llvm dev - Nov 2015 - The tbaa metadata for structure field array access

Hello,

I have one question regarding the tbaa metadata !8 used for the load
HdFree->name[0] in the following example.
Apparently the tbaa metadata !8 = !{!3, !3, i64 0}  has lost the structure field
information. Why not !8 = !{!6, !3, i64 16}?

If the structure field name is a scalar, the clang will generate accurate meta
data. Why does the clang generate
conservative metadata for the array fields?

Thanks,

Jin


typedef struct TypHeader {
    unsigned long       size;
    struct TypHeader    * * ptr;
    char                name[3];
    unsigned char       type;
}       * TypHandle;



TypHandle       HdFree;

void foo(unsigned long s, unsigned char c, struct TypHeader    * *p){
        HdFree->size = s;
        HdFree->ptr = p;
        HdFree->name[0] = 10;
        HdFree->type = c;

}

target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

%struct.TypHeader = type { i64, %struct.TypHeader**, [3 x i8], i8 }

@HdFree = common global %struct.TypHeader* null, align 8

; Function Attrs: nounwind uwtable
define void @foo(i64 %s, i8 zeroext %c) #0 {
entry:
  %0 = load %struct.TypHeader*, %struct.TypHeader** @HdFree, align 8, !tbaa !1
  %size = getelementptr inbounds %struct.TypHeader, %struct.TypHeader* %0, i64
0, i32 0
  store i64 %s, i64* %size, align 8, !tbaa !5
  %arrayidx = getelementptr inbounds %struct.TypHeader, %struct.TypHeader* %0,
i64 0, i32 2, i64 0
  store i8 102, i8* %arrayidx, align 8, !tbaa !8
  %1 = load %struct.TypHeader*, %struct.TypHeader** @HdFree, align 8, !tbaa !1
  %arrayidx2 = getelementptr inbounds %struct.TypHeader, %struct.TypHeader* %1,
i64 0, i32 2, i64 1
  store i8 114, i8* %arrayidx2, align 1, !tbaa !8
  %2 = load %struct.TypHeader*, %struct.TypHeader** @HdFree, align 8, !tbaa !1
  %arrayidx4 = getelementptr inbounds %struct.TypHeader, %struct.TypHeader* %2,
i64 0, i32 2, i64 2
  store i8 101, i8* %arrayidx4, align 2, !tbaa !8
  %3 = load %struct.TypHeader*, %struct.TypHeader** @HdFree, align 8, !tbaa !1
  %type = getelementptr inbounds %struct.TypHeader, %struct.TypHeader* %3, i64
0, i32 3
  store i8 %c, i8* %type, align 1, !tbaa !9
  ret void
}

attributes #0 = { nounwind uwtable
"disable-tail-calls"="false"
"less-precise-fpmad"="false"
"no-frame-pointer-elim"="false"
"no-infs-fp-math"="false"
"no-nans-fp-math"="false"
"stack-protector-buffer-size"="8"
"target-cpu"="x86-64"
"target-features"="+sse,+sse2"
"unsafe-fp-math"="false"
"use-soft-float"="false" }

!llvm.ident = !{!0}

!0 = !{!"clang version 3.8.0 (trunk 1545)"}
!1 = !{!2, !2, i64 0}
!2 = !{!"any pointer", !3, i64 0}
!3 = !{!"omnipotent char", !4, i64 0}
!4 = !{!"Simple C/C++ TBAA"}
!5 = !{!6, !7, i64 0}
!6 = !{!"TypHeader", !7, i64 0, !2, i64 8, !3, i64 16, !3, i64 19}
!7 = !{!"long", !3, i64 0}
!8 = !{!3, !3, i64 0}
!9 = !{!6, !3, i64 19}


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