, .L__unnamed_11]
    mov     rdx, rsp
    mov     esi, 74
    call    qword, ptr, [rip, +, _ZN4core6result13unwrap_failed17hc28f4ee1a6255957E@GOTPCREL]
   ```
   
   The generated assembly for the new implementation looks like this:
   ```asm
   x86_64::structures::paging::page::Page::from_page_table_indices:
    shl     rdi, 55
    movzx   esi, si
    shl     rsi, 46
    or      rsi, rdi
    movzx   edx, dx
    shl     rdx, 37
    movzx   eax, cx
    shl     rax, 28
    or      rax, rdx
    or      rax, rsi
    sar     rax, 16
    ret
   ```

   

   Freax13 committed Feb 9, 2024
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