LD: ALIGN vs SUBALIGN in linker scripts - gcc

Question

How do they differ?
I read that SUBALIGN() somehow forces a certain alignment. Are there other differences?
When should I use ALIGN() and when should I use SUBALIGN()?

Answer 1

SUBALIGN is
specifically for adjusting the alignment of the input sections within an output section.
To illustrate:
$ cat one.c
char a_one __attribute__((section(".mysection"))) = 0;
char b_one __attribute__((section(".mysection"))) = 0;
$ cat two.c
char a_two __attribute__((section(".mysection"))) = 0;
char b_two __attribute__((section(".mysection"))) = 0;
$ gcc -c one.c two.c
Case 1
$ cat foo_1.lds
SECTIONS
{
. = 0x10004;
.mysection ALIGN(8) : {
*(.mysection)
}
}
$ ld -T foo_1.lds one.o two.o -o foo1.out
$ readelf -s foo1.out
Symbol table '.symtab' contains 9 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000010008 0 SECTION LOCAL DEFAULT 1
2: 0000000000000000 0 SECTION LOCAL DEFAULT 2
3: 0000000000000000 0 FILE LOCAL DEFAULT ABS one.c
4: 0000000000000000 0 FILE LOCAL DEFAULT ABS two.c
5: 000000000001000b 1 OBJECT GLOBAL DEFAULT 1 b_two
6: 0000000000010008 1 OBJECT GLOBAL DEFAULT 1 a_one
7: 0000000000010009 1 OBJECT GLOBAL DEFAULT 1 b_one
8: 000000000001000a 1 OBJECT GLOBAL DEFAULT 1 a_two
$ readelf -t foo1.out | grep -A3 mysection
[ 1] .mysection
PROGBITS PROGBITS 0000000000010008 0000000000010008 0
0000000000000004 0000000000000000 0 1
[0000000000000003]: WRITE, ALLOC
Here, ALIGN(8) aligns .mysection to the next 8-byte boundary, 0x10008,
after 0x10004.
The char symbol a_one, coming from input section one.o(.mysection), is at the start of .mysection
followed at the next byte by b_two, also coming from input section one.o(.mysection). At the next byte,
is a_two, from input section two.o(.mysection), then b_two, also from two.o(.mysection). All 4
objects from all input sections *(.mysection) are just placed end to end from the start of output section .mysection.
Case 2
$ cat foo_2.lds
SECTIONS
{
. = 0x10004;
.mysection ALIGN(8) : SUBALIGN(16) {
*(.mysection)
}
}
$ ld -T foo_2.lds one.o two.o -o foo2.out
$ readelf -s foo2.out
Symbol table '.symtab' contains 9 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000010008 0 SECTION LOCAL DEFAULT 1
2: 0000000000000000 0 SECTION LOCAL DEFAULT 2
3: 0000000000000000 0 FILE LOCAL DEFAULT ABS one.c
4: 0000000000000000 0 FILE LOCAL DEFAULT ABS two.c
5: 0000000000010021 1 OBJECT GLOBAL DEFAULT 1 b_two
6: 0000000000010010 1 OBJECT GLOBAL DEFAULT 1 a_one
7: 0000000000010011 1 OBJECT GLOBAL DEFAULT 1 b_one
8: 0000000000010020 1 OBJECT GLOBAL DEFAULT 1 a_two
$ readelf -t foo2.out | grep -A3 mysection
[ 1] .mysection
PROGBITS PROGBITS 0000000000010008 0000000000010008 0
000000000000001a 0000000000000000 0 16
[0000000000000003]: WRITE, ALLOC
This time, the 8-byte aligned address of .mysection is unchanged. But the
effect of SUBALIGN(16) is that symbol a_one, coming from input
section one.o(.mysection) is placed at the next 16-byte
boundary, 0x10010, after the start of .mysection, and symbol b_one, coming from the
same input section is at the next byte. But symbol a_two, coming from input section
two.o(.mysection) is at the next 16-byte boundary, 0x10020; and b_two, coming
also from two.o(.mysection), is 1 byte after that.