[llvm-dev] [GlobalISel] A Proposal for global instruction selection

[Daniel Sanders via llvm-dev]

Hi,

I haven't found much time to look into the LLVM-IR-level optimizations yet so I'm not sure how they handle bitcasts. With that disclaimer in mind, I expect it's fine for the LLVM-IR level optimizations to handle them using either definition since they are equivalent at the LLVM-IR level. My thinking is that LLVM-IR is consistent about how virtual bits are assigned to types and that non-zero instruction nops arise when there is inconsistency.

At the LLVM-IR level, bits 0-127 of <4 x i32> map directly onto bits 0-127 of <2 x i64> using the identity map. It's therefore ok to interpret such bitcasts as zero-instruction no-ops. As far as I can tell, LLVM-IR has been defined such that the identity map can be used for bitcasts between all same-sized types, and also such that bitcasting between different-sized types is invalid.

Similarly, most targets have a single mapping of virtual bit numbers to physical bit numbers for each size that is applied consistently when mapping a type to memory. For example 32-bits map like so:
Little Endian Targets: virtual register bits {0..7,8..15,16..23,24..31} map to physical memory bits {0..7,8..15,16..23,24..31}
Big Endian Targets: virtual register bits {0..7,8..15,16..23,24..31} map to physical memory bits {24..31,16..23,8..15,0..7}
regardless of whether it's a float, or an i32. We therefore need zero instructions to re-map physical memory bits for one type onto another type.

The same idea holds for physical register classes. There's a single consistent mapping from physical memory bits to physical register bits that applies for all types that can be stored in that class. As long as this is the case the load/store and zero-instruction interpretation of bitcasts are equivalent.
In the case of big-endian MSA and NEON, there isn't a single consistent mapping from physical memory bits to physical register bits so the equivalence in the two definitions breaks down:
                i128: virtual register bits {0..31, 32..63, 64..95, 96...127} map to physical memory bits {96..127, 64..95, 32..63, 0..31}
                <4 x i32>: virtual register bits {0..31, 32..63, 64..95, 96...127} map to physical memory bits {0..31, 32..63, 64..95, 96..127}
                <2 x i64>: virtual register bits {0..31, 32..63, 64..95, 96...127} map to physical memory bits {32..63, 0..31, 96..127, 64..95}
with these inconsistent mappings we require instructions to bitcast between the types.

I found this thinking quite difficult to explain. Does it make sense?

> I am fine with treating bit casts as equivalent store/load pairs in GISel, I just want to be sure we do not have a semantic gap between the LLVM-IR and the backend if we do.

I think a gap would arise from not having a GISel equivalent to ISD::BITCAST (gBITCAST?) available when it's necessary for correctness. However, I agree that GISel should delete bitcasts for the common case where the store/load and zero-instruction definitions are equivalent.

From: Quentin Colombet [mailto:qcolombet at apple.com]
Sent: 11 January 2016 17:23
To: Daniel Sanders
Cc: Tim Northover (t.p.northover at gmail.com); llvm-dev
Subject: Re: [llvm-dev] [GlobalISel] A Proposal for global instruction selection

Hi Daniel,

Thanks for the pointers, I wasn’t aware of the second thread you’ve mentioned.

I may be wrong but I think LLVM-IR optimizations really treat bistcasts as no-op casts, in the sense of no instructions are required.

Is there anyone that could chime in on that?

However, it seems SelectionDAG sticks to the load/store semantic:
"BITCAST - This operator converts between integer, vector and FP values, as if the value was stored to memory with one type and loaded from the same address with the other type (or equivalently for vector format conversions, etc)."

I am fine with treating bit casts as equivalent store/load pairs in GISel, I just want to be sure we do not have a semantic gap between the LLVM-IR and the backend if we do.

Thanks,
-Quentin

On Jan 11, 2016, at 7:43 AM, Daniel Sanders <Daniel.Sanders at imgtec.com<mailto:Daniel.Sanders at imgtec.com>> wrote:

Hi,

It was a comment by Tim that first made me aware of it (see http://lists.llvm.org/pipermail/llvm-dev/2013-August/064714.html but I think he commented on one of my patches before that).

I asked about it on llvm-dev a couple weeks later (http://lists.llvm.org/pipermail/llvm-dev/2013-August/064919.html) highlighting the contradiction and was told that 'no-op cast' referred to the lack of math rather than a requirement that zero instructions are used. It's therefore my understanding that shuffling the bits to preserve the load/store based definition isn't considered to be changing the bits.

I think the main thing the current definition is unclear on is whether it refers to the bits in a physical machine register or the bits in the LLVM-IR virtual register. Most of the time these two views are the same but this doesn't quite work for big-endian MSA/NEON. For example:
%0 = bitcast <4 x i32> <i32 1, i32 2, i32 3, i32 4> to <2 x i64>
%0 = <2 x i64> <i64 (1 << 32) | 2, i64 (3 << 32) | 4>
are equivalent to each other in LLVM-IR terms but the constants are physically laid out in MSA registers as:
0x00000004000000030000000200000001 # <4 x i32> <i32 1, i32 2, i32 3, i32 4>
0x00000003000000040000000100000002 # <2 x i64> <i64 (1 << 32) | 2, i64 (3 << 32) | 4>
and we must therefore shuffle the bits to preserve LLVM-IR's point of view.

