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Hi MirceaFrom the RFC you mentioned, that is a Darwin specific implementation, which later got extended to support other targets. The main use case for the embed bitcode option is to allow compiler passing intermediate IR and command flags in the object file it produced for later use. For Darwin, it is used for bitcode recompilation, and some might use it to achieve other goals.In order to use this information properly, you needs to have tools that understand the layout and sections for embedded bitcode. You can't just use an ordinary linker, because like you said, an ELF linker will just append the bitcode. Depending on what you are trying to achieve, you need to implement the downstream tools, like linker, binary analysis tools, etc. to understand this concept.Steven
On Aug 24, 2020, at 7:10 PM, Mircea Trofin via llvm-dev <llvm...@lists.llvm.org> wrote:
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You should probably pull in some folks who implemented/maintain the feature for Darwin.
I guess they aren't linking this info, but only communicating in the object file between tools - maybe they flag these sections (either in the object, or by the linker) as ignored/dropped during linking. That semantic could be implemented in ELF too by marking the sections SHF_IGNORED or something (same-file split DWARF uses this technique).
So maybe the goal/desire is to have a different semantic, rather than the equivalent semantic being different on ELF compared to MachO.
So if it's a different semantic - yeah, I'd guess a flag that prefixes the module metadata with a length would make sense, then it can be linked naturally on any platform. (if the "don't link these sections" support on Darwin is done by the linker hardcoding the section name - then maybe this flag would also put the data in a different section that isn't linker stripped on Darwin, so users interested in getting everything linked together can do so on any platform)
But if this data is linked, then it'd be hard to know which command line goes with which module, yes? So maybe it'd make sense then to have the command line as a header before the module, in the same section. So they're kept together.
The .llvmbc / .llvmcmd section does not have the SHF_EXCLUDE flag. It
will be retained in the linked image.
>> So maybe the goal/desire is to have a different semantic, rather than the
>> equivalent semantic being different on ELF compared to MachO.
>>
>> So if it's a different semantic - yeah, I'd guess a flag that prefixes the
>> module metadata with a length would make sense, then it can be linked
>> naturally on any platform. (if the "don't link these sections" support on
>> Darwin is done by the linker hardcoding the section name - then maybe this
>> flag would also put the data in a different section that isn't linker
>> stripped on Darwin, so users interested in getting everything linked
>> together can do so on any platform)
>>
>> But if this data is linked, then it'd be hard to know which command line
>> goes with which module, yes? So maybe it'd make sense then to have the
>> command line as a header before the module, in the same section. So they're
>> kept together.
>>
>This last point was my follow-up :)
A module has a source_filename field.
clang -fembed-bitcode=all -c d/a.c
llvm-objcopy --dump-section=.llvmbc=a.bc a.o /dev/null
llvm-dis < a.bc => source_filename = "d/a.c"
The missing piece is a mechanism to extract a module from concatenated
bitcode (llvm-dis supports multi-module bitcode but not concatenated
bitcode https://reviews.llvm.org/D70153). I'll be happy to look into it:)
---
.llvmcmd may need the source file to be more useful.
>>>> <http://lists.llvm.org/pipermail/llvm-dev/2016-February/094851.html>,
>>>> and reading through the implementation, I'm piecing together that the goal
>>>> was to enable capturing IR right after clang and before passing it to
>>>> LLVM's optimization passes, as well as the command line options needed for
>>>> later compiling that IR to the same native object it was compiled to
>>>> originally (with the same compiler).
>>>>
>>>> Here's what I don't understand: say you have a.o and b.o compiled with
>>>> -fembed-bitcode=all. They are linked into a binary called my_binary. How do
>>>> you re-create the corresponding IR for modules a and b (let's call them
>>>> a.bc and b.bc), and their corresponding command lines? From what I can
>>>> tell, the linker just concatenates the IR for a and b in my_binary's
>>>> .llvmbc, and the same for the command line in .llvmcmd. Is there a
>>>> separator maybe I missed? For .llvmcmd, I could see how *maybe* -cc1 could
>>>> be that separator, what about the .llvmbc part? The magic number?
>>>>
>>>> Thanks!
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On 2020-08-28, Mircea Trofin via llvm-dev wrote:
>On Fri, Aug 28, 2020 at 11:22 AM David Blaikie <dbla...@gmail.com> wrote:
>
>> You should probably pull in some folks who implemented/maintain the
>> feature for Darwin.
>>
>> I guess they aren't linking this info, but only communicating in the
>> object file between tools - maybe they flag these sections (either in the
>> object, or by the linker) as ignored/dropped during linking. That semantic
>> could be implemented in ELF too by marking the sections SHF_IGNORED or
>> something (same-file split DWARF uses this technique).
The .llvmbc / .llvmcmd section does not have the SHF_EXCLUDE flag. It
will be retained in the linked image.
Here's the format I would suggest:
1. Put command-line flags in the module metadata instead of .llvmcmd.
2. Put each module in the bitcode wrapper supported by SkipBitcodeWrapperHeader, which includes a length field. I think LLVM only generates the wrapper for Darwin, but it can read the wrapper correctly on all platforms.
3. Change the .llvmbc section alignment so that no extra zeros are added between modules.
My use case: I'm using -fembed-bitcode on Linux as an alternative to the wllvm/whole-program-llvm tool. For my purposes, it'd be nice to also keep track of linker flags and other linker input files, but I can get most of what I need from the modules alone.
Sean
I investigated a bit about the bitcode file format today. The bitcode
is streaming style and I think an optional size field may be useful.
https://reviews.llvm.org/D86847 proposes to add a
BITCODE_SIZE_BLOCK_ID block. We actually don't need a container
because
the MODULE_CODE_SOURCE_FILENAME record encodes the source filename. We
can do a lightweight parse and obtain the field.
This should be fast because there are typically very few
blocks/records preceding MODULE_CODE_SOURCE_FILENAME.
For .llvmcmd, I am on the fence moving it into the bitcode. Downside:
retrieving the command line will be more difficult...
I'd like to mention that the functionality duplicates the existing
-frecord-command-line a bit...
% readelf -p .GCC.command.line a.o
String dump of section '.GCC.command.line':
[ 1] /tmp/clang-12 -c -frecord-command-line a.c
(GCC -frecord-gcc-switches uses a different format (some folks
consider it inferior to clang's format; and worse, the section is
SHF_MERGE|SHF_STRINGS):
% readelf -p .GCC.command.line a.o
String dump of section '.GCC.command.line':
[ 0] -imultiarch x86_64-linux-gnu
[ 1d] a.c
[ 21] -mtune=generic
[ 30] -march=x86-64
[ 3e] -frecord-gcc-switches
[ 54] -fasynchronous-unwind-tables