Re: ARM Inline Assembler and 64 bit variables

[Richard Earnshaw <Richard dot Earnshaw at buzzard dot freeserve dot co dot uk>]

On Thu, 01 Jun 2006 08:56:59 BST, Richard Earnshaw wrote:
> 
> > is it possible to use the inline assembler of GCC for arm elf target in
> > conjunction with 64 bit variables?
> 
> > I tried the following example (arithmetic average), but with little
> > success! 
> 
> > uint64_t        a,b,r; asm(
> >         "adds   %A0,    %A1,    %A2"            "\n\t"
> >         "adds   %B0,    %B1,    %B2"            "\n\t"
> >         "mov    %B0,    %B0,    rrx"            "\n\t"
> >         "mov    %A0,    %A0,    rrx"
> >         : "=r" (r) : "r" (a), "r" (b) : "cc" );
> 
> 
> Yes, it is, but you need to use the right operand modifiers (these vary by 
> target).
> 
> On ARM the modifiers for accessing 64-bit types are
> 
> %<n> The lowest numbered register of a pair
> %H<n> The highest numbered register of a pair
> %Q<n> The register containing the least significant part of the 32-bit 
> value
> %R<n> The register containing the most significant part of the 32-bit value
> 
> Why so many?  Well it depends on whether you want your code to compile 
> correctly for big-endian as well as little-endian systems.
> 
> So your test case above should probably read something like
> 
> int64_t        a,b,r;
> asm(
>         "adds   %Q0,    %Q1,    %Q2"            "\n\t"
>         "adds   %R0,    %R1,    %R2"            "\n\t"
>         "mov    %R0,    %R0,    rrx"            "\n\t"
>         "mov    %Q0,    %Q0,    rrx"
>         : "=&r" (r) : "r" (a), "r" (b) : "cc"
> );
> 
> Note the use of =&r in the constraint for the variable 'r'.  This ensures 
> that your input operands won't be corrupted before they have been fully 
> read.
> 
> R.
> 

Incidentally, if you want the arithmetic average, the code you want is

int64_t        a,b,r;
asm(
        "adds   %Q0,    %Q1,    %Q2"            "\n\t"
        "adcs   %R0,    %R1,    %R2"            "\n\t"
        "movs   %R0,    %R0,    rrx"            "\n\t"
        "mov    %Q0,    %Q0,    rrx"
        : "=&r" (r) : "r" (a), "r" (b) : "cc"
);

R.