[llvm-dev] [GSoC 2016] Interprocedural Register Allocation - Introduction and Feedback

[Hal Finkel via llvm-dev]

----- Original Message -----

> From: "vivek pandya" <vivekvpandya at gmail.com>
> To: "Mehdi Amini" <mehdi.amini at apple.com>
> Cc: "Hal Finkel" <hfinkel at anl.gov>, "Quentin Colombet"
> <qcolombet at apple.com>, "llvm-dev" <llvm-dev at lists.llvm.org>,
> "Matthias Braun" <matze at braunis.de>
> Sent: Wednesday, May 11, 2016 3:15:03 AM
> Subject: Re: [GSoC 2016] Interprocedural Register Allocation -
> Introduction and Feedback

> Vivek Pandya

> On Wed, May 11, 2016 at 10:02 AM, vivek pandya <
> vivekvpandya at gmail.com > wrote:

> > Vivek Pandya
> 

> > On Wed, May 11, 2016 at 9:43 AM, Mehdi Amini <
> > mehdi.amini at apple.com
> > > wrote:
> 

> > > > On May 10, 2016, at 6:06 PM, Hal Finkel < hfinkel at anl.gov >
> > > > wrote:
> > > 
> > 
> 

> > > > > From: "vivek pandya" < vivekvpandya at gmail.com >
> > > > 
> > > 
> > 
> 
> > > > > To: "llvm-dev" < llvm-dev at lists.llvm.org >, "Tim Amini
> > > > > Golling"
> > > > > <
> > > > > mehdi.amini at apple.com >, "Hal Finkel" < hfinkel at anl.gov >
> > > > 
> > > 
> > 
> 
> > > > > Cc: "Quentin Colombet" < qcolombet at apple.com >
> > > > 
> > > 
> > 
> 
> > > > > Sent: Tuesday, May 10, 2016 2:59:16 PM
> > > > 
> > > 
> > 
> 
> > > > > Subject: [GSoC 2016] Interprocedural Register Allocation -
> > > > > Introduction and Feedback
> > > > 
> > > 
> > 
> 

> > > > > Hello LLVM Community,
> > > > 
> > > 
> > 
> 

> > > > > Sorry for delay as I was busy in final exams.
> > > > 
> > > 
> > 
> 

> > > > > I am Vivek from India. Thanks for choosing my proposal for
> > > > > Interprocedural Register Allocation (IPRA) in LLVM. Mehdi
> > > > > Amini
> > > > > and
> > > > > Hal Finkel will be mentoring me for this project.
> > > > 
> > > 
> > 
> 

> > > > > IPRA can reduce code size and runtime of programs by
> > > > > allocating
> > > > > register across the module and procedure boundaries.
> > > > 
> > > 
> > 
> 

> > > > > I have identified some old but effective research work on
> > > > > this
> > > > > area.
> > > > 
> > > 
> > 
> 
> > > > > I want community's feedback for feasibility of these approach
> > > > > and
> > > > > I
> > > > > am targeting to implement two of them during this project.
> > > > 
> > > 
> > 
> 

> > > > > Here is list of the papers, I have read first two papers and
> > > > > I
> > > > > would
> > > > > like to discuss those approach first, I will read other two
> > > > > paper
> > > > > then initiate discussion for them as well. All I want is to
> > > > > find
> > > > > out
> > > > > a concrete implementation plan before 23 May, 2016 and for
> > > > > that
> > > > > I
> > > > > need community's help.
> > > > 
> > > 
> > 
> 

> > > > > 1) Compile time ----- Minimizing register usage penalty at
> > > > > procedure
> > > > > calls - http://dl.acm.org/citation.cfm?id=53999
> > > > 
> > > 
> > 
> 
> > > > > ====================================================================In
> > > > > this approach intra-procedural register allocation is used as
> > > > > base
> > > > > but machine code generation order is bottom up traversal of
> > > > > call
> > > > > graph and inter-procedural effect is achieved by propagating
> > > > > register usage information of callee function to caller (i.e
> > > > > child
> > > > > to parent in CallGraph) so that caller can use different
> > > > > registers
> > > > > than callee and can save load store cost at procedure call,
> > > > > this
> > > > > is
> > > > > not trivial as it seems due to recursive calls, library
> > > > > function
> > > > > usage etc. Also for upper region of the graph in this
> > > > > technique
> > > > > available number of registers might become zero in that case
> > > > > it
> > > > > should fall back to normal load store at procedure call.
> > > > > Apart
> > > > > from
> > > > > these difficulties other difficulties have been identified
> > > > > please
> > > > > follow this mail-chain
> > > > > https://groups.google.com/d/topic/llvm-dev/HOYAXv3m1LY/discussion
> > > > 
> > > 
> > 
> 
> > > > > My mentor has already provided me a patch that alters code
> > > > > generation
> > > > > order as per bottom up call graph traversal, I am working
> > > > > from
> > > > > that
> > > > > point now. Any other help/suggestion is always welcomed.
> > > > 
> > > 
> > 
> 

