pcala64.c

#include <stdio.h>
#include <stdlib.h>

int
main (int argc, char *argv[])
{
	long target;
	long dest;
	long pc;
	long delta;
	long pcala_hi20;
	long pcala_lo12;
	long pcala64_lo20;
	long pcala64_hi12;

	dest = strtoul (argv[1], NULL, 16);
	pc = strtoul (argv[2], NULL, 16);

	delta = dest - (pc & ~0xfffUL);
	delta = delta + ((delta & 0x800UL) << 1);
	delta = delta + ((delta & 0x80000000UL) << 1);
	delta = delta - ((delta & 0x800UL) << 21);

	pcala_hi20 = (delta >> 12) & 0xfffffUL;
	pcala_lo12 = delta & 0xfffUL;
	pcala64_lo20 = (delta >> 32) & 0xfffffUL;
	pcala64_hi12 = (delta >> 52) & 0xfffUL;

	printf ("R_LARCH_PCALA_HI20   = 0x%lx\n", pcala_hi20);
	printf ("R_LARCH_PCALA_LO12   = 0x%lx\n", pcala_lo12);
	printf ("R_LARCH_PCALA64_LO20 = 0x%lx\n", pcala64_lo20);
	printf ("R_LARCH_PCALA64_HI12 = 0x%lx\n", pcala64_hi12);

	target = (pc & ~0xfffUL) + ((pcala_hi20 << 44) >> 32) +
	         ((pcala64_hi12 << 52) | (pcala64_lo20 << 32) | (((pcala_lo12 << 52) >> 52) & 0xffffffffUL));
	if (target != dest) {
		printf ("%0lx != %0lx\n", dest, target);
		return -1;
	}

	return 0;
}

For the most cases, it works well. But for some corner cases, it may work badly.
a), the pc of these four instructions is not the same.
b), at least, in GCC, instructions are not even adjacent if they are scheduled.
Thus, we may need 4 pc as input.

@MQ-mengqing Even after scheduling, if 4 instructions are still in the same 4k aligned space, there is actually only one pc. If they are in different 4k, then all 4 instructions have a way to calculate the pc of the pcalau12i instruction when relocating. Is this feasible?

It is feasible if we find that way. But it seems difficult.

a),
Refer to RISC-V, we can use

.L1:
  pcalau12i $xx, sym
  addi.d $xx, $r0, .L1
  lu32i $xx, .L1
  lu52i $xx, $xx, .L1

They are not current abi.

b),
Mark they are always adjacent. Like call36.

c),
Set imm in instructions to indicate the bytes distance with pcalau12i.

.L1:
  pcalau12i $xx, sym
  addi.d $xx, $r0, (. - .L1), sym
  lu32i $xx, (. - .L1), sym
  lu52i $xx, $xx, (. - .L1), sym

It looks even more difficult to implement with linker relaxation enabling. They are not current abi, too.

Yes. The instruction's free encoding bit-field can be used to maintain the relationship before relocation. But this is no longer compatible with what it used to be.