[llvm-commits] [PATCH] TableGen: reduce the size of ContextDecision tables in X86GenDisassemblerTables

[Manman Ren]

Hi Craig,

I meant this data structure:
struct ModRMDecision {
  uint8_t     modrm_type;

  /* The macro below must be defined wherever this file is included. */
  INSTRUCTION_IDS 
};

Currently, modRMTable has 43k entries, each entry is 16-bit, and we have 6 ContextDecision tables, a total of 47k entries, where each entry is a struct ModRMDecision.
So if we reduce INSTRUCTION_IDS to 16-bit, we will reduce the size by 47k*2 bytes, and if we can further encode modrm_type and INSTRUCTION_IDS to a single 16-bit, we can save another 47k*1 bytes.
The current size of modRMTable is 43k*2 bytes.

I assume you meant modRMTable by "the ModRM decision table".

Thanks,
Manman

On Sep 10, 2012, at 7:17 PM, Craig Topper <craig.topper at gmail.com> wrote:

> I don't follow how you would merge modrm_type and instructionIDs as those fields are in different tables.
> 
> On Mon, Sep 10, 2012 at 7:12 PM, Manman Ren <mren at apple.com> wrote:
> 
> Yes, I noticed that the max value is 43459 after I spent the effort to add an encoder :(
> But we can potentially combine the modrm_type with instructionIDs to fit into a single 16-bit number, thus saving 8-bit for each entry.
> 
> Let me know whether that is worth a try.
> 
> I will look into how to uniquely the ModRM decision table if possible. Do you have specific suggestion on how to?
> 
> Thanks,
> Manman
> 
> On Sep 10, 2012, at 7:09 PM, Craig Topper <craig.topper at gmail.com> wrote:
> 
>> Far as I can tell there's no reason you can't just change the existing index to 16-bits. The largest index is 43459. I'd rather to see effort spent uniquifying the ModRM decision table which will provide a much larger bang.
>> 
>> On Mon, Sep 10, 2012 at 12:28 PM, Manman Ren <mren at apple.com> wrote:
>> 
>> TableGen: reduce the size of ContextDecision tables in X86GenDisassemblerTables
>> by adding an encoder for instructionIDs. The main motivation is that in 83% of
>> the cases, the value is zero, so we should be able to use a 16-bit index
>> into the encoder and the encoder will translate the index into the actual
>> 32-bit value.
>> 
>> Later on, we may combine the modrm_type with instructionIDs to fit into a single
>> 16-bit number.
>> 
>> The size of libLTO in a Release build is reduced by 200k. The downside is one
>> more memory access to the encoder when decoding the instruction ID.
>> 
>> Thanks for reviewing the patch! Comments are greatly appreciated.
>> 
>> Manman
>> 
>> 
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>> 
>> 
>> 
>> 
>> -- 
>> ~Craig
> 
> 
> 
> 
> -- 
> ~Craig

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