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MadVocoder42
MadVocoder42

Reputation: 93

Is there a better way to define a preprocessor macro for doing bit manipulation?

Take macro:

GPIOxMODE(gpio,mode,port)    ( GPIO##gpio->MODER   = ((GPIO##gpio->MODER &   ~((uint32_t)GPIO2BITMASK << (port*2))) | (mode << (port * 2))) )

Assuming that the reset value of the register is 0xFFFF.FFFF, I want to set a 2 bit width to an arbitrary value. This was written for an STM32 MCU that has 15 pins per port. GPIO2BITMASK is defined as 0x3. Is there a better way for clearing and setting a random 2 bits in anywhere in the 32-bit wide register.

Valid range for port 0 - 15
Valid range for mode 0 - 3

The method I came up with is to bit shift the mask, invert it, logically AND it with the existing register value, logically OR the result with a bit shifted new value.

I am looking to combine the mask and new value to reduce the number of logical operations bit shift operations. The goal is also keep the process generic enough so that I can use for bit operations of 1,2,3 or 4 bit widths.

Is there a better way?

In the long and sort of it, is there a better way is really an opened question. I am looking specifically for a method that will reduce the number of logical operations and bit shift operations, while being a simple one lined statement.

The answer is NO.

You MUST do reset/set to ensure that the bit field you are writing to has the desired value.

The answers received can be better (in a matter of opinion/preference/philosophy/practice) in that they aren't necessary a macros and have have parameter checking. Also pit falls of this style have been pointed out in both the comments and responses.

Upvotes: 2

Views: 931

Answers (2)

John Bollinger
John Bollinger

Reputation: 181724

The method I came up with is to bit shift the mask, invert it, logically AND it with the existing register value, logically OR the result with a bit shifted new value.

That or an equivalent is the way to do it.

I am looking to combine the mask and new value to reduce the number of logical operations bit shift operations. The goal is also keep the process generic enough so that I can use for bit operations of 1,2,3 or 4 bit widths.

Is there a better way?

You must accomplish two basic objectives:

  1. ensure that the bits that should be off in the affected range are in fact off, and
  2. ensure that the bits that should be on in the affected range are in fact on.

In the general case, those require two separate operations: a bitwise AND to force bits off, and a bitwise OR (or XOR, if the bits are first cleared) to turn the wanted bits on. There may be ways to shortcut for specific cases of original and target values, but if you want something general-purpose, as you say, then your options are limited.

Personally, though, I think I would be inclined to build it from multiple pieces, separating the GPIO selection from the actual computation. At minimum, you can separate out a generic macro for setting a range of bits:

#define SETBITS32(x,bits,offset,mask) ((((uint32_t)(x)) & ~(((uint32_t)(mask)) << (offset))) | (((uint32_t)(bits)) << (offset)))

#define GPIOxMODE(gpio,mode,port) (GPIO##gpio->MODER = SETBITS32(GPIO##gpio->MODER, mode, port * 2, GPIO2BITMASK)

But do note that there appears to be no good way to avoid such a macro evaluating some of its arguments more than once. It might therefore be safer to write SETBITS32 as a function instead. The compiler will probably inline such a function in any case, but you can maximize the likelihood of that by declaring it static and inline:

static inline uint32_t SETBITS32(uint32_t x, uint32_t bits, unsigned offset, uint32_t mask) {
    return x & ~(mask << offset) | (bits << offset);
}

That's easier to read, too, though it, like the macro, does assume that bits has no set bits outside the mask region.

Of course there are other, similar formulations. For instance, if you do not need to support discontinuous bit ranges, you might specify a bit count instead of a bit mask. This alternative does that, protects against the user providing bits outside the specified range, and also has some parameter validation:

static inline uint32_t set_bitrange_32(uint32_t x, uint32_t bits, unsigned width,
        unsigned offset) {
    if (width + offset > 32) {
        // error: invalid parameters
        return x;
    } else if (width == 0) {
        return x;
    }
    uint32_t mask = ~(uint32_t)0 >> (32 - width);
    return x & ~(mask << offset) | ((bits & mask) << offset);
}

Upvotes: 1

0___________
0___________

Reputation: 68013

This kind of macros should be avoided as a plaque for many reasons:

  1. They are not debuggable
  2. They are hard to find error prone

and many other reasons

The same result you can archive using inline functions. The resulting code will be the same effective

static inline __attribute__((always_inline)) void GPIOMODE(GPIO_TypeDef *gpio, unsigned mode, unsigned pin)
{
    gpio -> MODER &= ~(GPIO_MODER_MODE0_Msk << (pin * 2));
    gpio -> MODER |= mode << (pin * 2);
}

but if you love macros

#define GPIOxMODE(gpio,mode,port)    {volatile uint32_t *mdr = &GPIO##gpio->MODER; *mdr &= ~(GPIO_MODER_MODE0_Msk << (port*2)); *mdr |= mode << (port * 2);}

I am looking to combine the mask and new value to reduce the number of logical operations bit shift operations.

you cant. You need to reset and then set the bits.

Upvotes: 1

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