Unexpected effects of --src-prefix on code generation

[hal...@prenav.com]

Hi folks,

I just hit an unexpected effect of --src-prefix when using capnp compile (capnp version 0.5.3): it appears that using --src-prefix can cause structs to end up different sizes in generated code. I've attached a simple test case; with --src-prefix, the struct ends up being one byte shorter than without using --src-prefix:

$ capnp --version

Cap'n Proto version 0.5.3

$ cat test/common.capnp                                                                                                                  
@0xc2be90a8da0c2971;


using Cxx = import "/capnp/c++.capnp";
$Cxx.namespace("common");


struct S
{
    x @0 : UInt64;
}

$ capnp compile -oc++:res-no-prefix test/common.capnp

$ capnp compile --src-prefix=test -oc++:res-prefix test/common.capnp

$ diff res-prefix/common.capnp.c++ res-no-prefix/test/common.capnp.c++
8c8
< static const ::capnp::_::AlignedData<31> b_afcc81ad15d9043d = {
---
> static const ::capnp::_::AlignedData<32> b_afcc81ad15d9043d = {
11c11
<      13,   0,   0,   0,   1,   0,   1,   0,
---
>      18,   0,   0,   0,   1,   0,   1,   0,
15,16c15,16
<      21,   0,   0,   0, 122,   0,   0,   0,
<      25,   0,   0,   0,   7,   0,   0,   0,
---
>      21,   0,   0,   0, 162,   0,   0,   0,
>      29,   0,   0,   0,   7,   0,   0,   0,
18c18
<      21,   0,   0,   0,  63,   0,   0,   0,
---
>      25,   0,   0,   0,  63,   0,   0,   0,
21,22c21,23
<      99, 111, 109, 109, 111, 110,  46,  99,
<      97, 112, 110, 112,  58,  83,   0,   0,
---
>     116, 101, 115, 116,  47,  99, 111, 109,
>     109, 111, 110,  46,  99,  97, 112, 110,
>     112,  58,  83,   0,   0,   0,   0,   0,
46c47
<   0xafcc81ad15d9043d, b_afcc81ad15d9043d.words, 31, nullptr, m_afcc81ad15d9043d,
---
>   0xafcc81ad15d9043d, b_afcc81ad15d9043d.words, 32, nullptr, m_afcc81ad15d9043d

This difference is unexpected, given that the documentation for the src-prefix flag implies that the only thing that should be effected by it is the location of the generated file. That said, I'm not sure if it's a documentation bug or a compiler bug, so I didn't dig further into the compiler to figure out where the difference is being introduced. Am I misunderstanding src-prefix, or is this a bug?

Thanks so much!

Haldean

[David Renshaw]

Hi,

The generated code contains the full schema information from the code generator request, including the displayName field, which will differ depending on the value --src-prefix.

There should be no observable effects of having different values for `displayName`, except possibly in error messages.

You can examine the schema information yourself by doing this:

capnp compile -o - test/common.capnp | capnp decode /usr/local/include/capnp/schema.capnp CodeGeneratorRequest

- David

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[Haldean Brown]

Thanks for the quick response! This stems from my own misunderstanding then. We had diagnosed a new failure to the diff between those two outputs, but the error must be elsewhere. Sorry for the noise!