Allocate aligned heap space

   Recently, I used a written by others FFT Hardware acceleration module , requirement DDR Data alignment in 4k The border , It is estimated that the data bus used by this module is AXI Agreed ,AXI Burst transmission of cannot exceed 4k The border . The platform used is C6678DSP, Compiler supported C++ The version is C++98, No, aligned new, So I can only write by myself aligned_malloc function .

Extra space overhead

  C++ Of new What is called essentially is malloc function ,malloc You can apply for a certain size of space in the heap area , Then return to the first address of this space ; Corresponding free Function to free the allocated heap space .
   As shown in the figure above , Calling malloc Post return pointer $p$,B Represents the aligned boundary , The number of bytes aligned is L. Generally speaking, we get $p$ The address pointed to by the pointer is not aligned . But we can allocate a little more space , Make the data stored in the aligned position . So if you do that , The worst-case overhead is L-1, here $p$ The pointer is assigned to “ The border +1” Location .
   How to calculate the first aligned address from the assigned address . Just add... To the assigned address L-1, Then clear the low order of the address . For example, to align 4k The border ：
$$add{r}_{aligned}=(p+0\mathrm{x}\mathrm{F}\mathrm{F}\mathrm{F})\&\sim (0\mathrm{x}\mathrm{F}\mathrm{F}\mathrm{F})$$
   So if $p$ Already aligned to the boundary , Then the result will be $p$; and $p$ If not aligned to the boundary , The result will be the first aligned address above the current address . This aligned address can be used as aligned_malloc The return value of the function .

Space release

  
   In order to properly free the space just allocated , We still need to get ,malloc The address actually assigned $p$ preserved .$p$ yes void* type , So we need one more sizeof(void*) The size of the space used to store $p$. This storage location can be placed just below the aligned address .

   The worst case scenario at this time is that additional L-1 Byte space ,malloc The assigned address is located at “ The border -（sizeof(void*)-1）” The location of .

Sample code

void *aligned_malloc(size_t num, size_t alignment){
    
	void *pOrig, pAligned;
	size_t offset;

	// check alignment is power of 2
	if(alignment & (alignment - 1) != 0) pAligned = NULL;
	else{
    
		offset = alignment - 1 + sizeof(void *);
		pOrig = malloc(num + offset);
		pAligned = (void *)(((size_t)pOrig + (size_t)offset) & (~(alignment-1)));
		*((void **)((size_t)pAligned-sizeof(void *))) = pOrig;
	}

	return pAligned;
}

void aligned_free(void *pAligned){
    
	void **p = (void **)((size_t)pAligned- sizeof(void *));
	free(*p);
}