[LLVMdev] TBAA metadata

[Xin Tong]

On Thu, Apr 9, 2015 at 1:58 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
> On Thu, Apr 9, 2015 at 1:11 PM, Xin Tong <trent.tong at gmail.com> wrote:
>> Hi
>>
>> I do not really understand why frontend generated TBAA metadata is
>> needed for the TBAA pass to work. It seems to me that we can always go
>> up the IR chain and find the base type from which the pointer is
>> derived from. Take the following example.
>>
>
> LLVM Types != C types
>
>
>> I know %0 = load i32, i32* %a, align 4, !tbaa !1 and   store i32
>> %i.02, i32* %b, align 4, !tbaa !6
>> do not alias as their metadata !1 = !{!2, !3, i64 0} and !6 = !{!7,
>> !3, i64 0} tell me that they are derived from different (incompatible)
>> basetypes.
>> However i can also walk up the IR chain and find out %a is
>> point to basetype object of struct A and %b pointing to basetype
>> object of  struct B.
>
> Which tells you precisely nothing since LLVM has no inherent language,
> and thus, no inherent rules about aliasing :)
>
> It is perfectly legal to inttoptr, cast, whatever.

I see, but should not users be discouraged (or prohibited) from
writing a LLVM function in which they cast a struct A* to a struct B*
or vice versa and then linking with a LLVM IR generated from a C
program and still claim this is strict-C-aliasing compliant ?

I guess i am not getting how this is possible, the only ways i can
think of are to write something in LLVM IR directly or compile some C
programs (with castings) with fno-strict-aliasing and then link it
with a C program compiled with strict-aliasing.

I apologize if i am not making much sense.

-Trent
>
>
>>
>> Maybe, the metadata simplified the TBAA pass ?
>>
>> Thanks,
>> -Trent
>>
>> ; ModuleID = 'tbaa.ll'
>> target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
>> target triple = "x86_64-unknown-linux-gnu"
>>
>> %struct.A = type { i32 }
>> %struct.B = type { i32 }
>>
>> ; Function Attrs: nounwind uwtable
>> define i32 @foo(%struct.A* nocapture readonly %sa, %struct.B*
>> nocapture %sb) #0 {
>> entry:
>>   br label %for.body
>>
>> for.body:                                         ; preds = %entry, %for.body
>>   %i.02 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
>>   %sum.01 = phi i32 [ 0, %entry ], [ %add, %for.body ]
>>   %a = getelementptr inbounds %struct.A, %struct.A* %sa, i64 0, i32 0
>>   %0 = load i32, i32* %a, align 4, !tbaa !1
>>   %b = getelementptr inbounds %struct.B, %struct.B* %sb, i64 0, i32 0
>>   store i32 %i.02, i32* %b, align 4, !tbaa !6
>>   %add = add nsw i32 %0, %sum.01
>>   %inc = add nsw i32 %i.02, 1
>>   %cmp = icmp slt i32 %inc, 1024
>>   br i1 %cmp, label %for.body, label %for.end, !llvm.loop !8
>>
>> for.end:                                          ; preds = %for.body
>>   %sum.0.lcssa = phi i32 [ %add, %for.body ]
>>   ret i32 %sum.0.lcssa
>> }
>>
>> attributes #0 = { nounwind uwtable "less-precise-fpmad"="false"
>> "no-frame-pointer-elim"="false" "no-infs-fp-math"="false"
>> "no-nans-fp-math"="false" "stack-protector-buffer-size"="8"
>> "unsafe-fp-math"="false" "use-soft-float"="false" }
>>
>> !llvm.ident = !{!0}
>>
>> !0 = !{!"clang version 3.7.0 (trunk 524)"}
>> !1 = !{!2, !3, i64 0}
>> !2 = !{!"A", !3, i64 0}
>> !3 = !{!"int", !4, i64 0}
>> !4 = !{!"omnipotent char", !5, i64 0}
>> !5 = !{!"Simple C/C++ TBAA"}
>> !6 = !{!7, !3, i64 0}
>> !7 = !{!"B", !3, i64 0}
>> !8 = distinct !{!8, !9}
>> !9 = !{!"llvm.loop.unroll.disable"}
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