llvm dev - Nov 2012 - [LLVMdev] Dynamic optimalization passes in LLVM based compiler

Hi!
I'm new to LLVM but I've read tons of articles, I want to implement my
own
compiler and I came across a big problem.
I have several questions, that I cannot answer myself:

1) If I'm writing custom compiler do I have to "hardcode" passes
that it
uses (like in Kaleidoscope example:
http://llvm.org/docs/tutorial/LangImpl4.html) or I have to generate LLVM IR
and then use the 'opt' tool to run selected passes on generated code?
I think the solution with opt is not quite good, because the opt tool has
to parse the LLVM IR (or BC) input file, which is not needed, because we
are generating it, so we have had it in memory before.
Maybe there is another better solution allowing for enabling and disabling
passes in custom compiler with argument options like in opt?

2) I want to write compiler that does NOT generate LLVM IR by its own, it
should simply run one of available module passes and such pass will
generate LLVM IR.
The motivation behind this decision is that I want to have a graph (C++
serialized structure) as compiler input and I want to load this graph as
pass, run other passes (which will modify this graph) and then run a
"conversion module pass", which will convert this graph into LLVM IR.
Additional I want to be able to read several formats and because of that I
want to load this graph as a pass. (This pass will be of course grouped
with other "load passes")

Could you please tell me what will be the best (most flexible and easy)
solution to do this, keeping in mind the first question?

I have an idea of solution (which does not work completely) - the idea is
to create an compiler which will initialize the base module and will do
nothing at all. Then I can use the opt tool with my module passes, which
will load, modify graph and convert it to LLVM IR (with IRBUilder) - the
problem is if the opt could be run without input file and if it will handle
correctly this situation.

I was researching very long and I have not found any good answer for these
problems.
I would be very thankful for any help!
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On Sat, Nov 17, 2012 at 4:44 AM, Wojciech Daniło
<wojtek.danilo.ml at gmail.com> wrote:
> Hi!
> I'm new to LLVM but I've read tons of articles, I want to implement
my own
> compiler and I came across a big problem.
> I have several questions, that I cannot answer myself:
>
> 1) If I'm writing custom compiler do I have to "hardcode"
passes that it
> uses (like in Kaleidoscope example:
> http://llvm.org/docs/tutorial/LangImpl4.html) or I have to generate LLVM IR
> and then use the 'opt' tool to run selected passes on generated
code?
> I think the solution with opt is not quite good, because the opt tool has
to
> parse the LLVM IR (or BC) input file, which is not needed, because we are
> generating it, so we have had it in memory before.
> Maybe there is another better solution allowing for enabling and disabling
> passes in custom compiler with argument options like in opt?
I believe Clang just hardcodes passes. If you a user wants to
experiment with different pass options they can use the option to
generate LLVM bitcode from Clang then pass that to opt themselves.
> 2) I want to write compiler that does NOT generate LLVM IR by its own, it
> should simply run one of available module passes and such pass will
generate
> LLVM IR.
> The motivation behind this decision is that I want to have a graph (C++
> serialized structure) as compiler input and I want to load this graph as
> pass, run other passes (which will modify this graph) and then run a
> "conversion module pass", which will convert this graph into LLVM
IR.
> Additional I want to be able to read several formats and because of that I
> want to load this graph as a pass. (This pass will be of course grouped
with
> other "load passes")
LLVM's pass system is for IR transformations only. Anything else you
want to do you'll have to build separately/in front of LLVM. Once your
other system generates IR, then you can pass it to LLVM.
>
> Could you please tell me what will be the best (most flexible and easy)
> solution to do this, keeping in mind the first question?
>
> I have an idea of solution (which does not work completely) - the idea is
to
> create an compiler which will initialize the base module and will do
nothing
> at all. Then I can use the opt tool with my module passes, which will load,
> modify graph and convert it to LLVM IR (with IRBUilder) - the problem is if
> the opt could be run without input file and if it will handle correctly
this
> situation.
>
> I was researching very long and I have not found any good answer for these
> problems.
> I would be very thankful for any help!
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>

Thank you for yours response :)
I know that LLVM Pass was designed to transform IR, but lets focus on an
example - LLVM Pass is a function that transform some set of input into
output. It can transform IR into graph of lets say strongly connected
components and then other passes can use it (that data - not IR) to
generate other data OR to manipulate the IR.

So why I can not create passes, that would need data generated by other
passes (ie. graph loaded from disk) and then transform it into LLVM IR? I
do not see any difference between these cases.
Am I wrong?

2012/11/17 David Blaikie <dblaikie at gmail.com>
> On Sat, Nov 17, 2012 at 4:44 AM, Wojciech Daniło
> <wojtek.danilo.ml at gmail.com> wrote:
> > Hi!
> > I'm new to LLVM but I've read tons of articles, I want to
implement my
> own
> > compiler and I came across a big problem.
> > I have several questions, that I cannot answer myself:
> >
> > 1) If I'm writing custom compiler do I have to
"hardcode" passes that it
> > uses (like in Kaleidoscope example:
> > http://llvm.org/docs/tutorial/LangImpl4.html) or I have to generate
> LLVM IR
> > and then use the 'opt' tool to run selected passes on
generated code?
> > I think the solution with opt is not quite good, because the opt tool
> has to
> > parse the LLVM IR (or BC) input file, which is not needed, because we
are
> > generating it, so we have had it in memory before.
> > Maybe there is another better solution allowing for enabling and
> disabling
> > passes in custom compiler with argument options like in opt?
>
> I believe Clang just hardcodes passes. If you a user wants to
> experiment with different pass options they can use the option to
> generate LLVM bitcode from Clang then pass that to opt themselves.
>
> > 2) I want to write compiler that does NOT generate LLVM IR by its own,
it
> > should simply run one of available module passes and such pass will
> generate
> > LLVM IR.
> > The motivation behind this decision is that I want to have a graph
(C++
> > serialized structure) as compiler input and I want to load this graph
as
> > pass, run other passes (which will modify this graph) and then run a
> > "conversion module pass", which will convert this graph into
LLVM IR.
> > Additional I want to be able to read several formats and because of
that
> I
> > want to load this graph as a pass. (This pass will be of course
grouped
> with
> > other "load passes")
>
> LLVM's pass system is for IR transformations only. Anything else you
> want to do you'll have to build separately/in front of LLVM. Once your
> other system generates IR, then you can pass it to LLVM.
>
> >
> > Could you please tell me what will be the best (most flexible and
easy)
> > solution to do this, keeping in mind the first question?
> >
> > I have an idea of solution (which does not work completely) - the idea
> is to
> > create an compiler which will initialize the base module and will do
> nothing
> > at all. Then I can use the opt tool with my module passes, which will
> load,
> > modify graph and convert it to LLVM IR (with IRBUilder) - the problem
is
> if
> > the opt could be run without input file and if it will handle
correctly
> this
> > situation.
> >
> > I was researching very long and I have not found any good answer for
> these
> > problems.
> > I would be very thankful for any help!
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
>
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