Re: [RFC] Documentation about unaligned memory access

Denys Vlasenko
2007-11-25T08:27:39Z

On Thursday 22 November 2007 16:15, Daniel Drake wrote:
> In summary: if your code causes unaligned memory accesses to happen, your
> code will not work on some platforms, and will perform *very* badly on
> others.

Although understanding alignment is important, there is another
extreme - what I call "sadistic alignment". It's when data is being
aligned even if it will definitely run on an arch which doesn't require
this (arch/x86/*), or data being aligned to ridiculously large boundary.

Like gcc aligning any char array bigger that 31 byte to 32 bytes.
Bytes, not bits. Try to compile this with -O2:

static char s1[] = "12345678901234567890123456789012";
static char s2[] = "12345678901234567890123456789012";
void f(char*);
void g() {
f(s1);
f(s2);
}

$ hexdump -Cv t.o
00000000 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 |.ELF............|
00000010 01 00 03 00 01 00 00 00 00 00 00 00 00 00 00 00 |................|
00000020 38 01 00 00 00 00 00 00 34 00 00 00 00 00 28 00 |8.......4.....(.|
00000030 0a 00 07 00 55 89 e5 83 ec 08 c7 04 24 40 00 00 |....U.......$@..|
00000040 00 e8 fc ff ff ff c7 04 24 00 00 00 00 e8 fc ff |........$.......|
00000050 ff ff c9 c3 00 00 00 00 00 00 00 00 00 00 00 00 |................| <=== HERE
00000060 31 32 33 34 35 36 37 38 39 30 31 32 33 34 35 36 |1234567890123456|
00000070 37 38 39 30 31 32 33 34 35 36 37 38 39 30 31 32 |7890123456789012|
00000080 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| <=== HERE
00000090 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| <=== HERE
000000a0 31 32 33 34 35 36 37 38 39 30 31 32 33 34 35 36 |1234567890123456|
000000b0 37 38 39 30 31 32 33 34 35 36 37 38 39 30 31 32 |7890123456789012|
000000c0 00 00 00 00 00 47 43 43 3a 20 28 47 4e 55 29 20 |.....GCC: (GNU) |
000000d0 34 2e 30 2e 33 20 28 55 62 75 6e 74 75 20 34 2e |4.0.3 (Ubuntu 4.|
000000e0 30 2e 33 2d 31 75 62 75 6e 74 75 35 29 00 00 2e |0.3-1ubuntu5)...|
000000f0 73 79 6d 74 61 62 00 2e 73 74 72 74 61 62 00 2e |symtab..strtab..|

43 bytes wasted!

Thankfully, it is fixed in later gcc versions.

Please do not succumb to "alignment scare" in your doc.
--
vda