On 08/07/18 06:03, Jeffrey Walton wrote:
> Hi Everyone,
>
> I'm working from an ARM guide. The doc provides this code:
>
> $ cat move.c
> int main(int argc, char* argv[])
> {
> int a;
> asm volatile("movw %0,%L1 \n"
> "movt %0,%H1 \n"
> : "=r"(a) : "i"(0x12345678));
> return a;
> }
>
> It results in:
>
> $ gcc -march=armv7 move.c
> move.c: In function â??mainâ??:
> move.c:4:5: error: invalid 'asm': invalid operand for code 'H'
> asm volatile("movw %0,%L1 \n"
> ^
>
> The guide says this about the modifiers:
>
> L - The lowest-numbered register of a register pair, or the low 16
> bits of an immediate constant.
> H - The highest-numbered register of a register pair, or the high 16
> bits of an immediate constant
> ....
>
> Is this an ARM extension not present in GCC? Or am I doing something wrong?
>
> Jeff
>
The L and H modifiers are for dealing with 64-bit /register/ quantities
where you need two registers to hold the entire value. Your example
only has a single 32-bit value. You don't need qualifiers in this case.
For an immediate like this, you'll have to hand-code the reduction into
the appropriate fields, either in the operands you pass to the ASM or
within the ASM expansion itself. Something like:
asm volatile ("movw %0, %1;movt %0, %2": "=r"(a) : "i"(imm & 0xffff),
"i" (imm & 0xffff0000));
R.
