llvm dev - Aug 2015 - How to write a interprocedural optimization in LLVM backend?

Hi,

I'm wondering how can I write a global optimization pass in the LLVM
backend.
What I'm going to do is to merge constant pool entries that are shared
between different machine functions and emit these shared entries
in proper places between the functions.

For example, if `foo` and `bar` use the same global variable `var`, I'd
like to
generate something like the following:

foo:
    # code of foo
    # ....
    load r0, .LCPI_0
    load r0, r0    # load var
    # other code of foo

.LCPI_0:
    .long var # address of var

bar:
    # code of bar
    # ....
    load r0, .LCPI_0
    load r0, r0    # load var
    # other code of bar

The reason doing this is to reduce the code size as much as possible.
Our backend has a very limited PC-relative displacement for load/store
instructions.
I've tried the way that ARM and MIPS backend do (ConstantIslandPass),
but it does not share constant pool entries between functions and
increases code size in general.

It is possible to write a moudle pass in machine instruction level? If
possible,
how to do it? Any help is welcome. Thanks.

Cheers,
Huang
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