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From: Kuoping Hsu<kuoping@s...>
Date: Fri Sep 30 09:00:02 CEST 2005
Subject: [openrisc] GNU C keyword volatile, GNU compiler/optimizer and instruction re-ordering
Hi all,

I got one problem about the GNU compiler/optimizer on the instruction
re-ordering. To explain my question, there's a simple device on the
WISHBONE bus with 4 MMIO addressing I/O. When I write 0x8000000c to 1,
it will calculate some arithmetic algorithms from source buffer 'src',
and write the result back to destination buffer 'dst'.

some_funtion(char *src, char *dst, int size) {
int val;

....

*(int volatile)(0x80000000) = (int)(src);
*(int volatile)(0x80000004) = (int)(dst);
*(int volatile)(0x80000008) = size;

*(int volatile)(0x8000000c) = 1;

....

val = dst[0];

}

My question is, the C compiler may be re-order the assignment of 'val'
and '0x8000000c'. The variable 'val' will get the older value before the
simple device do. How can I solve the problem. My solution is write
another C function to program these MMIO, and put the C function to
another C file to prevent C compiler in-line it. Does anyone have better
solution?

There is also a discuses article on the following URL link. It more
precisely declare the re-ordering problem.

http://compilers.iecc.com/comparch/article/02-09-118

The re-ordering problem had greatly perplexed with me for a long time. I
very appreciated any suggestion.

Thanks and regards

Kuoping Hsu