Messages in this thread |  | | Date | Mon, 17 Dec 2001 13:45:26 +0100 | From | Jan-Benedict Glaw <> | Subject | xchg and GCC's optimisation:-( |
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Hi!
I recently posted a patch against 2.2.19 which eliminates most printk()s by (in short words):
#define printk(format, arg...) do {} while(0)
However, I got trouble using the floppy driver, because ./kernel/dma.c:free_dma() seems to get miscompiled:
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: 00000088 <free_dma>: 88: 83 ec 0c sub $0xc,%esp 8b: 8b 54 24 10 mov 0x10(%esp,1),%edx 8f: 83 fa 07 cmp $0x7,%edx 92: 77 1e ja b2 <free_dma+0x2a> 94: 31 c0 xor %eax,%eax 96: 87 04 d5 00 00 00 00 xchg %eax,0x0(,%edx,8) 9d: 85 c0 test %eax,%eax 9f: 75 11 jne b2 <free_dma+0x2a> a1: 83 c4 f8 add $0xfffffff8,%esp a4: 52 push %edx a5: 68 11 00 00 00 push $0x11 aa: e8 fc ff ff ff call ab <free_dma+0x23> af: 83 c4 10 add $0x10,%esp b2: 83 c4 0c add $0xc,%esp b5: c3 ret b6: 8d 76 00 lea 0x0(%esi),%esi b9: 8d bc 27 00 00 00 00 lea 0x0(%edi,1),%edi
...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:-(
MfG, JBG
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