[ADT][CMake][AutoConf] Fail-fast iterators for DenseMap

Summary:
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.

Enabling fail-fast iterators breaks the LLVM C++ ABI, so they are
predicated on `LLVM_ENABLE_ABI_BREAKING_CHECKS`.
`LLVM_ENABLE_ABI_BREAKING_CHECKS` by default flips with
`LLVM_ENABLE_ASSERTS`, but can be clamped to ON or OFF using the CMake /
autoconf build system.

Reviewers: chandlerc, dexonsmith, rnk, zturner

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8351

llvm-svn: 233310

1 files changed, 16 insertions, 0 deletions

diff --git a/llvm/docs/ProgrammersManual.rst b/llvm/docs/ProgrammersManual.rst
index 3b05280fb16..2c7e4a9954a 100644
--- a/llvm/docs/ProgrammersManual.rst
+++ b/llvm/docs/ProgrammersManual.rst
@@ -2553,6 +2553,22 @@ section on :ref:`isa and dyn_cast <isa>` and our :doc:`detailed document
 <HowToSetUpLLVMStyleRTTI>` which describes how you can implement this
 pattern for use with the LLVM helpers.
 
+.. _abi_breaking_checks:
+
+ABI Breaking Checks
+-------------------
+
+Checks and asserts that alter the LLVM C++ ABI are predicated on the
+preprocessor symbol `LLVM_ENABLE_ABI_BREAKING_CHECKS` -- LLVM
+libraries built with `LLVM_ENABLE_ABI_BREAKING_CHECKS` are not ABI
+compatible LLVM libraries built without it defined.  By default,
+turning on assertions also turns on `LLVM_ENABLE_ABI_BREAKING_CHECKS`
+so a default +Asserts build is not ABI compatible with a
+default -Asserts build.  Clients that want ABI compatibility
+between +Asserts and -Asserts builds should use the CMake or autoconf
+build systems to set `LLVM_ENABLE_ABI_BREAKING_CHECKS` independently
+of `LLVM_ENABLE_ASSERTIONS`.
+
 .. _coreclasses:
 
 The Core LLVM Class Hierarchy Reference