On Mon, Jul 9, 2018 at 5:51 AM, Richard Earnshaw (lists)
<Richard.Earnshaw@xxxxxxx> wrote:
> On 09/07/18 10:37, Richard Earnshaw (lists) wrote:
>> On 08/07/18 06:03, Jeffrey Walton wrote:
>>> ...
>>> 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?
>>>
>> 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));
>>
> Correction. Looking at the source code, the L modifier only appears to
> apply to 32-bit integer immediate values, the H modifier only appears to
> apply to 64-bit registers.
>
> So the guide is wrong for both cases, but in different ways. At least
> when it comes to GCC.
>
> Which document are you referring to?
Thanks Richard.
The guide I was using is
http://www.iarsys.co.jp/download/LMS2/arm/7304/ewarm7304doc/arm/doc/EWARM_DevelopmentGuide.ENU.pdf
(p.147).
Jeff
