> 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.
