I'll take a look at these.
On Wed, Sep 17, 2014 at 11:42 AM, Robert Edmonds wrote:
> Hi,
>
> Any chance someone could take a look at PR #20 ("Remove
> GOOGLE_PROTOBUF_ARCH_PPC"):
>
> https://github.com/google/protobuf/pull/20
>
> and PR #21 ("Expose generic atomicops on Clang")?
>
> https://github.com/google/protobuf/pull/21
>
> I'm planning on uploading a patched protobuf package to Debian in the
> very near future containing my patches in PR #20 and #21, as the 'mips'
> build failure described below blocks the transition of protobuf 2.6.0
> into the Debian archive. So if someone from Google could look at these
> patches I'd appreciate it.
>
> [PR #20]
>
> The GOOGLE_PROTOBUF_ARCH_PPC macro does not appear to be used anywhere
> in the protobuf code base. There is no Power-specific atomicops
> implementation. I think this would cause a build failure on Power
> systems where __ppc__ is actually defined. Notably, __ppc__ is not
> defined on either 32-bit or 64-bit Power on gcc, but *is* defined on
> 32/64-bit Power on clang. By luck, on Debian we happen to build
> protobuf and all of its reverse build dependencies (which are also
> affected, since this occurs in the headers compiled by client code) with
> gcc on the 'powerpc' (32-bit Power) and 'ppc64el' (64-bit Power,
> little-endian) architectures, so we have not run into this particular
> build failure corner case yet.
>
> It appears to be safe to simply remove the __ppc__ check and the
> GOOGLE_PROTOBUF_ARCH_PPC define, as it is not used anywhere in protobuf.
> This makes sure the generic atomicops implementation is used on Power.
> This is done in PR #20.
>
> [PR #21]
>
> A related issue is that the generic atomicops implementation is only
> exposed on GCC >= 4.7. However, Clang also supports the __atomic_*()
> intrinsics, but does not claim to be GCC >= 4.7. So in PR #21 I
> modified the preprocessor check to also support Clang. I have an
> example of a Debian build failure of one of protobuf's reverse build
> dependencies because of this bug, in the 'shogun' package:
>
>
> https://buildd.debian.org/status/fetch.php?pkg=shogun&arch=mips&ver=3.2.0-7.2%2Bb1&stamp=1410641206
>
> The 'shogun' package happens to be built with Clang on the Debian 'mips'
> architecture, so it needs to fall back to the generic atomicops
> implementation. Note, this is big-endian MIPS, not the little-endian
> 'mipsel' Debian architecture, which has a custom atomicops
> implementation in protobuf. (Confusingly, what Debian calls 'mipsel',
> protobuf calls 'GOOGLE_PROTOBUF_ARCH_MIPS'.) Compiling protobuf
> generated code on 'mips' with Clang results in the familiar error
> message:
>
> In file included from
> /usr/lib/../include/google/protobuf/generated_message_util.h:44:
> In file included from
> /usr/lib/../include/google/protobuf/stubs/once.h:81:
> In file included from
> /usr/lib/../include/google/protobuf/stubs/atomicops.h:59:
> /usr/lib/../include/google/protobuf/stubs/platform_macros.h:77:2:
> error: Host architecture was not detected as supported by protobuf
>
> [Issue #25]
>
> However, building protobuf with Clang on a generic atomicops
> architecture generates new compiler diagnostics that do not appear to
> have an equivalent in gcc, so I've filed issue #25:
>
> https://github.com/google/protobuf/issues/25
>
> This looks to be a genuine bug in the original generic atomicops
> implementation. These warnings appear to currently be harmless, since
> the affected functions do not appear to be used anywhere in protobuf or
> protobuf generated code.
>
> Thanks!
>
> --
> Robert Edmonds
>
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