[LLVMdev] TBAA metadata

[Daniel Berlin]

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.


>
> 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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