[LLVMdev] A few more questions about DIFactory and source-level debugging.

Talin
Wed, 21 Oct 2009 03:18:55 -0000

Well, I am much happier now that I understand about dsymutil, and can
actually step through my program in gdb. However, there are still some
issues that are puzzling me.
1) First off, the debugger appears to stop at odd points. The IR for my main
function looks correct to me:

define i32
@"main(tart.core.Array[tart.core.String])->int"(%"tart.core.Array[tart.core.String]"*
%args) {
entry:
  call void @llvm.dbg.func.start({ }* bitcast (%llvm.dbg.subprogram.type*
@llvm.dbg.subprogram to { }*))
  call void @llvm.dbg.stoppoint(i32 6, i32 22, { }* bitcast
(%llvm.dbg.compile_unit.type* @llvm.dbg.compile_unit to { }*))
  %testModuleReflection = call { } @testModuleReflection() ; <{ }>
[#uses=0]
  call void @llvm.dbg.stoppoint(i32 7, i32 19, { }* bitcast
(%llvm.dbg.compile_unit.type* @llvm.dbg.compile_unit to { }*))
  %testModuleMethods = call { } @testModuleMethods() ; <{ }> [#uses=0]
  call void @llvm.dbg.stoppoint(i32 8, i32 16, { }* bitcast
(%llvm.dbg.compile_unit.type* @llvm.dbg.compile_unit to { }*))
  %testFindMethod = call { } @testFindMethod()    ; <{ }> [#uses=0]
  call void @llvm.dbg.stoppoint(i32 9, i32 10, { }* bitcast
(%llvm.dbg.compile_unit.type* @llvm.dbg.compile_unit to { }*))
  call void @llvm.dbg.region.end({ }* bitcast (%llvm.dbg.subprogram.type*
@llvm.dbg.subprogram to { }*))
  ret i32 0
}


However, when I single step into the function, it stops at the the *second*
stop point. In fact, it appears to do this fairly consistently with all
functions - the very first statement is always skipped.

Here's the original source (with line numbers) for reference:

1 import tart.reflect.Module;
2 import tart.reflect.Method;
3
4 @EntryPoint
5 def main(args:String[]) -> int {
6  testModuleReflection();
7  testModuleMethods();
8  testFindMethod();
9  return 0;
10 }


2) Another weird thing is that I can't seem to declare function variables
that are not lvalues. The DIFactory::InsertDeclare method seems to require
that the Storage parameter be the result of an alloca. However, what about
function arguments that are passed by value, such as ints?

3) The same issue holds for immutable local variables. My language supports
the concept of a "assign once" variable (like 'final' in
Java). for which I
use SSA values directly rather than storage created via alloca(). Does this
means that there is no way to debug such variables?

4) There seems to be something weird going on with DW_TAG_inheritance: When
I print out the type in the debugger I see <> symbols:

2 = {
  <> = {
    <> = {
      __tib = 0x0
    },
    members of tart.reflect.Member:
    _name = 0x0,
    _fullName = 0x0,
    _kind = 0,

   .. etc ...


I'd like to see an example of DW_TAG_inheritance in the doc.

5) I can't seem to get the structure offsets to work. Here's what my
generated IR for a struct member looks like (reformatted somewhat):

@llvm.dbg.derivedtype104 = internal constant %llvm.dbg.derivedtype.type {
  i32 458765,
  { }* bitcast (%llvm.dbg.compile_unit.type* @llvm.dbg.compile_unit to {
}*),
  i8* getelementptr inbounds ([10 x i8]* @.str103, i32 0, i32 0),
  { }* bitcast (%llvm.dbg.compile_unit.type* @llvm.dbg.compile_unit to {
}*),
  i32 27,
  i64 mul (i64 ptrtoint (%1** getelementptr (%1** null, i32 1) to i64), i64
8),
  i32 mul (i32 ptrtoint (%1** getelementptr (%38* null, i32 0, i32 1) to
i32), i32 8),
  i64 mul (i64 ptrtoint (%1** getelementptr (%tart.reflect.Member* null, i64
0, i32 2) to i64), i64 8),
  i32 0,
  { }* bitcast (%llvm.dbg.derivedtype.type* @llvm.dbg.derivedtype102 to {
}*) },
  section "llvm.metadata" ; <%llvm.dbg.derivedtype.type*>
[#uses=1]


You can see the offset calculation on line 9. However, here's what dwarfdump
reports for the entry:

    0x00001dcd:         member [10]
                    name( "_fullName" )
                    type( {0x00001bbb} ( tart.core.String* ) )
                    decl file(
"/Users/talin/Projects/tart/trunk/stdlib/tart/reflect/Module.tart" )
                    decl line( 27 )
                    data member location( +0 ) <-- huh?

6) It might be good to mention in the source-level debugging docs that line
and column numbers are 1-based, not 0-based. I know that might seem obvious
but it threw me off for a bit.

7) Another thing that confused me for a while is that the debugging APIs all
want size, offset, and alignment values to be in bits, not bytes - but
ConstantExpr::getSizeOf() et al return a size in bytes (which makse sense,
since it  behaves like C sizeof). More confusing, however, the comments for
getSizeOf() doesn't say what units its result is in - I automatically
assumed that getSizeOf and DIFactory were compatible. Probably the simplest
thing would be to add a "getSizeOfInBits" method (and the same for
align and
offset) which could be used directly with DIFactory.

Note: All of the above results were produced with the current 2.6 branch
head.

-- Talin
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