[llvm-dev] [RFC] [Windows SEH][-EHa] Support Hardware Exception Handling

Ten Tzen via llvm-dev
Thu, 16 Apr 2020 04:14:48 -0000

Hi, Eli,

Why are you under the impression that threw_exception() will not be called if
optimizations are enabled?  I don’t know if the -EHa Spec is clearly described
in MSFT Webs.  At least this proposal has described the rules for both C &
C++ code.

The very first rule clearly said that “no exception can move in or out of _try
region., i.e., no potential faulty instruction can be moved across _try
boundary”.  As such the dereference of statement return *(C*)0  must be kept in
_try scope and the access-violation fault will be caught in _except handler
where threw_exception() will be called.

I don’t see why Register allocation plays a part in this topic. I do see a
serious problem in LLVM SJLJ today (All tests in MSVC’s Setjmp suite fail with
-O2 that I will look into it soon).  But I failed to see why HW exception is
corelated to setjmp/longjmp.  These are two totally different features and the
approaches employed are also totally different.
It would be helpful if you can give one example why this proposal need to care
about how registers are allocated.

Again what we intend to do in this feature is to achieve these two points below.
Please take a moment to read through it.  Let me know if there is anything
unclear.
Thanks

--Ten
**** The rules for C code: ****
    For C-code, one way (MSVC approach) to achieve SEH -EHa semantic is to
    follow three rules. First, no exception can move in or out of _try
    region., i.e., no "potential faulty instruction can be moved across
_try
    boundary. Second, the order of exceptions for instructions
'directly'
    under a _try must be preserved (not applied to those in callees).
    Finally, global states (local/global/heap variables) that can be read
    outside of _try region must be updated in memory (not just in register)
    before the subsequent exception occurs.

    **** The impact to C++ code: ****
    Although SEH is a feature for C code, -EHa does have a profound effect
    on C++ side. When a C++ function (in the same compilation unit with
    option -EHa ) is called by a SEH C function, a hardware exception occurs
    in C++ code can also be handled properly by an upstream SEH _try-handler
    or a C++ catch(...). As such, when that happens in the middle of an
    object's life scope, the dtor must be invoked the same way as C++
    Synchronous Exception during unwinding process.


From: Eli Friedman <efriedma at quicinc.com>
Sent: Wednesday, April 15, 2020 4:29 PM
To: Ten Tzen <tentzen at microsoft.com>; llvm-dev at lists.llvm.org
Cc: rnk at google.com; Aaron Smith <aaron.smith at microsoft.com>; Joseph
Tremoulet <jotrem at microsoft.com>
Subject: [EXTERNAL] RE: [RFC] [Windows SEH][-EHa] Support Hardware Exception
Handling

I still have basically the same concerns.  I’ll try to give more concrete
examples for what I’m concerned about.

Suppose I have something like the following:

typedef struct C { int x[2]; } C;
void threw_exception();
void z();
C f() {
    __try {
        z();
        return *(C*)0;
    } __except(1) {
        threw_exception();
    }
    C c = {0};
    return c;
}


Currently, under your proposal, this won’t call threw_exception() if
optimization is enabled, as far as I can tell.  I have no idea if this is
intentional: your proposal and your patch don’t contain or point to any
documentation, and I can’t find any documentation that describes this on
Microsoft’s website.  (I don’t really care what the answer is here; I care that
there’s some documented answer to this question, and other questions like it.)


Constructing a testcase for the register allocation issues I mentioned before is
hard because it’s sort of “random” based on the register allocation heuristics,
but see
https://reviews.llvm.org/D77767<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Freviews.llvm.org%2FD77767&data=02%7C01%7Ctentzen%40microsoft.com%7C334ed759562941d3f2ba08d7e194d0d6%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637225901600853320&sdata=hTvtxO9OVy3fwzqX2EfLRb76Qb%2ByDlmdTHHLvuIrupQ%3D&reserved=0>
for the sort of issues that come up.  Note that we mark setjmp returns_twice,
which turns off certain optimizations.  I don’t really like extending the usage
of this sort of construct further, but if we are going to, we should at least
mark the new intrinsics returns_twice, so they get the same protection as
setjmp.

