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    SubjectRe: xchg and GCC's optimisation:-(
    Date
    On Monday 17 December 2001 10:45, Jan-Benedict Glaw wrote:

    > void free_dma(unsigned int dmanr)
    > {
    > if (dmanr >= MAX_DMA_CHANNELS) {
    > printk("Trying to free DMA%d\n", dmanr);
    > return;
    > }
    >
    > if (xchg(&dma_chan_busy[dmanr].lock, 0) == 0) {
    > /* ERROR */ printk("Trying to free free DMA%d\n", dmanr);
    > return;
    > }
    >
    > } /* free_dma */
    >
    > Including a real_printk() at the line marked with ERROR will
    > result in:
    [snip]
    > ...which is fine and contains the needed xchg call. However,
    > substituting the printk() with "do {} while (0)" above,
    > the "if" path seems to be completely removed by the optimizer:
    >
    > 00000088 <free_dma>:
    > 88: c3 ret
    > 89: 8d b4 26 00 00 00 00 lea 0x0(%esi,1),%esi
    >
    >
    > I've looked at ./include/asm-i386/system.h which does some black
    > magic with it, and I don't really understand that. However, the
    > result is that the xchg gets optimized away, rendering at least
    > the floppy module unuseable:-(

    There is a comment that asm is not 100% valid.
    My GCC 3.0.1 does not produce buggy code, guess why?
    It does _not_ inline __xchg() even at -O99!

    So much of compiler improvement 8-(
    --
    vda
    -
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