Re: Attribute const and inline functions

[Sebastian Huber <sebastian.huber@xxxxxxxxxxxxxxxxxx>]

I found a way to hide some information from GCC using inline assembler:

extern volatile const int *ip;

__attribute__((const)) static inline volatile const int *get_ip_asm(void)
{
    volatile const int *p;

    __asm__ ("ld [%1], %0" : "=r" (p) : "r" (&ip));

    return p;
}

int a(void)
{
    return ((int) &ip) & 0xfffffc00;
}

static inline int i_asm(void)
{
    return *get_ip_asm();
}

void f(void);

void g_asm(void)
{
    i_asm();
    f();
    i_asm();
}

        .file   "test.c"
        .section        ".text"
        .align 4
        .global a
        .type   a, #function
        .proc   04
a:
        sethi   %hi(ip), %o0
        or      %o0, %lo(ip), %o0
        jmp     %o7+8
         and    %o0, -1024, %o0
        .size   a, .-a
        .align 4
        .global g_asm
        .type   g_asm, #function
        .proc   020
g_asm:
        save    %sp, -96, %sp
        sethi   %hi(ip), %i5
        or      %i5, %lo(ip), %i5
! 7 "test.c" 1
        ld [%i5], %i5
! 0 "" 2
        ld      [%i5], %g1
        call    f, 0
         nop
        ld      [%i5], %g1
        jmp     %i7+8
         restore
        .size   g_asm, .-g_asm
        .ident  "GCC: (GNU) 4.9.0 20140103 (experimental)"

This is next to optimal.  Unfortunately GCC doesn't optimize a() to

a:
        sethi   %hi(ip), %o0
        jmp     %o7+8
         nop

--
Sebastian Huber, embedded brains GmbH

Address : Dornierstr. 4, D-82178 Puchheim, Germany
Phone   : +49 89 189 47 41-16
Fax     : +49 89 189 47 41-09
E-Mail  : sebastian.huber@xxxxxxxxxxxxxxxxxx
PGP     : Public key available on request.

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