[PATCH] D36313: [llvm-dwarfdump] - Print section name and index when dumping .debug_info ranges

George Rimar via llvm-commits llvm-commits at lists.llvm.org
Tue Aug 8 01:40:05 PDT 2017

>Too much/slow? I'd be surprised if it was very expensive to do a single loop over the sections (~hundreds of sections in an object file? Fewer in a linked executable), >maybe lazily (when the first range is rendered) - it'd only happen once for the whole dump run (& add a lookup in the resulting map for each range entry dumping - >which, yeah, that's something, but still). dwarfdump's an interactive/user-focussed tool, it probably takes longer to print things to the terminal than most of this >computation. (& longer still for the user to read it, etc)

May be. What I was mean by "too much" is that I have feeling that scaning over all sections

anyways should be not neccessary for printing nice output here. I don't see the reasons for additional code compliction.

There are probably 2 possible cases:

Print ".section.name [index]" or "[index] .section.name". For latter case we do not need to scan over all sections to

columnize the "[index]" as we can just use total amount of sections to calculate padding. So why not to use this form ?

>>I would not omit something dependent on third conditions too, because it makes logic of output unclear forpeople that are not familar with tool. ("Why it prints >>section index for "foo" and does not for "bar" ? BUUUG !")
>Not sure it'd be particulary bug-like if the section numbers were only shown when the section names were ambiguous. Given two examples it would seem fairly >clear to me, I think, that the numbers were added to disambiguate two sections with the same name.

If somebody going to parse tool's output then it is easier to have consistent format.
Also (I'll show below) it can be not convinent or impossible to search for a section by name in a section table,
so if we are going to print section index at least in some cases, I think it worth then to print it in all cases.

>>Also I think often I am looking for section number in readelf/objdump/other tools,
>Really? That surprises me - why are the section numbers of interest to you? For myself I'm usually interested in the section name.

Ah, section name is often a final point of interest for sure, but numbers are very important to have. See:

When I do readelf -a for some file and look at symbol table, I see section index "1" for "main",
using it's index then I can look into section table.
Symbol table '.symtab' contains 16 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS main.c
    13: 0000000000000000    21 FUNC    GLOBAL DEFAULT    1 main

But if we would have only name here, like:
13: 0000000000000000    21 FUNC    GLOBAL DEFAULT    ".text.foobar" main

It would be harder or impossible to find it by name in section table.
For example, below is output of following:
ar -x libclangAnalysis.a
readelf -a ReachableCode.cpp.o

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .strtab           STRTAB           0000000000000000  003e8080
       00000000000094e9  0000000000000000           0     0     1
  [ 2] .text             PROGBITS         0000000000000000  00000040
       0000000000002c96  0000000000000000  AX       0     0     16
  [ 3] .rela.text        RELA             0000000000000000  002a7818
       0000000000001c38  0000000000000018          1486     2     8
  [ 4] .group            GROUP            0000000000000000  0029ede0
       0000000000000008  0000000000000004          1486   1136     4
  [ 5] .text             PROGBITS         0000000000000000  00002ce0
       0000000000000011  0000000000000000 AXG       0     0     16
  [ 6] .group            GROUP            0000000000000000  0029ede8
       000000000000000c  0000000000000004          1486   968     4
  [ 7] .text             PROGBITS         0000000000000000  00002d00
       00000000000000a7  0000000000000000 AXG       0     0     16
<~1500 the same sections here skpped.>

As you can see section name can be not enough.

>I'd skip printing it entirely if it's empty. 'Section: ""' doesn't seem helpful.
>I think printing it on the same line without the "Section: " prefix, as it was in your first version, is probably better when it is per-line.
>I guess maybe you're optimizing this for the case where some set of ranges share the same section (so it prints a "Section: "x"" header and then all the ranges in >that section? (or all the ranges in that section that are contiguous until another section change?)? Though I don't see a test case for that (multiple sections, some >ranges next to each other in the same section).

You earlier wrote: "& what about omitting the name or putting it somewhere else (like at the start on a separate line) if every entry is in the same section? (which will be the case for all ranges except the compile_unit ranges, most likely)", so that is what I tried to implement.

>But I don't think that's a scenario we realy need to optimize for - it's going to be pretty rare that a list contains both multiple sections and distinct ranges in the >same section. (function/scope ranges will contain ranegs all from the same section and CU ranges will /mostly/ contain all ranges from distinct sections - except in >cases of nodebug functions putting holes in the range)

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