>On 10 Apr 96 at 10:49, Tom May wrote:
>> I just sent a bug report and patch to bug-gcc concerning a problem in
>> gcc 2.7.2 common subexpression elimination that has possible
>> implications in the linux kernel.
[...]
>This is a classical aliasing situation; you are modifying *p
>indirectly via buf. I think almost every compiler assumes
>"no-pointer-aliasing" when optimizing; otherwise it's very hard to
>optimize.
When I was looking at the source to cse.c in gcc, it was apparent that
gcc does not assume no-pointer-aliasing. I suppose it could actually
make better code in some (many?) cases if it did. The cse stuff will
assume all memory is clobbered by a write to a non-fixed address, such
as through a pointer. In this case, the write is to a fixed address
but the __asm__ contains an explicit clobber for "memory". But that
clobber is falling through a crack in the logic and not being honored
because it is inside the `if' and the code to handle this situation is
slightly broken. It is clearly intended that it be honored.
It may not be difficult to add a -fassume-no-aliasing switch to the
compiler to get the behavior you mention, though.
Tom.