[llvm-dev] IPRA, interprocedural register allocation, question
vivek pandya via llvm-dev
llvm-dev at lists.llvm.org
Fri Jul 8 01:12:44 PDT 2016
On Fri, Jul 8, 2016 at 9:47 AM, Lawrence, Peter <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.
> 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]
> *Sent:* Wednesday, July 06, 2016 2:09 PM
> *To:* llvm-dev <llvm-dev at lists.llvm.org>; llvm-dev-request at lists.llvm.org;
> Lawrence, Peter <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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