> > > > > 2) Link time ----- Global register allocation at link time -
> > > > > http://dl.acm.org/citation.cfm?id=989415
> > > > 
> > > 
> > 
> 
> > > > > ====================================================================In
> > > > > this particular approach (sort of true IPRA) registers will
> > > > > be
> > > > > reallocated (this optimization will be optional if turned off
> > > > > still
> > > > > code will be compiled as per intra-procedural allocation) at
> > > > > link
> > > > > time. Here modules are first complied as per normal
> > > > > compilation
> > > > > but
> > > > > the object code is annotated with details so that linker can
> > > > > build
> > > > > call graph and also calculate usage information at link time.
> > > > > Compiler also write hints in object code that if particular
> > > > > variable
> > > > > is allocated in some other register ( due to new allocation)
> > > > > then
> > > > > how the code should be changed? Thus linker can use these
> > > > > information to decide which variables (global) need to be in
> > > > > same
> > > > > register through out the program execution and also according
> > > > > to
> > > > > register usage information in call graph which procedure will
> > > > > not
> > > > > be
> > > > > active simultaneously so that locals for that procedures can
> > > > > be
> > > > > in
> > > > > same registers with out load store at procedure calls.
> > > > 
> > > 
> > 
> 
> > > > > For these particular method help me to analyze feasibility:
> > > > 
> > > 
> > 
> 
> > > > > 1) Can llvm collects following information at module level in
> > > > > MachineIR? list of procedures in module, list of locals in
> > > > > procedures, list of procedures that a particular procedure
> > > > > can
> > > > > call,
> > > > > and a list of the variables this procedure references. Each
> > > > > entry
> > > > > in
> > > > > the last two lists includes an estimate of the number of
> > > > > times
> > > > > the
> > > > > procedure is called or the variable is referenced in each
> > > > > execution
> > > > > of this procedure
> > > > 
> > > 
> > 
> 
> > > > > 2) Can llvm write informative commands to object files?
> > > > 
> > > 
> > 
> 
> > > > > 3) Can LTO is capable of leveraging those commands?
> > > > 
> > > 
> > 
> 

> > > > In terms of scoping the project for the summer, I definitely
> > > > recommend that you focus on (1) first. If you finish that, we
> > > > can
> > > > certainly move on to other things.
> > > 
> > 
> 
> > > I'll add +1 here, but I already wrote the same thing on IRC when
> > > discussing with Vivek. True IPRA without a proper MachineModule
> > > infrastructure won't be doable in my opinion (even with such
> > > infrastructure, it may not be trivial in LLVM in general).
> > 
> 

> > > > Regarding link time, note that any such a design would likely
> > > > look
> > > > much different than in David Wall's paper however, because our
> > > > LTO
> > > > re-codegens everything anyway. The paper says, "Finally, it
> > > > keeps
> > > > us
> > > > honest as designers of the system; once we postpone anything
> > > > until
> > > > link time, the temptation is great to postpone everything, ..."
> > > > -
> > > > Well, we've long-since succumb to that temptation when we LTO.
> > > > C'est
> > > > la vie.
> > > 
> > 
> 

> > > +1 as well, our LTO will benefit naturally from the leaf-to-root
> > > information propagation. ThinLTO will be more
> > > challenging/interesting though!
> > 
> 
> > > > > For the first part a mechanism similar to MachineModulePass
> > > > > would
> > > > > be
> > > > > desirable but that may not be possible during this project,
> > > > > but
> > > > > if
> > > > > we can make some sort of smaller version of that to suit our
> > > > > purpose.
> > > > 
> > > 
> > 
> 
> > > > I don't think we need to make any kind of MachineModulePass to
> > > > make
> > > > this work. Once we alter the visitation order based on the
> > > > CGSCC
> > > > iteration scheme, we can keep state in-between functions in the
> > > > pre-existing hacky way (using static members of the relevant
> > > > function passes).
> > > 
> > 
> 

