[LLVMdev] Customize Standard C Library Using LLVM (to support llvm backend optimization)

[Chao Yan]

Purpose:

I implemented a pass on LLVM backend that changes the output format of
*ARM* assembly/binary
(e.g add a jump at the end of each basic block to eliminate fall through).
By calling:

llc -march=arm somefile.bc

it generates expected arm assembly/binary that runs properly on arm gnu
linux (I use qemu-arm and gem5 to simulate it). Now I want to do the same
thing on standard c library, but here are problems.
Problems:

According to:

http://article.gmane.org/gmane.comp.compilers.llvm.devel/77025
https://llvm.org/svn/llvm-project/llvm/tags/RELEASE_14/docs/OpenProjects.html#glibc

compiling glibc using llvm may not be a proper option. On the other hand,
according to:

http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-January/047088.html

llvm could be able to compile newlib, thus people consider newlib as an
alternative. However, according to:

http://www.embecosm.com/appnotes/ean9/ean9-howto-newlib-1.0.html#id2711887

newlib intends to support binaries for bare metal (no OS) software. It
implements only the hardware independent parts (e.g libc and libm) and
leave a stub for each hardware dependent syscall (e.g everything in
libgloss).

In fact I tried to compile a simple "hello world" c program using
arm-none-eabi-gcc which was configured with "--with-newlib" option, the
program execution ends up with segmentation faults on both qemu-arm and
gem5.
Questions:

I'm not sure if the newlib is compatible with glibc. I'm wondering if I
could use llvm to cross-compile the machine independent parts (at the same
time change the arm output format) from newlib and use
arm-none-linux-gnueabi-gcc to cross-compile the machine dependent parts
from glibc and put these two parts together to generate my own standard c
library?

There might be mistakes/misunderstandings in my work. Are there any other
possible methods that could add my changes to at least part of the standard
c libraries, and make the program run on qemu-arm or gem5?
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