[llvm] r262811 - Bitcode reader: Inline readAbbreviatedField in readRecord and move the enclosing loop in each case (NFC)

[Mehdi Amini via llvm-commits]

> On Mar 7, 2016, at 8:20 AM, Rafael Espíndola <rafael.espindola at gmail.com> wrote:
> 
> With the gold plugin I got a smaller but measurable speedup. Linking
> clang with -plugin-opt=disable-output went from 15.99 seconds to 15.85
> seconds.

Thanks for double-checking!

0.9% improvement right? 

I reported earlier that readRecord() accounts for ~%3 of the total link time, and that it is ~20% faster itself, so you're seeing even more improvements  than I did.
ThinLTO is stressing it more with ~10% of the total link time and thus ~2% improvement on the total.

-- 
Mehdi




> 
> Cheers,
> Rafael
> 
> 
> On 6 March 2016 at 19:38, Mehdi Amini via llvm-commits
> <llvm-commits at lists.llvm.org> wrote:
>> Author: mehdi_amini
>> Date: Sun Mar  6 18:38:09 2016
>> New Revision: 262811
>> 
>> URL: http://llvm.org/viewvc/llvm-project?rev=262811&view=rev
>> Log:
>> Bitcode reader: Inline readAbbreviatedField in readRecord and move the enclosing loop in each case (NFC)
>> 
>> Summary: This make readRecord 20% faster, measured on an LTO build
>> 
>> Reviewers: rafael
>> 
>> Subscribers: llvm-commits
>> 
>> Differential Revision: http://reviews.llvm.org/D17911
>> 
>> From: Mehdi Amini <mehdi.amini at apple.com>
>> 
>> Modified:
>>    llvm/trunk/lib/Bitcode/Reader/BitstreamReader.cpp
>> 
>> Modified: llvm/trunk/lib/Bitcode/Reader/BitstreamReader.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Bitcode/Reader/BitstreamReader.cpp?rev=262811&r1=262810&r2=262811&view=diff
>> ==============================================================================
>> --- llvm/trunk/lib/Bitcode/Reader/BitstreamReader.cpp (original)
>> +++ llvm/trunk/lib/Bitcode/Reader/BitstreamReader.cpp Sun Mar  6 18:38:09 2016
>> @@ -131,8 +131,25 @@ void BitstreamCursor::skipRecord(unsigne
>>       const BitCodeAbbrevOp &EltEnc = Abbv->getOperandInfo(++i);
>> 
>>       // Read all the elements.
>> -      for (; NumElts; --NumElts)
>> -        skipAbbreviatedField(*this, EltEnc);
>> +      // Decode the value as we are commanded.
>> +      switch (EltEnc.getEncoding()) {
>> +      default:
>> +        report_fatal_error("Array element type can't be an Array or a Blob");
>> +      case BitCodeAbbrevOp::Fixed:
>> +        assert((unsigned)Op.getEncodingData() <= MaxChunkSize);
>> +        for (; NumElts; --NumElts)
>> +          Read((unsigned)EltEnc.getEncodingData());
>> +        break;
>> +      case BitCodeAbbrevOp::VBR:
>> +        assert((unsigned)Op.getEncodingData() <= MaxChunkSize);
>> +        for (; NumElts; --NumElts)
>> +          ReadVBR64((unsigned)EltEnc.getEncodingData());
>> +        break;
>> +      case BitCodeAbbrevOp::Char6:
>> +        for (; NumElts; --NumElts)
>> +          Read(6);
>> +        break;
>> +      }
>>       continue;
>>     }
>> 
>> @@ -206,13 +223,23 @@ unsigned BitstreamCursor::readRecord(uns
>>       if (!EltEnc.isEncoding())
>>         report_fatal_error(
>>             "Array element type has to be an encoding of a type");
>> -      if (EltEnc.getEncoding() == BitCodeAbbrevOp::Array ||
>> -          EltEnc.getEncoding() == BitCodeAbbrevOp::Blob)
>> -        report_fatal_error("Array element type can't be an Array or a Blob");
>> 
>>       // Read all the elements.
>> -      for (; NumElts; --NumElts)
>> -        Vals.push_back(readAbbreviatedField(*this, EltEnc));
>> +      switch (EltEnc.getEncoding()) {
>> +      default:
>> +        report_fatal_error("Array element type can't be an Array or a Blob");
>> +      case BitCodeAbbrevOp::Fixed:
>> +        for (; NumElts; --NumElts)
>> +          Vals.push_back(Read((unsigned)EltEnc.getEncodingData()));
>> +        break;
>> +      case BitCodeAbbrevOp::VBR:
>> +        for (; NumElts; --NumElts)
>> +          Vals.push_back(ReadVBR64((unsigned)EltEnc.getEncodingData()));
>> +        break;
>> +      case BitCodeAbbrevOp::Char6:
>> +        for (; NumElts; --NumElts)
>> +          Vals.push_back(BitCodeAbbrevOp::DecodeChar6(Read(6)));
>> +      }
>>       continue;
>>     }
>> 
>> 
>> 
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