Jeffrey Walton wrote on 07/09/2018 12:49 PM:
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).
I'm not an assembler guy, but a quick search revealed the following:
https://gcc.gnu.org/onlinedocs/gcc/Constraints.html
https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html
(or in this case ../gcc_trunk/gcc/config/arm/constraints.md )
https://gcc.gnu.org/onlinedocs/gcc/Modifiers.html
So, the said L and H modifiers of ARM seem not among the listed modifiers in
the 3nd link above.
As the table on the said page in the above pdf shows, there are many more
modifiers available that seem to be not implemented yet in gcc.
If modifying the source is an option, then one surely can find replacements,
like here:
https://stackoverflow.com/questions/7800055/movw-and-movt-in-arm-assembly
