[LLVMdev] inttoptr and basicaa

[Arnold Schwaighofer]

BasicAA treats it conservatively if used on its own. It will return
mayalias for the two pointers.

TBAA operates based on the guarantee that pointers to different types
cannot alias (think C's strict aliasing rules).
Therein lies its power but also its danger, that is, nothing prevents the
programmer to write code that violates these rules (That's why we have
-fno-strict-aliasing).

So when basica gives up returning mayalias we query tbaa, which will return
- based on strict aliasing rules of our language - noalias.

opt -basicaa -print-alias-sets basicaa.ll -S -o -
Alias Set Tracker: 1 alias sets for 2 pointer values.
  AliasSet[0x109808c90, 2] may alias, Mod       Pointers: (i64* %sunkaddr4,
18446744073709551615), (i32* %sunkaddr2, 18446744073709551615)

opt -basicaa -tbaa -print-alias-sets basicaa.ll -S -o -
Alias Set Tracker: 2 alias sets for 2 pointer values.
  AliasSet[0x112d07cf0, 1] must alias, Mod       Pointers: (i64*
%sunkaddr4, 18446744073709551615)
  AliasSet[0x112d07d40, 1] must alias, Mod       Pointers: (i32*
%sunkaddr2, 18446744073709551615)

define void @basicaatest(%str* %a) {
  %sunkaddr1 = ptrtoint %str* %a to i32
  %sunkadd = add i32 %sunkaddr1, 4
  %sunkaddr2 = inttoptr i32 %sunkadd to i32*
  %sunkaddr3 = ptrtoint %str* %a to i32
  %sunkadd2 = add i32 %sunkaddr1, 4
  %sunkaddr4 = inttoptr i32 %sunkadd2 to i64*
  store i64 1, i64* %sunkaddr4, !tbaa !0
  store i32 2, i32* %sunkaddr2, !tbaa !2
  ret void
}

!0 = metadata !{metadata !"int64", metadata !1}
!1 = metadata !{}
!2 = metadata !{metadata !"int32", metadata !1}


On Thursday, November 8, 2012, Pranav Bhandarkar wrote:
>
> 2) If the answer to the above is no, then shouldn't basicaa treat the above
> conservatively before it looks at TBAA ?
>
>
> Thanks,
> Pranav
>
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