#2816 Explicit alignment boundaries shifted while linked

[Henk]

Explicit alignment boundaries fail due to linker shifting assembled piece of code off the alignment boundary.

Steps to reproduce the issue:

1. sdasstm8 -los test.asm
2. sdcc main.c -mstm8 --out-fmt-ihx test.rel -o main.hex

In the test.lst shows the alignment boundaries are taking into account while assembling:

                                      1 .area CODE
      000000                          2 .bndry 4
      000000 9D               [ 1]    3 nop
      000004                          4 .bndry 4
      000004 9D               [ 1]    5 nop

When looking at the .rst file showing the linker results, the alignment boundaries are no longer taking into account:

                                      1 .area CODE
      008026                          2 .bndry 4
      008026 9D               [ 1]    3 nop
      00802A                          4 .bndry 4
      00802A 9D               [ 1]    5 nop

sdcc -v: mcs51/z80/z180/r2k/r3ka/gbz80/tlcs90/ds390/pic16/pic14/TININative/ds400/hc08/s08/stm8 3.6.9 #10220 (MINGW32)

[Henk]

Since STM8 fetches flash in absolute aligned chunks of 4 bytes, an quickfix would be to allign every .rel on a 4 bytes alignment boundary, so that timing critical code is executed as expected.

[Henk]

There is a work around for this issue, place explicit 4 byte alignment boundaries to every source file.

For assembly source files this is quite simple, just adding the 4 byte alignment boundary at end of file:

.area CODE
.bndry 4

SDCC only allows inline assembly within function scopes in C source files, therefore the 4 byte alignment boundary must be wrapped in a __naked function:

static void _4_byte_alignment_boundary(void) __naked
{
  __asm__ (".bndry 4");
}