[LLVMdev] TBAA metadata

[Xin Tong]

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.

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.

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"}