[CodeGen] Introduce a FAULTING_LOAD_OP pseudo-op.
Summary:
This instruction encodes a loading operation that may fault, and a label
to branch to if the load page-faults. The locations of potentially
faulting loads and their "handler" destinations are recorded in a
FaultMap section, meant to be consumed by LLVM's clients.
Nothing generates FAULTING_LOAD_OP instructions yet, but they will be
used in a future change.
The documentation (FaultMaps.rst) needs improvement and I will update
this diff with a more expanded version shortly.
Depends on D10196
Reviewers: rnk, reames, AndyAyers, ab, atrick, pgavlin
Reviewed By: atrick, pgavlin
Subscribers: llvm-commits
Differential Revision: http://reviews.llvm.org/D10197
llvm-svn: 239740
diff --git a/llvm/docs/FaultMaps.rst b/llvm/docs/FaultMaps.rst
new file mode 100644
index 0000000..d6274fa
--- /dev/null
+++ b/llvm/docs/FaultMaps.rst
@@ -0,0 +1,54 @@
+==============================
+FaultMaps and implicit checks
+==============================
+
+.. contents::
+ :local:
+ :depth: 2
+
+Motivation
+==========
+
+Code generated by managed language runtimes tend to have checks that
+are required for safety but never fail in practice. In such cases, it
+is profitable to make the non-failing case cheaper even if it makes
+the failing case significantly more expensive. This asymmetry can be
+exploited by folding such safety checks into operations that can be
+made to fault reliably if the check would have failed, and recovering
+from such a fault by using a signal handler.
+
+For example, Java requires null checks on objects before they are read
+from or written to. If the object is ``null`` then a
+``NullPointerException`` has to be thrown, interrupting normal
+execution. In practice, however, dereferencing a ``null`` pointer is
+extremely rare in well-behaved Java programs, and typically the null
+check can be folded into a nearby memory operation that operates on
+the same memory location.
+
+The Fault Map Section
+=====================
+
+Information about implicit checks generated by LLVM are put in a
+special "fault map" section. On Darwin this section is named
+``__llvm_faultmaps``.
+
+The format of this section is
+
+.. code-block:: none
+
+ Header {
+ uint8 : Fault Map Version (current version is 1)
+ uint8 : Reserved (expected to be 0)
+ uint16 : Reserved (expected to be 0)
+ }
+ uint32 : NumFunctions
+ FunctionInfo[NumFunctions] {
+ uint64 : FunctionAddress
+ uint32 : NumFaultingPCs
+ uint32 : Reserved (expected to be 0)
+ FunctionFaultInfo[NumFaultingPCs] {
+ uint32 : FaultType = FaultMaps::FaultingLoad (only legal value currently)
+ uint32 : FaultingPCOffset
+ uint32 : handlerPCOffset
+ }
+ }