[llvm-dev] Preparing BOLT for LLVM monorepo

Mon Jan 10 14:02:12 PST 2022

On Mon, Jan 10, 2022 at 1:12 PM Amir Aupov via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi Shengchen,
>
> Thank you for the code review and your suggestions!
> We've addressed some of them (see inline) but still in the process of
> addressing them all.
>
> On Tue, Jan 4, 2022 at 7:56 PM Kan, Shengchen <shengchen.kan at intel.com>
> wrote:
>
>> Hi Amir
>>
>>
>>
>> I read the MC-related code and it looks fine to me in general. I’m also
>> supportive for the merge. The following are some suggestions.
>>
>>
>>
>>    1. https://llvm.org/docs/CodingStandards.html#braced-initializer-lists
>>     If you use a braced initializer list when initializing a variable, use an
>>    equals before the open curly brace. e.g BinaryBasicBlock.h
>>
>> Addressed, soon to be published.
>
>
>>
>>    1.
>>    https://llvm.org/docs/ProgrammersManual.html#fine-grained-debug-info-with-debug-type-and-the-debug-only-option
>>    DEBUG_TYPE should not be target specific,
>>
>> e.g, the definition of DEBUG_TYPE should be same in X86MCPlusBuilder.cpp
>> and AArch64MCPlusBuilder.cpp
>>
> Addressed in
> https://github.com/facebookincubator/BOLT/commit/7ca0c5a26714754ec1dd72576ddecaa8b459842c
>
>
>>
>>    1. Functions getShortArithOpcode, getShortBranchOpcode in
>>    X86MCPlusBuilder.cpp use reversed mappings as functions
>>    getRelaxedOpcodeBranch, getRelaxedOpcodeArith in X86AsmBackend.cpp. Writing
>>    twice in two places makes the code difficult to maintain, could we generate
>>    the table with tablgen? The memory folding and unfolding tables in
>>    X86InstrFoldTables.cpp give an example.
>>
>> To make sure I get it right: the tables in X86InstrFoldTables.cpp were
> generated with tablegen at some point, but are checked in as source code?
> Is there a way to rebuild these tables, so I can use that as an example?
>

There is a tablegen backend that can generate it, but it has some bugs.
Especially around some oddities with the SSE movd/movq instructions if I
remember right. I used to run it after adding new instructions and picked
the table entries I thought were ok. The code is in
llvm/utils/TableGen/X86FoldTablesEmitter.cpp
but I'm not sure how helpful it is. It's mostly trying to find pairs of
instructions that different in the register vs memory form of the modrm
byte with rest of the encoding being the same. For your case, the
instructions differ in the opcode byte. I guess you'd have to write a
tablegen backend that knew rules like if opcode==0x81 or 0x83, the other
instruction is the one with the other opcode, but rest of the encoding is
the same.

>
>>    1. It would better if functions isADD, isAND, isCMP, isDEC, isINC,
>>    isSUB and isTEST can be generated with tablgen.
>>
>> Removed our definitions of isINC/isDEC:
> https://github.com/facebookincubator/BOLT/commit/9fec2d58964d643a7436ba2804f8fb967f23881e
> How do we approach generating isADD/... with tablegen? There are
> appropriate definitions (`defm ADD`, etc)
> in llvm/lib/Target/X86/X86InstrArithmetic.td but I'm not sure how to get
> tablegen to generate the tables.
>

I think you'd have to add isAdd, isCmp, etc. fields to the X86Inst class
and set them on the appropriate instructions. Then have a tablegen backend
collect all the instructions with each property. Or you could figure out
the different encoding rules like if the opcode is 0x80-0x83 than the
format being MRM4r/MRM4m/MRM4X mean the opcode is an AND.