> > Sorry my mistake here by first part I mean 1) requirement in the
> > link
> > time approach.
> 
> > > I also don't see where/why we need a MachineModule(Pass) for the
> > > CGSCC scheme, that said I'd rather avoid using a function pass
> > > with
> > > static members, if we can have a ModuleAnalysis that is
> > > bookkeeping
> > > the results for functions in the module and queries by the
> > > register
> > > allocator somehow.
> > 
> 
> > > Matthias/Quentin may have other inputs on this aspect.
> > 
> 
> @Hal do you mean to add a simple MachineFunction pass that will just
> operate on register allocated function and prepare a BitVector to
> indicate which register is being used by MachineFunction, and then
> use this pass as analysis pass (i.e just simply return static
> BitVector for clobbered register when register allocation for next
> function begins. This part is not much clear to me) this thing can
> be done by scheduling a pass post register allocation in
> lib/CodeGen/Passes.cpp

> void TargetPassConfig::addMachinePasses() {
> .
> .
> .
> // Run pre-ra passes.
> addPreRegAlloc();

> // Run register allocation and passes that are tightly coupled with
> it,
> // including phi elimination and scheduling.
> if (getOptimizeRegAlloc())
> addOptimizedRegAlloc(createRegAllocPass(true));
> else
> addFastRegAlloc(createRegAllocPass(false));

> // Run post-ra passes.
> addPostRegAlloc();
> // Adding a new pass here which keeps register mask information
> across function calls.
> .
> .
> .
> }

> But this also requires current register allocators to use this
> information in someway because RegMaskBits in
> LiveIntervalAnalysis.cpp is not static across calls. I mean I am not
> clear for how to propagate static info to Intra-procedural Register
> allocators (if possible without disturbing their code )
First, my hope is that we won't need to change the register allocators, as such, in order to make use of this information. Instead, we'll simply be able to alter the register masks generated for the call instructions. These masks will indicate fewer clobbers than might otherwise be present based on the ABI because of information gathered during the codegen of the callee. These masks are generally constructed by target based on the calling convention. The PowerPC backend, for example, looks like this: 

// Add a register mask operand representing the call-preserved registers. 
const TargetRegisterInfo *TRI = Subtarget.getRegisterInfo(); 
const uint32_t *Mask = 
TRI->getCallPreservedMask(DAG.getMachineFunction(), CallConv); 
assert(Mask && "Missing call preserved mask for calling convention"); 
Ops.push_back(DAG.getRegisterMask(Mask)); 

but it can be more complicated. If you look for uses of 'getRegisterMask' in Target/*/*ISelLowering.cpp, you'll see what I mean. Regardless, the code ends up calling some method is the targets TargetRegisterInfo subclass. These methods generally look something like this: 

const uint32_t * 
PPCRegisterInfo::getCallPreservedMask(const MachineFunction &MF, 
CallingConv::ID CC) const { 
const PPCSubtarget &Subtarget = MF.getSubtarget<PPCSubtarget>(); 
... 
return TM.isPPC64() ? (Subtarget.hasAltivec() ? CSR_SVR464_Altivec_RegMask 
: CSR_SVR464_RegMask) 
: (Subtarget.hasAltivec() ? CSR_SVR432_Altivec_RegMask 
: CSR_SVR432_RegMask); 
} 

In any case, the fundamental idea here is that, when someone calls getCallPreservedMask in order to set the regmask on a call, we might not have to use the CC at all. Instead, if we've already codegened the function, we might use a cache of 'exact' register masks computed during codegen of the potential callees instead. 

In order to do this, I think we'll need to provide a function callable from the target's getCallPreservedMask implementation, which can return such an 'exact' regmask when available. I think we need to do it this way for two reasons: 

1. Not all of the target code calls getCallPreservedMask, but sometimes calls other similar target-specific functions (e.g. getTLSCallPreservedMask). 
2. The targets need to opt-in to this behavior because only the target can know that all register uses are really tagged correctly post "pre-emit". 

Because the target is free to introduce uses of registers at essentially any time, we need to do the scanning for used registers after the "pre-emit" passes run. This can be done by scheduling some simple register-use scanning pass after the call to addPreEmitPass in lib/CodeGen/Passes.cpp. 

> I think this also applies in someway to Mehdi Amini's idea to keep a
> ModulePass for bookkeeping but then existing register allocators
> will be required to change so that the code can query the ModulePass
> for RegMaskBits for particular function.
I think that the simplest way to do this is to create an immutable analysis pass (e.g. BasicAA) that keeps the cache of the computed register masks. This is somewhat similar in spirit to how the 'AssumptionCache' analysis works at the IR level. This analysis can then be created by lib/CodeGen/Passes.cpp early, and then queried and passed around later by the CodeGen/Target code. Because it is an immutable analysis, it won't get destroyed until the very end, which is also important because, I imagine, it will need to own the memory associated with the generated register masks. 

-Hal 

> Vivek

> > Yes for propagating register usage approach we don't need
> > MachineModulePass
> 

> > Vivek
> 
> > > --
> > 
> 
> > > Mehdi
> > 
> 

-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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