[PATCH] [opaque pointer type] Bitcode support for explicit type parameter on GEP.

[Duncan P. N. Exon Smith]

> On 2015-Feb-24, at 15:55, David Blaikie <dblaikie at gmail.com> wrote:
> 
> Use an explicit abbreviation with an Array type for operands.
> 
> 
> http://reviews.llvm.org/D7736
> 
> Files:
>  include/llvm/Bitcode/LLVMBitCodes.h
>  include/llvm/IR/Instructions.h
>  lib/Bitcode/Reader/BitcodeReader.cpp
>  lib/Bitcode/Writer/BitcodeWriter.cpp
>  test/Bitcode/function-encoding-rel-operands.ll
>  tools/llvm-bcanalyzer/llvm-bcanalyzer.cpp
> 
> EMAIL PREFERENCES
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> <D7736.20634.patch>

(A little confused about how your response came in two separate
emails -- does phabricator always do that? -- but I'm responding to
the one with the patch and copying in relevant comments.) 

> Ah, thanks - updated the patch to use this. I'm not sure what array type is ideal (since it'll be a combination of types and value references - is there a way to figure out the maximum size I need for those? I know there's the trick for computing the bit size for type references (& use that for the pointee type of the gep) but maybe there's a way to do that for value numbers too, and take the max of the two? Assuming that produces optimal bit encoding (maybe it's better to use a smaller VBR encoding so we only pay extra bits for some of the higher indicies? I've no idea))

VBR makes sense here.  I don't think a similar trick for values would
be particularly helpful -- since there are so many -- and values are
stored relative to the instruction id anyway (see implementation of
`PushValueAndType()`).

> Index: lib/Bitcode/Writer/BitcodeWriter.cpp
> ===================================================================
> --- lib/Bitcode/Writer/BitcodeWriter.cpp
> +++ lib/Bitcode/Writer/BitcodeWriter.cpp
> @@ -2284,6 +2288,19 @@
>                                     Abbv) != FUNCTION_INST_UNREACHABLE_ABBREV)
>        llvm_unreachable("Unexpected abbrev ordering!");
>    }
> +  {
> +    BitCodeAbbrev *Abbv = new BitCodeAbbrev();
> +    Abbv->Add(BitCodeAbbrevOp(bitc::FUNC_CODE_INST_GEP));
> +    Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1));
> +    Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed,       // dest ty
> +                              Log2_32_Ceil(VE.getTypes().size()+1)));
> +    Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Array));
> +    Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));
> +    if (Stream.EmitBlockInfoAbbrev(bitc::FUNCTION_BLOCK_ID,
> +                                   Abbv) != FUNCTION_INST_GEP_ABBREV)
> +      llvm_unreachable("Unexpected abbrev ordering!");
> +  }
> +
>  
>    Stream.ExitBlock();
>  }
> 

I also don't know what the best VBR is, but since the "default"
encoding seems to be VBR-6, I think that's a better choice in the
absence of data.

Otherwise this LGTM.