[SOLVED] Loading a 64-bit data into a 32-bit register

Loading a 64-bit data into a 32-bit register

Let's say I have a 3-element array of 64-bit data:

   src    DCQ    0x0200200AD00236DD
          DCQ    0x00003401AAC4D097
          DCQ    0X0001FC219AC931BE

assuming that I know the address of "src" (named srcAdr), I can load the lower 32-bit content of an element of src at a certain index into a register named srcLo by saying:

   LDR    srcLo, [srcAdr, index, LSL#3]

In order to get the higher 32-bit content of this element, I know I can:

   ADD    srcAdrHi, srcAdr, #4
   LDR    srcHi, [srcAdrHi, index, LSL#3]

Question is, is there a more elegant way to do this? Say, for example, in one instruction?

Solution

Following my comment: I do not think you can do without an extra instruction here if for whatever reason you must work with the data as with a uint64_t array, using an index.

For a 'C' function:

int foo(unsigned long long *srcT, int index) {
  unsigned int temp=0;
  temp = (unsigned int)(srcT[index]);
  temp += (unsigned int)(srcT[index] >> 32);
  return temp;
}

The compiler (ARM gcc 8.2 -O3 -mcpu=arm7tdmi) produced:

foo:
    add     r3, r0, r1, lsl #3
    ldr     r3, [r3, #4]
    ldr     r0, [r0, r1, lsl #3]
    add     r0, r0, r3
    bx      lr

As you can see it also produced an extra instruction (add) to access the 'high half'. The exact sequence of instructions would, of course, depend on what manipulations are performed on the array. If you'd walk through it in a loop you'd most likely get ldm + add Rx,#8, etc.