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From: vda <vda@port.imtp.ilyichevsk.odessa.ua>
To: Jan-Benedict Glaw <jbglaw@microdata-pos.de>, linux-kernel@vger.kernel.org
Subject: Re: xchg and GCC's optimisation:-(
Date: Mon, 17 Dec 2001 15:33:47 -0200
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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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