[llvm-dev] Understanding how to use of debugger intrinsic functions (original) (raw)

via llvm-dev llvm-dev at lists.llvm.org
Tue Jan 29 06:55:41 PST 2019

-----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: "../testdata/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: DWATEsigned) !20 = !DIDerivedType(tag: DWTAGpointertype, baseType: !21, size: 64) !21 = !DIDerivedType(tag: DWTAGpointertype, baseType: !22, size: 64) !22 = !DIBasicType(name: "char", size: 8, encoding: DWATEsignedchar) !24 = !DILocation(line: 8, column: 14, scope: !17) -- Regards, Peng

LLVM Developers mailing list llvm-dev at lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

More information about the llvm-dev mailing list