mirror of
https://github.com/c64scene-ar/llvm-6502.git
synced 2024-12-13 20:32:21 +00:00
e1e95c11b8
Summary: This change introduces a !make.implicit metadata that allows the frontend to pre-select the set of explicit null checks that will be considered for transformation into implicit null checks. The reason for not using profiling data instead of !make.implicit is explained in the change to `FaultMaps.rst`. Reviewers: atrick, reames, pgavlin, JosephTremoulet Subscribers: llvm-commits Differential Revision: http://reviews.llvm.org/D10824 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241116 91177308-0d34-0410-b5e6-96231b3b80d8
128 lines
4.2 KiB
ReStructuredText
128 lines
4.2 KiB
ReStructuredText
==============================
|
|
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 : FaultKind = FaultMaps::FaultingLoad (only legal value currently)
|
|
uint32 : FaultingPCOffset
|
|
uint32 : HandlerPCOffset
|
|
}
|
|
}
|
|
|
|
|
|
The ``ImplicitNullChecks`` pass
|
|
===============================
|
|
|
|
The ``ImplicitNullChecks`` pass transforms explicit control flow for
|
|
checking if a pointer is ``null``, like:
|
|
|
|
.. code-block:: llvm
|
|
|
|
%ptr = call i32* @get_ptr()
|
|
%ptr_is_null = icmp i32* %ptr, null
|
|
br i1 %ptr_is_null, label %is_null, label %not_null, !make.implicit !0
|
|
|
|
not_null:
|
|
%t = load i32, i32* %ptr
|
|
br label %do_something_with_t
|
|
|
|
is_null:
|
|
call void @HFC()
|
|
unreachable
|
|
|
|
!0 = !{}
|
|
|
|
to control flow implicit in the instruction loading or storing through
|
|
the pointer being null checked:
|
|
|
|
.. code-block:: llvm
|
|
|
|
%ptr = call i32* @get_ptr()
|
|
%t = load i32, i32* %ptr ;; handler-pc = label %is_null
|
|
br label %do_something_with_t
|
|
|
|
is_null:
|
|
call void @HFC()
|
|
unreachable
|
|
|
|
This transform happens at the ``MachineInstr`` level, not the LLVM IR
|
|
level (so the above example is only representative, not literal). The
|
|
``ImplicitNullChecks`` pass runs during codegen, if
|
|
``-enable-implicit-null-checks`` is passed to ``llc``.
|
|
|
|
The ``ImplicitNullChecks`` pass adds entries to the
|
|
``__llvm_faultmaps`` section described above as needed.
|
|
|
|
``make.implicit`` metadata
|
|
--------------------------
|
|
|
|
Making null checks implicit is an aggressive optimization, and it can
|
|
be a net performance pessimization if too many memory operations end
|
|
up faulting because of it. A language runtime typically needs to
|
|
ensure that only a negligible number of implicit null checks actually
|
|
fault once the application has reached a steady state. A standard way
|
|
of doing this is by healing failed implicit null checks into explicit
|
|
null checks via code patching or recompilation. It follows that there
|
|
are two requirements an explicit null check needs to satisfy for it to
|
|
be profitable to convert it to an implicit null check:
|
|
|
|
1. The case where the pointer is actually null (i.e. the "failing"
|
|
case) is extremely rare.
|
|
|
|
2. The failing path heals the implicit null check into an explicit
|
|
null check so that the application does not repeatedly page
|
|
fault.
|
|
|
|
The frontend is expected to mark branches that satisfy (1) and (2)
|
|
using a ``!make.implicit`` metadata node (the actual content of the
|
|
metadata node is ignored). Only branches that are marked with
|
|
``!make.implicit`` metadata are considered as candidates for
|
|
conversion into implicit null checks.
|
|
|
|
(Note that while we could deal with (1) using profiling data, dealing
|
|
with (2) requires some information not present in branch profiles.)
|