[PATCH] [ADT][CMake][AutoConf] fail-fast iterators for DenseMap

[Zachary Turner]

As far as I know this is fine, because LLDB always builds the clang that it
links against.  It's certainly the case for the Windows and Linux
buildbots.  +Greg Clayton in case he knows something different on the apple
side.

On Mon, Mar 16, 2015 at 10:45 AM Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:

>
> > On 2015 Mar 16, at 09:03, Zachary Turner <zturner at google.com> wrote:
> >
> > If I understand correctly, this broke before because LLDB would redefine
> _NDEBUG sometimes, but not other times when including clang headers.
> >
>
> I think LLDB's NDEBUG inconsistency is a separate issue, and should
> probably be cleaned up/removed (as I pointed out, the original reason
> for redefining NDEBUG was cleaned up years ago).
>
> If the LLVM/clang libraries that LLDB links against are built with
> FAIL_FAST=false, and LLDB builds with FAIL_FAST=true, then you're
> still going to have an ABI problem.
>
> IIUC, with this patch LLDB would need to match FAIL_FAST with the
> LLVM/clang libraries it's linking against.
>
> > Now, since this isn't based on _NDEBUG, this is no longer an issue. If
> so, looks good to me.
> > On Mon, Mar 16, 2015 at 8:35 AM David Blaikie <dblaikie at gmail.com>
> wrote:
> > On Sun, Mar 15, 2015 at 10:48 PM, Sanjoy Das <
> sanjoy at playingwithpointers.com> wrote:
> > Hi chandlerc, dexonsmith, rnk, zturner,
> >
> > This is a re-do of D7931 with the following changes:
> >
> >  * the `EpochTracker` class is now conditional on
> >    `LLVM_ENABLE_FAIL_FAST_ITERATORS` and not `NDEBUG`.
> >
> > It'd be nice to have a more general conditional name so we can add other
> ABI-breaking debug checks under the same flag. (strawman:
> LLVM_ABI_BREAKING_DEBUG ?)
> >
> >
> >  * **The most significant change**: `LLVM_ENABLE_FAIL_FAST_ITERATORS`
> >    can be forced to be set or unset independent of `NDEBUG`.  By
> >    default, cmake and autoconf set it (thus enabling fail-fast
> >    iterators) for with-asserts builds and unset it for without-asserts
> >    builds.
> >
> > *fingers crossed that this is sufficient for other users* (that they're
> willing to manually unset this value for their debug builds if they need
> ABI compatibility)
> >
> >
> > With these changes, projects that require ABI compatibility between
> > `defined(NDEBUG)` builds and `!defined(NDEBUG)` builds can get that by
> > clamping `LLVM_ENABLE_FAIL_FAST_ITERATORS` to on or off.
> >
> > == Minor changes from D7931 ==
> >
> >  * `HandleBase::getEpochAddress` returns a `void *` to emphasise that
> >    the result is for comparison, not derefernecing.
> >
> >  * `HandleBase::getEpochAddress` was missing in the fail-fast-disabled
> >    version of `HandleBase`.  This was not a problem earlier since the
> >    function is only called from `assert`s and when `assert`s were
> >    enabled, we'd always choose the fail-fast-enabled version of
> >    `HandleBase`.  Since this is no longer the case (a with-asserts
> >    build may be using the fail-fast-disabled version of `HandleBase`),
> >    the missing method shows up as an obvious compiler error.
> >
> > Original summary for D7931:
> > This patch is an attempt at making `DenseMapIterator`s "fail-fast".
> > Fail-fast iterators that have been invalidated due to insertion into
> > the host `DenseMap` deterministically trip an assert (in debug mode)
> > on access, instead of non-deterministically hitting memory corruption
> > issues.
> >
> > http://reviews.llvm.org/D8351
> >
> > Files:
> >   CMakeLists.txt
> >   Makefile.config.in
> >   Makefile.rules
> >   autoconf/configure.ac
> >   cmake/modules/HandleLLVMOptions.cmake
> >   configure
> >   docs/CMake.rst
> >   include/llvm/ADT/DenseMap.h
> >   include/llvm/ADT/EpochTracker.h
> >
> > EMAIL PREFERENCES
> >   http://reviews.llvm.org/settings/panel/emailpreferences/
> >
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> >
> >
>
>
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