> Thanks,
> -Amir
>
>
>>
>> Best
>>
>> Shengchen
>>
>> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *Wang,
>> Phoebe via llvm-dev
>> *Sent:* Thursday, December 30, 2021 5:33 PM
>> *To:* Amir Aupov <amir.aupov at gmail.com>
>> *Cc:* llvm-dev at lists.llvm.org
>> *Subject:* Re: [llvm-dev] Preparing BOLT for LLVM monorepo
>>
>>
>>
>> Hi Amir,
>>
>>
>>
>> Amazing job! Thanks for the quick and perfect changes. I don’t have any
>> other problems now. Hoping seeing the code in main trunk soon.
>>
>>
>>
>> Thanks
>>
>> Phoebe
>>
>>
>>
>> *From:* Amir Aupov <amir.aupov at gmail.com>
>> *Sent:* Thursday, December 30, 2021 4:32 PM
>> *To:* Wang, Phoebe <phoebe.wang at intel.com>
>> *Cc:* llvm-dev at lists.llvm.org
>> *Subject:* Re: [llvm-dev] Preparing BOLT for LLVM monorepo
>>
>>
>>
>> Hi Phoebe,
>>
>>
>>
>> Thank you again for reviewing the codebase and for your feedback. We've
>> addressed it in recent commits:
>>
>> >  1.  Use the monorepo of LLVM in the example for convenience in https://github.com/facebookincubator/BOLT/blob/main/bolt/docs/OptimizingClang.md#getting-clang-7-sources
>>
>> E.g, git clone --branch=release/7.x https://github.com/llvm/llvm-project.git
>>
>> https://github.com/facebookincubator/BOLT/commit/c4ed6a11eb264660f5cfe7f5165df35b63d52608
>>
>>
>>
>> >  1.  I found there are 142 code with “not implemented”, most of which are in Core/MCPlusBuilder.h.
>>
>>      *   Do they affect the functionality of BOLT?
>>
>>      *   Do you have plan to implement them recently or can they be removed instead?
>>
>> Explained above.
>>
>> >  2.  I noticed some inconsistent use of braces in the code. Maybe better to follow with LLVM coding standard<https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements>.
>>
>>      *   https://github.com/facebookincubator/BOLT/blob/main/bolt/lib/Core/BinaryBasicBlock.cpp#L241, L289<https://github.com/facebookincubator/BOLT/blob/main/bolt/lib/Core/BinaryBasicBlock.cpp#L289>, etc.
>>
>>      *   https://github.com/facebookincubator/BOLT/blob/main/bolt/lib/Core/DebugData.cpp#L193
>>
>> https://github.com/facebookincubator/BOLT/commit/fe3fcf094e320af4da4a98a5387820cd0ca826b9
>>
>> https://github.com/facebookincubator/BOLT/commit/b40699ed90882465e49ec46e4d3ad28623e54c69
>>
>> https://github.com/facebookincubator/BOLT/commit/c21ee425083276a7af700e232df15e2a3d31dacb
>>
>> https://github.com/facebookincubator/BOLT/commit/ebc06afcfc35fbb4360b3788adeb6228c27ad830
>>
>> https://github.com/facebookincubator/BOLT/commit/dca3003fce711daee387bbbf66bf4ecbe1b9ab9b
>>
>> https://github.com/facebookincubator/BOLT/commit/cd526a4550455b0f203771088cef6e512b0dd1a2
>>
>> >  3.  Some files don’t have a descriptions in the first line, e.g.  DynoStats.cpp<https://github.com/facebookincubator/BOLT/blob/main/bolt/lib/Core/DynoStats.cpp#L1>, ParallelUtilities.cpp<https://github.com/facebookincubator/BOLT/blob/main/bolt/lib/Core/ParallelUtilities.cpp#L1>, etc.
>>
>> >  4.  Leaving without descriptions might be fine, but the format should be consistent. Leaving with spaces like in BinaryFunctionProfile.cpp<https://github.com/facebookincubator/BOLT/blob/main/bolt/lib/Core/BinaryFunctionProfile.cpp#L1> doesn’t make sense.
>>
>>
>> https://github.com/facebookincubator/BOLT/commit/349c8abc696919e989418895f4ade24a4c910617
>>
>>
>>
>> On Tue, Dec 21, 2021 at 4:22 AM Wang, Phoebe <phoebe.wang at intel.com>
>> wrote:
>>
>> Hi Amir,
>>
>>
>>
>> It makes sense to me. Thanks!
>>
>>
>>
>> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *Amir
>> Aupov via llvm-dev
>> *Sent:* Tuesday, December 21, 2021 8:57 AM
>> *To:* llvm-dev at lists.llvm.org
>> *Subject:* Re: [llvm-dev] Preparing BOLT for LLVM monorepo
>>
>>
>>
>> Hi Phoebe,
>>
>>
>>
>> Thanks again for taking the time to look at the codebase. We've added
>> your feedback to our upstreaming tracking issue on github:
>> https://github.com/facebookincubator/BOLT/issues/248. We've started
>> addressing the issues.
>>
>>
>>
>> Regarding "not implemented” in Core/MCPlusBuilder.h: it's a design
>> decision to prevent the use of non-implemented target-specific methods
>> (these are virtual methods). Target-specific implementation goes to
>> X86MCPlusBuilder and AArch64MCPlusBuilder. Some methods only make sense for
>> specific targets, so we intend to keep llvm_unreachable statements in
>> MCPlusBuilder.h.
>>
>> _______________________________________________
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