[J-core] PC-relative loads and delay slots

[Robert Ou]

On Mon, Jul 18, 2016 at 6:52 AM, Rich Felker <dalias at libc.org> wrote:
> On Mon, Jul 18, 2016 at 02:37:55AM -0700, Robert Ou wrote:
>> Hi,
>>
>> What is the correct behavior of PC-relative instructions such as
>> "mov.l @(disp, PC), Rn" in a branch delay slot? Is this even allowed?
>> From my testing, GAS seems to think it is "disp is multiplied by 4 and
>> added to the address of the mov.l opcode + 2" but J-core seems to
>> execute it as "disp is multiplied by 4 and added to the address of the
>> branch target + 2". I discovered this while working on my MyHDL
>> demonstration, and you can compare the difference in my demonstration
>> by running the master branch and the branch_delay_test branch.
>
> If true I think it's a bug. The original SH ISA documentation
> specifies the behavior for "PC-relative" mov instructions as:
>
>   "The PC points to the starting address of the second instruction
>   after this MOV instruction"
>
> (as opposed to the actual current value of the program counter). This
> text is found on page 202 of document REJ09B0171-0500O.
>
> I'm quite surprised we haven't run into this bug, since I would expect
> gcc to generate code with immediate loads in branch delay slots (e.g.
> when making function calls with constant arguments).
>
> Rich

I did some testing on an actual Mimas v2 board using the mimas_v2.bin
on the j-core website (bootrom says Mon Apr 18 22:36:49 UTC 2016, md5
is 390768a7ef9061a19f163061c4e15ac3), and PC-relative loads in the
delay slot of "bra label" seem to work correctly but PC-relative loads
in the delay slot of "bsr label" or "rts" do not seem to work
correctly. I didn't test any other instructions with delay slots. I
tested it with this program (replace
sources/root-filesystem/src/hello.c in the Aboriginal Linux source):

#include <stdio.h>

asm (
"test_delay_slot_1:         \n"
"   sts pr, r1              \n"

"   bsr dummysub_1          \n"
"    mov.l testval_1, r0    \n"

"   lds r1, pr              \n"
"   rts                     \n"
"    nop                    \n"
"dummysub_1:                \n"
"   rts                     \n"
"    nop                    \n"
".align 4                   \n"
"testval_1:                 \n"
"   .long 0xabcddcba        \n"
);

asm (
"test_delay_slot_2:         \n"
"   sts pr, r1              \n"

"   mov.l testval_2, r0     \n"
"   bsr dummysub_2          \n"
"    nop                    \n"

"   lds r1, pr              \n"
"   rts                     \n"
"    nop                    \n"
"dummysub_2:                \n"
"   rts                     \n"
"    nop                    \n"
".align 4                   \n"
"testval_2:                 \n"
"   .long 0xabcddcba        \n"
);

asm (
"test_delay_slot_3:         \n"

"   bra testret_3           \n"
"    mov.l testval_3, r0    \n"

"testret_3:                 \n"
"   rts                     \n"
"    nop                    \n"
".align 4                   \n"
"testval_3:                 \n"
"   .long 0xfeedface        \n"
);

asm (
"test_delay_slot_4:         \n"

"   mov.l testval_4, r0     \n"
"   bra testret_4           \n"
"    nop                    \n"

"testret_4:                 \n"
"   rts                     \n"
"    nop                    \n"
".align 4                   \n"
"testval_4:                 \n"
"   .long 0xfeedface        \n"
);

asm (
"test_delay_slot_5:         \n"

"   rts                     \n"
"    mov.l testval_5, r0    \n"

".align 4                   \n"
"testval_5:                 \n"
"   .long 0xcafed00d        \n"
);

asm (
"test_delay_slot_6:         \n"

"   mov.l testval_6, r0     \n"
"   rts                     \n"
"    nop                    \n"

".align 4                   \n"
"testval_6:                 \n"
"   .long 0xcafed00d        \n"
);

int main(int argc, char *argv[])
{
  printf("Hello world!\n");
  printf("Result 1 is %08X\n", test_delay_slot_1());
  printf("Result 2 is %08X\n", test_delay_slot_2());
  printf("Result 3 is %08X\n", test_delay_slot_3());
  printf("Result 4 is %08X\n", test_delay_slot_4());
  printf("Result 5 is %08X\n", test_delay_slot_5());
  printf("Result 6 is %08X\n", test_delay_slot_6());
  return 0;
}

The output of this program is:
Hello world!
Result 1 is 012AD006
Result 2 is ABCDDCBA
Result 3 is FEEDFACE
Result 4 is FEEDFACE
Result 5 is 0009D412
Result 6 is CAFED00D

but I believe the expected output should be:
Hello world!
Result 1 is ABCDDCBA
Result 2 is ABCDDCBA
Result 3 is FEEDFACE
Result 4 is FEEDFACE
Result 5 is CAFED00D
Result 6 is CAFED00D