[llvm-dev] Understanding how to use of debugger intrinsic functions

[via llvm-dev]

> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Peng
> Yu via llvm-dev
> Sent: Monday, January 28, 2019 7:37 PM
> To: llvm-dev
> Subject: [llvm-dev] Understanding how to use of debugger intrinsic
> functions
> 
> I see debugger intrinsic functions like @llvm.dbg.declare appear in
> the .ll generated with -g. But I still don't understand how it is
> used.
> 
> https://llvm.org/docs/SourceLevelDebugging.html#debugger-intrinsic-
> functions
> 
> For the following C program, I got the relevant text in the .ll file
> pasted below and the relevant metadata (listed using depth-first
> search).
> 
> Does @llvm.dbg.declare actually translate to some machine code similar
> to other user-defined functions? It seems that it only describes the
> data types. Will they be translated to a native debugging format to
> store along with the executable?

@llvm.dbg.declare associates the `alloca` for a variable with its
debug-info metadata description.  It does not translate into machine
code, but provides information for the production of DWARF or CodeView 
debug info in the object file.

The existence of debug info is not supposed to change anything about 
machine code generation. This "supposed to" is not perfect, but that
is the goal.  Some of us believe that using IR instructions to supply
debug information is an anti-pattern, and it has certainly been a source
of bugs over the years where optimization passes forget to accommodate
or ignore debug-info instructions correctly, but so far nobody has had
the time available to design a different mechanism.

> 
> Can they be used for binary instrumentations? Thanks.

I don't see how, but that's not really an area I'm familiar with.
--paulr

> 
> //////
> #include <stdio.h>
> 
> int f(int i) {
>     return i + 1;
> }
> 
> int main(int argc, char *argv[1]) {
>     if(f(argc)) {
>         puts("Hello World!");
>     }
>     return 0;
> }
> //////
> 
> define i32 @main(i32 %arg, i8** %arg1) #0 !dbg !17 {
> ...
>   store i32 %arg, i32* %tmp2, align 4
>   call void @llvm.dbg.declare(metadata i32* %tmp2, metadata !23,
> metadata !DIExpression()), !dbg !24
>  ...
> 
> !23 = !DILocalVariable(name: "argc", arg: 1, scope: !17, file: !1,
> line: 8, type: !11)
> 
> !17 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line:
> 8, type: !18, isLocal: false, isDefinition: true, scopeLine: 8, flags:
> DIFlagPrototyped, isOptimized: false, unit: !0, retainedNodes: !2)
> !1 = !DIFile(filename: "../test_data/main.c", directory:
> "/Users/pengy/linux/bin/wrappercomposite/src/xplat/llvmxplat/src/lvll2/lvl
> l2fundbi/llin2funbodyraw0/main")
> !18 = !DISubroutineType(types: !19)
> !19 = !{!11, !11, !20}
> !11 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
> !20 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !21, size: 64)
> !21 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !22, size: 64)
> !22 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
> 
> !24 = !DILocation(line: 8, column: 14, scope: !17)
> 
> --
> Regards,
> Peng
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