Re: [Xen-devel] [PATCH 2/2] x86/E820: improve insn selection

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 12/12/17 11:10, Jan Beulich wrote:
> ..., largely to shrink code size a little:
> - use TEST instead of CMP with zero immediate
> - use MOVZWL instead of AND with 0xffff immediate
> - compute final highmem_bk value in registers, accessing memory just
>   once
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, altbeit it
preferably with space in the first hunk.

Any chance we can drop redundant size suffixes as we go?

>
> --- a/xen/arch/x86/boot/mem.S
> +++ b/xen/arch/x86/boot/mem.S
> @@ -29,8 +29,8 @@ get_memory_map:
>          movw    %di,%ax
>          addw    $20,%ax
>          movw    %ax,%di
> -        cmpl    $0,%ebx                         # check to see if
> -        jne     1b                              # %ebx is set to EOF
> +        testl   %ebx,%ebx                       # check to see if
> +        jnz     1b                              # %ebx is set to EOF
>  
>  .Lmem88:
>          movb    $0x88, %ah
> @@ -48,17 +48,17 @@ get_memory_map:
>          int     $0x15
>          jc      .Lint12
>  
> -        cmpw    $0x0, %cx                       # Kludge to handle BIOSes
> -        jne     1f                              # which report their extended
> -        cmpw    $0x0, %dx                       # memory in AX/BX rather than
> -        jne     1f                              # CX/DX.  The spec I have 
> read
> +        testw   %cx, %cx                        # Kludge to handle BIOSes
> +        jnz     1f                              # which report their extended
> +        testw   %dx, %dx                        # memory in AX/BX rather than
> +        jnz     1f                              # CX/DX.  The spec I have 
> read
>          movw    %ax, %cx                        # seems to indicate AX/BX 
>          movw    %bx, %dx                        # are more reasonable 
> anyway...
> -1:      andl    $0xffff,%edx                    # clear sign extend
> +1:      movzwl  %dx, %edx
>          shll    $6,%edx                         # and go from 64k to 1k 
> chunks
> +        movzwl  %cx, %ecx
> +        addl    %ecx, %edx                      # add in lower memory
>          movl    %edx,bootsym(highmem_kb)        # store extended memory size
> -        andl    $0xffff,%ecx                    # clear sign extend
> -        addl    %ecx,bootsym(highmem_kb)        # and add lower memory into
>  
>  .Lint12:
>          int     $0x12
>
>
>


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