[LLVMdev] PROPOSAL: struct-access-path aware TBAA

Shuxin Yang shuxin.llvm at gmail.com
Wed Mar 13 11:07:42 PDT 2013

> The program I gave was well typed :)

Hi, Daniel:
    Thank you for sharing your insight.  I didn't realized it is 
well-typed -- I'm basically a big nut of any std.
I'd admit std/spec is one of the most boring material on this planet:-).

    So, if I understand correct, your point is:
        if a std call a type-casting (could be one which is in 
bad-taste:-), TBAA has to respect such std.

   If that is strictly true, TBAA has to reply on point-to analysis. 
However, that would virtually disable
TBAA as most point-to set has "unknown" element.

    Going back to my previous mail,
> In the below example, GCC assumes p and q point to anything because
> they are incoming arguments.
>> ------------------------------
>> typedef struct {
>>      int x;
>> }T1;
>> typedef struct {
>>      int y;
>> }T2;
>> int foo(T1 *p, T2 *q) {
>>      p->x = 1;
>>      q->y = 4;
>>      return p->x;
>> }
>> --------------------------
Yes, gcc should assume p and q point to anything, however, the result 
contradict to the assumption --
It promote the p->x expression.

   If I fabricate a caller by stealing some code from your previous 
example, see bellow.
I think these code & your previous example (about placement new) share 
the same std.  I'm wondering
if gcc can give a correct result.

    foo_caller() {
        T1 t1;
        T1 *pt1;
        T2 *pt2 = new (pt1) T2;
        foo(pt1, pt2);

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