[LLVMdev] Trying to access the user defined variable name

[Devang Patel]

On Wed, Jul 14, 2010 at 3:03 PM, Arvind Sudarsanam
<arvind.sudarsanam at aggiemail.usu.edu> wrote:
> Dear all,
>
> I have the following source code:
>
> long f(long x, long y)
> {
>        long u;
>
>   u = x+y;
>   return u;
> };
>
> After clang (with debug option set) and llvm opt using mem2reg, I get
> the following .ll file
> ***************************************************************************
> ; ModuleID = '<stdin>'
> target datalayout =
> "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:32:32-n8:16:32"
> target triple = "i686-pc-linux-gnu"
>
> define i32 @f(i32 %x, i32 %y) nounwind {
> entry:
>  call void @llvm.dbg.value(metadata !{i32 %x}, i64 0, metadata !0)
>  call void @llvm.dbg.value(metadata !{i32 %y}, i64 0, metadata !7)
>  %add = add nsw i32 %x, %y, !dbg !8              ; <i32> [#uses=1]
>  call void @llvm.dbg.value(metadata !{i32 %add}, i64 0, metadata !10), !dbg !8
>  ret i32 %add, !dbg !11
> }
>
> declare void @llvm.dbg.declare(metadata, metadata) nounwind readnone
>
> declare void @llvm.dbg.value(metadata, i64, metadata) nounwind readnone
>
> !0 = metadata !{i32 524545, metadata !1, metadata !"x", metadata !2,
> i32 1, metadata !6} ; [ DW_TAG_arg_variable ]
> !1 = metadata !{i32 524334, i32 0, metadata !2, metadata !"f",
> metadata !"f", metadata !"f", metadata !2, i32 2, metadata !4, i1
> false, i1 true, i32 0, i32 0, null, i1 false, i1 false} ; [
> DW_TAG_subprogram ]
> !2 = metadata !{i32 524329, metadata !"testadd.c", metadata
> !"/home/asudarsanam/perforce/llvmtop/setup", metadata !3} ; [
> DW_TAG_file_type ]
> !3 = metadata !{i32 524305, i32 0, i32 12, metadata !"testadd.c",
> metadata !".", metadata !"clang 2.0", i1 true, i1 false, metadata !"",
> i32 0} ; [ DW_TAG_compile_unit ]
> !4 = metadata !{i32 524309, metadata !2, metadata !"", metadata !2,
> i32 0, i64 0, i64 0, i64 0, i32 0, null, metadata !5, i32 0, null} ; [
> DW_TAG_subroutine_type ]
> !5 = metadata !{metadata !6}
> !6 = metadata !{i32 524324, metadata !2, metadata !"long", metadata
> !2, i32 0, i64 32, i64 32, i64 0, i32 0, i32 5} ; [ DW_TAG_base_type ]
> !7 = metadata !{i32 524545, metadata !1, metadata !"y", metadata !2,
> i32 1, metadata !6} ; [ DW_TAG_arg_variable ]
> !8 = metadata !{i32 5, i32 4, metadata !9, null}
> !9 = metadata !{i32 524299, metadata !1, i32 2, i32 1} ; [
> DW_TAG_lexical_block ] !10 = metadata !{i32 524544, metadata !9,
> metadata !"u", metadata !2, i32 3, metadata !6} ; [
> DW_TAG_auto_variable ]
> !11 = metadata !{i32 7, i32 1, metadata !9, null}
> ******************************************************************
>
> Llvm has replaced the name of the local variable (u) by a temporary
> name (%add). However, the debug information contains the information
> about the original variable name.  **** !10 = metadata !{i32 524544,
> metadata !9, metadata !"u", metadata !2, i32 3, metadata !6} ; [
> DW_TAG_auto_variable ] ******

Variable name 'u' is intentionally preserved in DW_TAG_auto_variable
because user debugging this code does not know anything about temp.
names used by compiler.

> I have been trying for a while and have been unsuccessful in accessing
> this information (mapping between the temporary name (%add) and the
> actual name (u)).

That's what llvm.dbg.value is doing.

 call void @llvm.dbg.value(metadata !{i32 %add}, i64 0, metadata !10), !dbg !8

First parameter is the compiler generated temp. and 3rd parameter
provides info about the variable, including its name 'u'.

-
Devang