[LLVMbugs] [Bug 11112] New: llvm-ld corrupts debugging information of local variables.

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=11112

           Summary: llvm-ld corrupts debugging information of local
                    variables.
           Product: libraries
           Version: trunk
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Linker
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: haohui.mai at gmail.com
                CC: llvmbugs at cs.uiuc.edu


Created an attachment (id=7440)
 --> (http://llvm.org/bugs/attachment.cgi?id=7440)
the bitcode file

I'm using clang+llvm to compile my project, and I found out llvm-ld somehow
made the debugging information of local variables unavailable in debugger.

Here is a simple C program:

extern int putchar(int c);

int main() {
        for (int debug_var = 0; debug_var < 26; ++debug_var)
                putchar(debug_var + 'A');
        return 0;
}

I compiled it with clang in mainline:

$clang -march=armv7 -mfloat-abi=soft -ccc-host-triple arm-elf -Wall -Werror
-emit-llvm t.c -c -o t.bc -O0 -g

The debugging information for "debug_var" is available after llc:

$llc -disable-fp-elim -march=arm -mcpu=cortex-a9 -mattr=+vfp3,+neon
-float-abi=soft -O=0 t.bc -o -|grep "debug_var" -a3
    .long    .Ltmp0                  @ DW_AT_low_pc
    .long    .Ltmp1                  @ DW_AT_high_pc
    .byte    4                       @ Abbrev [4] 0x7f:0x14 DW_TAG_variable
    .ascii     "debug_var"            @ DW_AT_name
    .byte    0
    .byte    1                       @ DW_AT_decl_file
    .byte    4                       @ DW_AT_decl_line

However, if I pass the bitcode file through llvm-ld, the information is no
longer available:

$llvm-ld -disable-opt -link-as-library t.bc -o -|/opt/local/ibos/bin/llc
-disable-fp-elim -march=arm -mcpu=cortex-a9 -mattr=+vfp3,+neon -float-abi=soft
-O=0|grep "debug_var" -a3

The diff of bitcode before and after llvm-ld shows slight difference:

$ diff -u before-llvm-ld.ll after-llvm-ld.ll
...
-  call void @llvm.dbg.declare(metadata !{i32* %debug_var}, metadata !12), !dbg
!15
-  store i32 0, i32* %debug_var, align 4, !dbg !16
-  br label %for.cond, !dbg !16
+  call void @llvm.dbg.declare(metadata !12, metadata !13), !dbg !16
+  store i32 0, i32* %debug_var, align 4, !dbg !17
+  br label %for.cond, !dbg !17
...
-!12 = metadata !{i32 721152, metadata !13, metadata !"debug_var", metadata !6,
i32 4, metadata !9, i32 0, i32 0} ; [ DW_TAG_auto_variable ]
-!13 = metadata !{i32 720907, metadata !14, i32 4, i32 2, metadata !6, i32 1} ;
[ DW_TAG_lexical_block ]
+!12 = metadata !{null}
+!13 = metadata !{i32 721152, metadata !14, metadata !"debug_var", metadata !6,
i32 4, metadata !9, i32 0, i32 0} ; [ DW_TAG_auto_variable ]

You can see the first argument of @llvm.dbg.declare is nullified for some
reason.

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