Inline assembly without inputs considered const/pure?

[Matthias Pfaller]

On 02/23/2016 09:20 AM, Matthias Pfaller wrote:
> Hi,
> 
> I'm using __asm on a cortex-m4 to enable/disable/restore interrupts.
> 
> I have defined a couple of macros to manipulate registers only available
> by using the msr/mrs opcodes:
> 
>> #define mrs(x)  ({                                                          \
>>         volatile unsigned int __r = 0;                                      \
>>         __asm__ __volatile__("" : : : "memory");                            \
>>         __asm__("mrs %[R]," __STRING(x) : [R] "=r" (__r) : : "cc", "memory");\
>>         __r;                                                                \
>>     })

After some tries I found an actually quite simple solution for my
problem. The compiler deletes the mrs because it has no input. So we
have to provide input that is constant, but different on each invocation
of the macro. Using the "J" constraints I can supply an integer constant
between -4095 and 4095 without forcing the compiler to produce any load
requests. __LINE__ is a source for unique integers:

> #define mrs(x)  ({                                                          \
>         unsigned int __r;                                                   \
>         __asm__ __volatile__("" : : : "memory");                            \
>         __asm__("mrs %[R]," __STRING(x) "/* %[D] avoid constant folding */" \
>             : [R] "=r" (__r)                                                \
>             : [D] "J" ((__LINE__ & 0x1fff) - 0xfff)                         \
>             : "cc", "memory"                                                \
>         );                                                                  \   
>         __r;                                                                \   
>     })

Matthias
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Matthias Pfaller                          Software Entwicklung
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