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    Subjectstack allocation and gcc
    Hello All!

    [ please cc: me ]

    I have observed funny behaviour of both gcc 2.95/322 on ppc32 and
    i686 platforms.

    Have written this routine and compiled it with 'gcc -O2':

    int a(int v)
    {
    char buf[32];

    if (v > 5) {
    char buf2[32];
    printf( buf, buf2 );
    } else {
    char buf2[32];
    printf( buf, buf2 );
    }
    return 1;
    }

    I expected that stack on every branch of 'if(v>5)' will be allocated
    later - but seems that gcc allocate stack space once and in this case it
    will 'overallocate' 32 bytes - 'char buf2' will be allocated twice for
    every branch. On i686 gcc allocates 108 bytes, on ppc32 it allocates 116
    bytes. (additional space seems to be induced by printf() call)
    Adding to this routine something like 'do { char a[32]; } while(0);'
    several times shows that stack buffers are not reused - and allocated
    for every this kind of context separately.

    As to my understanding - since this buffers do live in different
    mutually exclusive contextes - they can be reused. But this seems to be
    not case. Waste of precious kernel stack space - and waste of d-cache.

    I have read 'info gcc' - but found nothing relevant to this.
    I've checked ppc abi - but found no limitations to reuse of stack space.

    Is it expected behaviour of compiler? gcc feature?

    [ I have created macro which opens into inline function call which
    utilizes va_list - on ppc32 va_list adds at least 32 bytes to stack use.
    Seems to be bad idea for kernel-space, since every use if macro adds to
    stack use (10 macro calls == 320 bytes). Easy to rewrite to not to use
    va_list - but have I *NO* stack allocation check script in place - this
    stuff could easily get into production release. Not nice. ]

    disassembling outputs:

    --- objdump/ix86 -------------------
    00000000 <a>:
    0: 55 push %ebp
    1: 89 e5 mov %esp,%ebp
    3: 83 ec 68 sub $0x68,%esp
    6: 83 7d 08 05 cmpl $0x5,0x8(%ebp)
    a: 7e 1c jle 28 <a+0x28>
    c: 83 ec 08 sub $0x8,%esp
    f: 8d 45 b8 lea 0xffffffb8(%ebp),%eax
    12: 50 push %eax
    13: 8d 45 d8 lea 0xffffffd8(%ebp),%eax
    16: 50 push %eax
    17: e8 fc ff ff ff call 18 <a+0x18>
    18: R_386_PC32 printf
    1c: 83 c4 10 add $0x10,%esp
    1f: b8 01 00 00 00 mov $0x1,%eax
    24: c9 leave
    25: c3 ret
    26: 89 f6 mov %esi,%esi
    28: 83 ec 08 sub $0x8,%esp
    2b: 8d 45 98 lea 0xffffff98(%ebp),%eax
    2e: eb e2 jmp 12 <a+0x12>
    ------------------------------------

    --- objdump/ppc82xx ----------------
    00000000 <a>:
    0: 94 21 ff 90 stwu r1,-112(r1)
    4: 7c 08 02 a6 mflr r0
    8: 90 01 00 74 stw r0,116(r1)
    c: 2c 03 00 05 cmpwi r3,5
    10: 40 81 00 18 ble 28 <a+0x28>
    14: 38 61 00 08 addi r3,r1,8
    18: 38 81 00 28 addi r4,r1,40
    1c: 4c c6 31 82 crclr 4*cr1+eq
    20: 48 00 00 01 bl 20 <a+0x20>
    20: R_PPC_REL24 printf
    24: 48 00 00 14 b 38 <a+0x38>
    28: 38 61 00 08 addi r3,r1,8
    2c: 38 81 00 48 addi r4,r1,72
    30: 4c c6 31 82 crclr 4*cr1+eq
    34: 48 00 00 01 bl 34 <a+0x34>
    34: R_PPC_REL24 printf
    38: 38 60 00 01 li r3,1
    3c: 80 01 00 74 lwz r0,116(r1)
    40: 7c 08 03 a6 mtlr r0
    44: 38 21 00 70 addi r1,r1,112
    48: 4e 80 00 20 blr
    ------------------------------------

    --
    Ihar 'Philips' Filipau / with best regards from Saarbruecken.
    -- _ _ _
    "... and for $64000 question, could you get yourself |_|*|_|
    vaguely familiar with the notion of on-topic posting?" |_|_|*|
    -- Al Viro @ LKML |*|*|*|
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