Re: PCI Express support for 2.4 kernel

[Vladimir Kondratiev]

Kevin,
there is no black magic. Compilation goes .c -> .s -> .o; if assembly 
have nothing added, object have not as well.
To be sure, I did the following:

[tmp]$ gcc -c t.c; objdump -xs t.o

t.o:     file format elf32-i386
t.o
architecture: i386, flags 0x00000010:
HAS_SYMS
start address 0x00000000

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
 0 .text         00000000  00000000  00000000  00000034  2**2
                 CONTENTS, ALLOC, LOAD, READONLY, CODE
 1 .data         00000000  00000000  00000000  00000034  2**2
                 CONTENTS, ALLOC, LOAD, DATA
 2 .bss          00000008  00000000  00000000  00000034  2**2
                 ALLOC
 3 .note.GNU-stack 00000000  00000000  00000000  00000034  2**0
                 CONTENTS, READONLY
 4 .comment      00000033  00000000  00000000  00000034  2**0
                 CONTENTS, READONLY
SYMBOL TABLE:
00000000 l    df *ABS*    00000000 t.c
00000000 l    d  .text    00000000
00000000 l    d  .data    00000000
00000000 l    d  .bss    00000000
00000000 l     O .bss    00000004 a1
00000004 l     O .bss    00000004 a2
00000000 l    d  .note.GNU-stack    00000000
00000000 l    d  .comment    00000000


Contents of section .text:
Contents of section .data:
Contents of section .note.GNU-stack:
Contents of section .comment:
0000 00474343 3a202847 4e552920 332e332e  .GCC: (GNU) 3.3.
0010 31203230 30333038 31312028 52656420  1 20030811 (Red
0020 48617420 4c696e75 7820332e 332e312d  Hat Linux 3.3.1-
0030 312900                               1).            

Kevin P. Fleming wrote:

> Vladimir Kondratiev wrote:
>
> > To illustrate zero cost, I did the following test:
> > [tmp]$ cat t.c; gcc -S t.c; cat t.s
> > static int a1=0;
> > static int a2;
> > /* EOF */
> >
> >    .file    "t.c"
> >    .local    a1
> >    .comm    a1,4,4
> >    .local    a2
> >    .comm    a2,4,4
> >    .section    .note.GNU-stack,"",@progbits
> >    .ident    "GCC: (GNU) 3.3.1 20030811 (Red Hat Linux 3.3.1-1)"
> >
> > As you can see, assembly code is identical, compiler did this trivial 
> > optimization for me.
>
>
> You've missed the point, though. Initializing a static variable to 
> zero causes space to be consumed in the resulting object file (not 
> instruction code to be generated). This is wasted space, because if 
> you don't initialize to zero the variable will be allocated out of 
> space that is _automatically_ zeroed for you. This reduces the size of 
> the kernel image by not filling it with unnecessary zeroes.

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