From: Quentin Colombet [mailto:qcolombet at apple.com]
Sent: 07 January 2016 19:58
To: Daniel Sanders
Cc: llvm-dev
Subject: Re: [llvm-dev] [GlobalISel] A Proposal for global instruction selection

Hi Daniel,

I had a quick look at the language reference for bitcast and I have a different reading than what you were pointing out.
Indeed, my take away is:
"It is always a no-op cast because no bits change with this conversion."

In other words, deleting all bitcast instructions should be fine.

My understanding of the quote you’ve highlighted is that it tells C programmers that this is like a memcpy, not a cast :).

Cheers,
-Quentin
On Nov 20, 2015, at 6:53 AM, Daniel Sanders <Daniel.Sanders at imgtec.com<mailto:Daniel.Sanders at imgtec.com>> wrote:

Hi,

I haven't had chance to read all of this yet, but one minor thing occurred to me during your presentation that I want to mention. At one point you mentioned deleting all the bitcast instructions since they're equivalent to nops but this isn't always true.

The http://llvm.org/docs/LangRef.html definition of the bitcast instruction includes this sentence:
The conversion is done as if the value had been stored to memory and read back as type ty2.
For big-endian MSA, this is equivalent to a shuffling of the bits in the register because endianness only changes the byte order within each element. The order of the elements is unaffected by endianness. IIRC, big-endian NEON is the same way.

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Quentin Colombet via llvm-dev
Sent: 18 November 2015 19:27
To: llvm-dev
Subject: [llvm-dev] [GlobalISel] A Proposal for global instruction selection

Hi,

With this email, I would like to kick-off the development for the next instruction selector that I described during the last LLVM Dev’ Meeting.
For the motivations, see Jakob’s proposal (http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-August/064727.html) and for the proposal, see the slides (Keynote: http://llvm.org/viewvc/llvm-project/www/trunk/devmtg/2015-10/slides/Colombet-GlobalInstructionSelection.key?view=co or PDF: http://llvm.org/viewvc/llvm-project/www/trunk/devmtg/2015-10/slides/Colombet-GlobalInstructionSelection.pdf?revision=252430&view=co) or the talk (https://www.youtube.com/watch?v=F6GGbYtae3g&list=PL_R5A0lGi1AA4Lv2bBFSwhgDaHvvpVU21&index=2).

TL;DR This is happening now, feedbacks invited!

*** Context ***

During the last LLVM Dev’ Meeting, I have presented a proposal for the next instruction selector, GlobalISel. The proposal is basically summarized in "High Level Prototype Design” and “Roadmap”. (If you want further details, feel free to reach me.)

The first step of the development plan is to prototype the new framework on open source. The idea is to start prototyping now(!) and have the discussion ongoing in parallel. The reason of such approach is to have code that can be used to inform those discussions, e.g., by collecting data and trying different designs approaches. Regarding the discussion, I have listed a few points where your feedbacks would be particularly appreciated (see Feedback Invite).

Also, as I have mentioned in my talk, some issues are controversial but I expect them to be resolved during prototype development. Specifically theses concern aspects of legalization (should parts of it be done at the LLVM IR level or all at the MI level?) and code re-use for instruction combiner. Please feel free to bring up your specific concern as I move along with the development plan.

I expect the design to evolve with our experimental findings and your feedbacks and contributions.
Nonetheless, we expect to nail down some design decisions once and for all as the prototype progresses. I have highlighted them with the following pattern [final].



*** Feedback Invite ***

If you follow and support this work you need to be aware of three things and I am eager to hear your feedback and thoughts about them: the overall goals of Global ISel, the goals of the prototype, and the impact of the prototype work on backend design.

In the section “Goals", I defined (repeated for people that saw the talk) the goals for the Global ISel design.
- Do you see anything missing?
- Do you see something that should not be there?

The prototype will answer critical design questions (see “Design Questions the Prototype Addresses at the End of M1" for examples) before the actual design of Gobal ISel is finalized, but it cannot cover everything.
Specifically we will *not* look into improving TableGen or reuse InstCombine (see “ Proposed Approach” for the rational). Please let me know if you see any issue with that.

There is also basic ground work needed to prepare for Global ISel and I need to extend the core MachineInstr-level APIs as explained during the talk. For this, I prepared sketches of patches to illustrate them and describe the details in the “Implications” section below. Please have a look at the patches to have a better idea of the expected impact.

If there is anything else you want to discuss related to Global ISel feel free to reach me. In particular, several people expressed their interests during the LLVM Dev Meeting in contributing to the project. Let me know what is your area of interest, so that we can coordinate our efforts.
Anyhow, please add [GlobalISel] in the subject line to help categorizing the emails.



