llvm dev - Sep 2012 - [LLVMdev] Can clang generate the same bitcode with the toolchains that have same version of libraries but different targets

Hi,

I'm using clang to build my project as follows:

$ clang++  -ccc-host-triple arm-none-linux-gnueabi
--sysroot=/usr/local/arm-none-linux-gnueabi/arm-none-linux-gnueabi/libc/ -o
foo.o -emit-llvm -c foo.cpp

$ clang++  -ccc-host-triple arm-none-linux-gnueabi
--sysroot=/usr/local/arm-none-linux-gnueabi/arm-none-linux-gnueabi/libc/ -o
main.o -emit-llvm -c main.cpp

$ llvm-link foo.o main.o -o *main.bc*

$ llc main.bc -o main.ARM.s

$ arm-none-linux-gnueabi-g++ main.ARM.s -o main.ARM


1. Can clang generate the same bitcode (*main.bc *in the above
example) with the toolchains that have same version of libraries but
different targets?

2. Is there any possibility that the generated bitcode with the ARM
toolchain (*main.bc *in the above example) can be compiled to another
platform, like PowerPC or mips?

Currently, I tried the toolchains of ARM and PowerPC that have the
different versions of libraries, obviously the generated bitcode are
different.
So I wonder whether the version of library is the only reason why the
generated bitcode are different.


Thanks,
Kenia Kuo
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Kenia Kuo <kenkillerkuo at gmail.com> writes:

[snip]
> Currently, I tried the toolchains of ARM and PowerPC that have the
> different versions of libraries, obviously the generated bitcode are
> different.
> So I wonder whether the version of library is the only reason why the
> generated bitcode are different.
http://llvm.org/docs/FAQ.html#can-i-compile-c-or-c-code-to-platform-independent-llvm-bitcode

Hi Óscar,

Thank you for your reply.

It looks like the limitations are the platform's API and ABI (included the
size of variable).
So, if there are two platforms that have the same API, ABI but different
ISAs, the bitcode can be shared. Can I say that?

Thanks,
Kenia Kuo

2012/9/25 Óscar Fuentes <ofv at wanadoo.es>
> Kenia Kuo <kenkillerkuo at gmail.com> writes:
>
> [snip]
>
> > Currently, I tried the toolchains of ARM and PowerPC that have the
> > different versions of libraries, obviously the generated bitcode are
> > different.
> > So I wonder whether the version of library is the only reason why the
> > generated bitcode are different.
>
>
>
http://llvm.org/docs/FAQ.html#can-i-compile-c-or-c-code-to-platform-independent-llvm-bitcode
>
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Hi David,

Thank you for your reply.

In fact, my question originally came from the google native client.
I thought that the bitcode can be shared among the different targets as it
inherently is.
Therefore, I used the clang to generate the bitcode to prove that my
thought is right,
and I supposed that the different bitcode caused from the different
versions of libraries before.

Thanks for the above answering, I know the limitations of sharing the
bitcode now.


Best regards,
Kenia

2012/9/26 David Tweed <david.tweed at arm.com>
> Hi Kenia,****
>
> ** **
>
> What you might be trying to end up with (rather than the question
you're
> directly asking J ) is a platform independent compiled representation.****
>
> (Of course you might be aiming to do something else, like a cross-platform
> compile farm, in which case I have no input...)****
>
> If this is the goal, the google native client team are working on PNaCl,
> which is (in addition to "safety properties") designed to be****
>
> platform independent****
>
> ** **
>
>
>
http://www.chromium.org/nativeclient/pnacl/building-and-testing-portable-native-client
> ****
>
> ** **
>
> I've not kept up with this project, but I believe that this is only
> achieved by considering all the possible ABIs of the targets and****
>
> ensuring enough information is provided so that the final to-native
> compilation stage can resolve everything, so it's more like
"bitcode able
> to accommodate****
>
> all the listed targets" rather than "bitcode that's target
independent".**
> **
>
> ** **
>
> HTH,****
>
> Dave****
>
> ** **
>
> *From:* llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu] *On
> Behalf Of *Kenia Kuo
> *Sent:* 25 September 2012 15:49
> *To:* Óscar Fuentes
> *Cc:* llvmdev at cs.uiuc.edu
> *Subject:* Re: [LLVMdev] Can clang generate the same bitcode with the
> toolchains that have same version of libraries but different targets****
>
> ** **
>
> Hi Óscar,****
>
> Thank you for your reply.****
>
> ** **
>
> It looks like the limitations are the platform's API and ABI (included
the
> size of variable).****
>
> So, if there are two platforms that have the same API, ABI but different
> ISAs, the bitcode can be shared. Can I say that?****
>
> ** **
>
> Thanks,****
>
> Kenia Kuo****
>
> ** **
>
> 2012/9/25 Óscar Fuentes <ofv at wanadoo.es>****
>
> Kenia Kuo <kenkillerkuo at gmail.com> writes:
>
> [snip]****
>
>
> > Currently, I tried the toolchains of ARM and PowerPC that have the
> > different versions of libraries, obviously the generated bitcode are
> > different.
> > So I wonder whether the version of library is the only reason why the
> > generated bitcode are different.****
>
>
>
http://llvm.org/docs/FAQ.html#can-i-compile-c-or-c-code-to-platform-independent-llvm-bitcode
> ****
>
> ** **
>
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Hi,
I am interested in building some large projects to get single .bc files. Is
there an easy way to do this? Or do I have to go through and understand the
whole makefile script?
Thank you very much.
Andy
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On Thu, Nov 1, 2012 at 10:04 PM, Zhoujinguo <zhoujinguo1988 at yahoo.cn>
wrote:
> Hi,
>
> I am interested in building some large projects to get single .bc files.
> Is there an easy way to do this? Or do I have to go through and understand
> the whole makefile script?
>
This is what LLVM's "LTO" (Link Time Optimization) does,
basically. This is
triggered by building with -O4 and you'd need to use an LTO-aware linker
(like gold) to handle the link step. I'm not sure how you'd actually
dump
out bitcode from the linker, rather than a final binary - but that's
more-or-less where you'd get involved.

I realize this is a bit vague (as I'm by no means an expert on the matter)
but thought it might be at least a pointer to some ideas/approach.

- David
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