[llvm-dev] IPRA, interprocedural register allocation, question

[Lawrence, Peter via llvm-dev]

Vivek,
          Here’s the way I see it, let me know if you agree or disagree,

You cannot optimize a function’s calling convention (register-usage) unless
You can see and change every caller, and you only know this for non-static functions
if you know that all calls to external functions cannot call back into the current
compilation unit.

#1 gives you the info necessary to change the call-site to the external function

So you don’t need #2 to do RA around the call-site to the external function, instead
You need #2 before you can change any non-static function’s calling convention
within the current compilation unit, assuming you have this information for all
external functions.

To be more concrete, let foo() be a non-static function in the current compilation
Unit,  any calls to foo() from external functions will have to use the “default”
Calling convention, so foo’s calling convention cannot be changed.  We have to
Know that none of the external functions can call-back to the compilation unit
(they are “leaf” functions relative to the compilation unit) before we can change
Foo()’s calling convention.


Also, the issue of escaping-pointer-to-function is made clear by the example
Of the atexit() and exit() library functions,  IE even static functions can end up
Being called by external functions.  So exit() can never be declared “leaf”, and
To get the benefit of IPRA it needs to be within the compilation unit, either
By whole-program compilation or by LTO, if it is used.


--Peter.






From: vivek pandya [mailto:vivekvpandya at gmail.com]
Sent: Friday, July 08, 2016 9:26 PM
To: Lawrence, Peter <c_plawre at qca.qualcomm.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>; llvm-dev-request at lists.llvm.org; Hal Finkel <hfinkel at anl.gov>; Tim Amini Golling <mehdi.amini at apple.com>
Subject: Re: Re:[llvm-dev] IPRA, interprocedural register allocation, question



On Sat, Jul 9, 2016 at 8:15 AM, Lawrence, Peter <c_plawre at qca.qualcomm.com<mailto:c_plawre at qca.qualcomm.com>> wrote:
Vivek,
           IIUC it seems that we need two pieces of information to do IPRA,
1. what registers the callee clobbers
2. what the callee does to the call-graph
Yes I think this is enough, but in your case we don't require #2

And it is #2 that we are missing when we define an external function,
Even when we declare it with a preserves or a regmask attribute,

Because I think  once we have effect of attribute at IR/MI level then we can just parse it and populate register usage information vector for declared function and then we can propagate reg mask on each call site encountered.
But I am not user will it be easy to get new attribute working or we may need to hack clang for that too.

I would also like to have thoughts from my mentors (Mehdi Amini and Hal Finkel) about this.
So what I / we need is another attribute that says this is a leaf function,
At least in my case all I’m really concerned with are leaf functions

I am stating with a simple function  declaration which have a custom attribute.

-Vivek

Thoughts ?


--Peter Lawrence.



From: vivek pandya [mailto:vivekvpandya at gmail.com<mailto:vivekvpandya at gmail.com>]
Sent: Friday, July 08, 2016 10:24 AM
To: Lawrence, Peter <c_plawre at qca.qualcomm.com<mailto:c_plawre at qca.qualcomm.com>>
Cc: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>; llvm-dev-request at lists.llvm.org<mailto:llvm-dev-request at lists.llvm.org>
Subject: Re: Re:[llvm-dev] IPRA, interprocedural register allocation, question



On Fri, Jul 8, 2016 at 1:42 PM, vivek pandya <vivekvpandya at gmail.com<mailto:vivekvpandya at gmail.com>> wrote:


On Fri, Jul 8, 2016 at 9:47 AM, Lawrence, Peter <c_plawre at qca.qualcomm.com<mailto:c_plawre at qca.qualcomm.com>> wrote:
Vivek,
             I am looking into these function attributes in the clang docs
                Preserve_most
                Preserve_all
They are not available in the 3.6.2 that I am currently using, but I hope they exist in 3.8

These should provide enough info to solve my problem,
at the MC level calls to functions with these attributes
with be code-gen’ed  through different “calling conventions”,
and CALL instructions to them should have different register USE and DEF info,

Yes I believe that preserve_most or preserve_all should help you even with out IPRA. But just to note IPRA can even help further for example on X86 preserve_most cc will not preserve R11 (this can be verified from X86CallingConv.td and X86RegisterInfo.cpp) how ever IPAR calculates regmask based on the actual register usage and if procedure with preserve_most cc does not use R11 and none callsite inside of function body then IPRA will mark R11 as preserved. Also IPRA produces RegMask which is super set of RegMask due to calling convention.

I believe that __attribute__ ((registermask = ....))  can provide more flexibility compare to preserve_all or preserve_most CC in some case. So believe that we should try it out.