*** Goals ***

The high level goals of the new instruction selector are:
- Global instruction selector.
- Fast instruction selector.
- Shared code path for fast and good instruction selection.
- IR that represents ISA concepts better.
- More flexible instruction selector.
- Easier to maintain/understand framework, in particular legalization.
- Self contained machine representation, no back links to LLVM IR.
- No change to LLVM IR.

Note:  The goals are common to all targets. In particular, we do not intend to work on target specific feature for the prototype.
The bottom line is please make sure those goals are compatible with what you want to achieve for your target, even if your requirement does not get listed here.



*** Proposed Approach ***

In this section, I describe the approach I plan to pursue in the prototype and the roadmap to get there. The final design will flow out of it.

For this prototype, we purposely exclude any work to improve or use TableGen or InstCombine [final]. We will keep in mind however, that some of the C++ code we write will be table-generated at some point.
The rational is that we do not want to lay down a new TableGen/InstCombine infrastructure before being able to work on the ISel framework itself.

The prototype vehicle will be AArch64. None of the changes for GlobalISel will negatively impact the existing ISel.


** High Level Prototype Design **

As shown in the talk, the expected pipeline for the prototype is:
LLVM IR -> IRTranslator -> Generic (G) MachineInstr -> Legalizer -> RegBankSelect -> Select -> MachineInstr

Where:
- Terms in bold are intermediate representations.
-  Generic MachineInstrs are machine instructions with a generic opcode, e.g., ADD, COPY.
- IRTranslator: Translate LLVM IR to (G) MachineInstr.
- Legalizer: Legalize illegal (G) MachineInstr to legal (G) MachineInstr.
- RegBankSelect: Assign virtual register with size to virtual register with Register Bank.
- Select: Translate the remaining (G) MachineInstr to MachineIntr.



** Implications **

As part of the bring-up of the prototype, we need to extend some of the core MachineInstr-level APIs:
  - Need to remember FastMath flags for each MachineInstr.
  - Need to know the type of each MachineInstr. We don’t want ADD8, ADD16, etc.
  - Extend the MachineRegisterInfo to support size as well as register classes for virtual registers.

I have sketched the changes in the attached patches to help picturing how the changes would impact the existing APIs.

Note: I do not intend to commit those changes as they are. They will go the usual review process in due time.

The patches contain “// ***”-like comment that give a rough explanation on why those changes are needed w.r.t. the goals.
The order of the patches could be modified since the dependencies between those are not sequential. Anyhow, here are the patches:
1. Introduce (some of) the generic opcode.
2. Make MachineFunction more independent of LLVM IR to eventually be able to delete the LLVM IR instance from the memory.
3. Extend MachineInstr to represent additional information attached to generic opcode.
4. Teach MachineRegisterInfo about size for virtual registers.
5. Introduce a helper class to build MachineInstr related objects.
6. Add new target hooks to lower the ABI directly to MachineInstr.
7. Introduce the IRTranslator pass.


** Roadmap for the Prototype **

We plan to split the prototype in three main milestones:
1. Translation: LLVM IR to (G) MachineInstr translation.
2. Basic selector: Legal LLVM IR to target specific MachineInstr.
3. Simple legalization: Support scalar type legalization and some vector instructions.

Notes:
- For #1, we will not support any fancy instructions like landing pad or switch.
- Each milestone should take about 3-4 months.
- At the end of #2, we would have a FastISel like selector.

Each milestone will be detailed right before starting it. The rational is that we want to accommodate what we discovered with the prototype for the next milestone. In other words, in this email, I only describe the first milestone in detail and I will give more details on the next milestone shortly before we start it and so on. For your information, here is the remaining of the intended roadmap for the full project:
4. Productization: Clean up implementation, stabilize the APIs.
5. Complex legalization: Extend legalization support to everything missing.
6. Completeness: Fill the blanks, e.g., landing pad.
7. Clean-up and performance: Add the necessary bits to be at parity or beat SelectionDAG generated code.
8. Transition: Document how to switch, provide tools to help.


** Milestone 1 **

The first phase is focused on the IRTranslator pass.

The IRTranslator is responsible for translating the LLVM IR into Generic MachineInstr. The IRTranslator pass uses some target hooks to perform the ABI lowering. We can either define a new API for them, e.g., ABILoweringInfo, or extend the existing TargetLowering.
Moreover, the prototype will focus on simple instruction, i.e., we will not support switch or landing pad for this iteration.

At the end of M1, the prototype will not be able to produce code, since we would only have the beginning of the Global ISel pipeline. Instead, we will test the IRTranslator on the generic output that is produced from the tested IR.

* Design Decisions *

- The IRTranslator is a final class. Its purpose is to move away from LLVM IR to MachineInstr world [final].
- Lower the ABI as part of the translation process [final].

* Design Questions the Prototype Addresses at the End of M1 *

- Handling of aggregate types during the translation.
- Lowering of switches.
- What about Module pass for Machine pass?
- Introduce new APIs to have a clearer separation between:
  - Legalization (setOperationAction, etc.)
  - Cost/Combine related (isXXXFree, etc.)
  - Lowering related (LowerFormal, etc.)
- What is the contract with the backends? Is it still “should be able to select any valid LLVM IR”?

Thanks,
-Quentin

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