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Hi,
I would like to prevent structure members optimization. I used volatile keyword:
typedef volatile struct { volatile ... volatile uint32_t discard; volatile ... } regs_t;
I declared a member in another non-volatile struct:
typedef struct { ... volatile regs_t *regs; ... } peripheral_t;
I used an "empty" expression to just clear flags in register (the memory read access must be performed):
peripheral_t p; p.regs = 0x12345678; /* init to register block starting address */ p.regs->discard; /* gets optimized out when opt. is on */
The "empty" expression gets optimized out although I thought (hoped) I prevented it via the volatile keywords. - What is the right place to put the volatile? - Does the whole struct have to be volatile? - Does it make sense to define volatile type? (typedef volatile struct...)
Thanks for tips/explanations.
Regards Pavel
- Does it make sense to define volatile type? (typedef volatile struct...)
It is possible to declare a whole structure volatile instead of its single elements, however, for the sake of intuitiveness, it makes more sense to declare the various registers of a register set as volatile, because it really is the single register that is volatile.
You may have discovered a compiler bug here. Incorrect implementations of the volatile keyword appear to be rather common among compilers.
Christoph: wrote > You may have discovered a compiler bug here.
As I wrote earlier, the behavior seems to be valid according to the C standard. The volatile structure is a member of a non-volatile structure, a pointer to which is used to access a register. Following my advice in previous post may solve the issue.
-- Marcus http://www.doulos.com/arm/
I don't know much about this. I don't have any experience on ATMEL MCU.
But, maybe you can take a look at some ATMEL Example Code.
In AT91SAM7S32.h
#ifndef AT91SAM7S32_H #define AT91SAM7S32_H #ifndef __ASSEMBLY__ typedef volatile unsigned int AT91_REG;// Hardware register definition #define AT91_CAST(a) (a) #else #define AT91_CAST(a) #endif // ***************************************************************************** // SOFTWARE API DEFINITION FOR System Peripherals // ***************************************************************************** #ifndef __ASSEMBLY__ typedef struct _AT91S_SYS { AT91_REG AIC_SMR[32]; // Source Mode Register AT91_REG AIC_SVR[32]; // Source Vector Register AT91_REG AIC_IVR; // IRQ Vector Register AT91_REG AIC_FVR; // FIQ Vector Register AT91_REG AIC_ISR; // Interrupt Status Register AT91_REG AIC_IPR; // Interrupt Pending Register