-Vivek

This CALL instruction register USE and DEF info should already be useful
to the intra-procedural register allocator (allowing values live across these
calls to be in what are otherwise caller-save registers),
at least that’s how I read the MC dumps, every call instruction seems to have
every caller-save register flagged as “imp-def”, IE implicitly-defined by the instruction,
and hopefully what is considered a caller-save register at a call-site is defined by the callee.
And this should be the information that IPRA takes advantage of in its bottom-up analysis.

Yes that is expected help from IPRA.

Which leads me to this question, when compiling an entire whole program at one time,
so there is no linking and no LTO, will there ever be IPRA that works within LLC for this scenario,
and is this an objective of your project, or are you focusing only on LTO ?
The current IPRA infrastructure works at compile time so it's scope of optimization is restricted to a compilation unit. So IPRA can only construct correct register usage information if the procedure's code is generated by same compiler instance that means we can't optimize library calls or procedure defined in other module. This is because we can't keep register usage information data across two different compiler instance.

Now if we consider LTO, it eliminates above limitation by making a large IR module from smaller modules before generating code and thus we can have register usage information (at lest) for procedure which was previously defined in other module, because now with LTO every thing is in one module. So that also clarifies that IPRA does not do anything at link time.

Now coming to LLC, it can use IPRA and optimize for functions defined in current module. So yes while compiling whole program ( a single huge .bc file) IPRA can be used with LLC. Also just note that if a software is written in separate files per module (which is very common) and still you want to maximize benefits of IPRA, then we can use llvm-link tool to combine several .bc files to produce a huge .bc file and use that with LLC to get maximum benefits.

I know this is not the typical “linux” scenario (dynamic linking of not only standard libraries,
but also sometimes even application libraries, and lots of static linking because of program
size), but it is a typical “embedded” scenario, which is where I am currently.

I don't understand this use case but we can have further improvement in IPRA for example if you have several libraries which has already compiled and codegen, but you are able to provide information of register usage for the functions of that libraries than we can think about an approach were we can store register usage information into a file (which will obviously increase compile time) and use that information across different compiler instances so that we can provide register usage information with out having actual code while compiling.

Other thoughts or comments ?

I am looking for some ideas that can improve current IPRA. So if you feel anything relevant please let me know we can discuss and implement feasible ideas.

Thanks,
Vivek

--Peter Lawrence.


From: vivek pandya [mailto:vivekvpandya at gmail.com<mailto:vivekvpandya at gmail.com>]
Sent: Wednesday, July 06, 2016 2:09 PM
To: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>; llvm-dev-request at lists.llvm.org<mailto:llvm-dev-request at lists.llvm.org>; Lawrence, Peter <c_plawre at qca.qualcomm.com<mailto:c_plawre at qca.qualcomm.com>>
Subject: Re:[llvm-dev] IPRA, interprocedural register allocation, question

Hello Peter,

Thanks to pointing out this interesting case.
Vivek,
          I have an application where many of the leaf functions are
Hand-coded assembly language,  because they use special IO instructions
That only the assembler knows about.  These functions typically don't
Use any registers besides the incoming argument registers, IE they don't
Need to use any additional callee-save nor caller-save registers.
If inline asm template has specified clobbered list properly than IPRA is able to use that information and it propagates correct register mask (and that also means that skipping clobbers list while IPRA enabled may broke executable)
For example in following code:
int gcd( int a, int b ) {
    int result ;
    /* Compute Greatest Common Divisor using Euclid's Algorithm */
    __asm__ __volatile__ ( "movl %1, %%r15d;"
                          "movl %2, %%ecx;"
                          "CONTD: cmpl $0, %%ecx;"
                          "je DONE;"
                          "xorl %%r13d, %%r13d;"
                          "idivl %%ecx;"
                          "movl %%ecx, %%r15d;"
                          "movl %%r13d, %%ecx;"
                          "jmp CONTD;"
                          "DONE: movl %%r15d, %0;" : "=g" (result) : "g" (a), "g" (b) : "ecx" ,"r13", "r15"
    );

    return result ;
}
IPRA calculates and propagates correct regmask in which it marks CH, CL, ECX .. clobbered and R13, R15 is not marked clobbered as it is callee saved and LLVM code generators also insert spill/restores code for them.

Is there any way in your IPRA interprocedural register allocation project that
The user can supply this information for external functions ?
By external word do you here mean function defined in other module than being used?  In that case as IPRA can operate on only one module at time register usage propagation is not possible. But there is a work around for this problem. You can use IPRA with link time optimization enabled because the way LLVM LTO works it creates a big IR modules out of source files and them optimize and codegen it so in that case IPRA can have actual register usage info (if function will be compiled in current module).

In case you want to experiment with IPRA please apply http://reviews.llvm.org/D21395 this patch before you begin.

-Vivek

Perhaps using some form of __attribute__ ?
Maybe __attribute__ ((registermask = ....))  ?


--Peter Lawrence.



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