Looks fine to me.
2011/9/7 Wu Yongchong <wuyongch...@gmail.com>
> Hi all,
>
> Could a gatekeeper review the attached patch for bug 826, which is an
> output difference failure in accessing union member for the attached
> test case?
>
> Bug description:
> ----------------
> When the test case is compiled with the following command,
> $ opencc name.c
> the execution fails as follows.
> $ ./a.out
>
> failed
>
> As shown below, 'function' just returns the dereferenced value of the
> passed pointer to a structure.
> struct mystruct { char aa[32]; };
>
> struct mystruct function( struct mystruct *p)
> {
> return *p;
> }
>
> However, a problem occurs if the addresses of the passed function
> argument and the structure variable being defined by the function call
> overlap, because the fix for revision 3659 removes a copy into the
> intermediate buffer. Without this intermediate buffer, in the attached
> test case, the content of my_uni.un1.st1(function argument) is
> overwritten when 'function' is called to define my_uni.un2.st2. This
> is similar to a case where a call to memcpy(&my_uni.un2.st2,
> &my_uni.un1.st1, sizeof(struct mystruct) has two overlapping
> addresses.
> struct container1 { struct mystruct st1; int bi; };
> struct container2 { int ci; struct mystruct st2; };
> union myunion
> {
> struct container1 un1;
> struct container2 un2;
> };
> my_uni.un2.st2 = function(&my_uni.un1.st1);
>
> Before revision 3659, the caller reserved an intermediate space on
> stack and passed the address as the first 'hidden' argument together
> with the actual argument. Then, the callee copies the return value
> (the content of my_uni.un1.st1 in the example) to this intermediate
> buffer and then the caller copies the intermediate buffer to the
> variable being defined (my_uni.un2.st2 in the example).
>
> The fix for revision 3659 removes this intermediate buffer by passing
> the address of the variable being defined (my_uni.un2.st2 in the
> example) directly as the hidden first argument. This is a nice
> optimization but problem occurs if the function reads data from the
> variable being defined as in the above example.
>
> This optimization of removing the intermediate buffer can be done if
> the variable being defined is a local variable and its address is not
> taken. However, this couldn't be done in the current implementation
> location, which is 'wgen', without adding another pass through the
> input programs, because the 'address taken' information is not
> complete at that point.
>
> Patch:
> ------
> The fix is to partially undo the change for revision 3659. The change
> for revision 3659 includes other fix in addition to the one for
> removing 'structure copies'. This fix undoes only the part for
> optimization of removing structure copies.
>
> --
> yongchong
>
>
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>
--
Regards,
Lai Jian-Xin
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