-Eli

From: Ten Tzen <tentzen at microsoft.com<mailto:tentzen at
microsoft.com>>
Sent: Wednesday, April 15, 2020 1:51 PM
To: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
Cc: rnk at google.com<mailto:rnk at google.com>; Eli Friedman <efriedma
at quicinc.com<mailto:efriedma at quicinc.com>>; aaron.smith at
microsoft.com<mailto:aaron.smith at microsoft.com>; Joseph Tremoulet
<jotrem at microsoft.com<mailto:jotrem at microsoft.com>>
Subject: [EXT] [RFC] [Windows SEH][-EHa] Support Hardware Exception Handling

Hi,
This is a spin-off of previous Windows SEH RFC below. This RFC only focus on
supporting HW Exception Handling.

A detailed implementation can be seen in here:
https://github.com/tentzen/llvm-project/commit/8a2421c274b683051e456cbe12c177e3b934fb5e<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Ftentzen%2Fllvm-project%2Fcommit%2F8a2421c274b683051e456cbe12c177e3b934fb5e&data=02%7C01%7Ctentzen%40microsoft.com%7C334ed759562941d3f2ba08d7e194d0d6%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637225901600853320&sdata=Dx710jfERAD7dDrNvsuEFOYVOrYgeYcRLAlt3mmW8es%3D&reserved=0>
It passes all MSVC SEH suite (excluding those with “Jumping out of _finally” (
_Local_Unwind)).

Thanks,

--Ten

    **** The rules for C code: ****
    For C-code, one way (MSVC approach) to achieve SEH -EHa semantic is to
    follow three rules. First, no exception can move in or out of _try
    region., i.e., no "potential faulty instruction can be moved across
_try
    boundary. Second, the order of exceptions for instructions
'directly'
    under a _try must be preserved (not applied to those in callees).
    Finally, global states (local/global/heap variables) that can be read
    outside of _try region must be updated in memory (not just in register)
    before the subsequent exception occurs.

    **** The impact to C++ code: ****
    Although SEH is a feature for C code, -EHa does have a profound effect
    on C++ side. When a C++ function (in the same compilation unit with
    option -EHa ) is called by a SEH C function, a hardware exception occurs
    in C++ code can also be handled properly by an upstream SEH _try-handler
    or a C++ catch(...). As such, when that happens in the middle of an
    object's life scope, the dtor must be invoked the same way as C++
    Synchronous Exception during unwinding process.

    **** Design and Implementation: ****

    A natural way to achieve the rules above in LLVM today is to allow an EH
    edge added on memory/computation instruction (previous iload/istore
    idea) so that exception path is modeled in Flow graph preciously.
    However, tracking every single memory instruction and potential faulty
    instruction can create many Invokes, complicate flow graph and possibly
    result in negative performance impact for downstream optimization and
    code generation. Making all optimizations be aware of the new semantic
    is also substantial.

    This design does not intend to model exception path at instruction
    level. Instead, the proposed design tracks and reports EH state at
    BLOCK-level to reduce the complexity of flow graph and minimize the
    performance-impact on CPP code under -EHa option. Detailed
    implementation described below.

    -- Two intrinsic are created to track CPP object scopes;
    eha_scope_begin() and eha_scope_end(). _scope_begin() is immediately
    added after ctor() is called and EHStack is pushed. So it must be an
    invoke, not a call. With that it's also guaranteed an EH-cleanup-pad is
    created regardless whether there exists a call in this scope. _scope_end
    is added before dtor(). These two intrinsics make the computation of
    Block-State possible in downstream code gen pass, even in the presence
    of ctor/dtor inlining.

    -- Two intrinsic, seh_try_begin() and seh_try_end(), are added for
    C-code to mark _try boundary and to prevent from exceptions being moved
    across _try boundary.

    -- All memory instructions inside a _try are considered as
'volatile' to
    assure 2nd and 3rd rules for C-code above. This is a little
    sub-optimized. But it's acceptable as the amount of code directly under
    _try is very small.

    -- For both C++ & C-code, the state of each block is computed at the
    same place in BE (WinEHPreparing pass) where all other EH tables/maps
    are calculated. In addition to _scope_begin & _scope_end, the
    computation of block state also rely on the existing State tracking code
    (UnwindMap and InvokeStateMap).

    -- For both C++ & C-code, the state of each block with potential trap
    instruction is marked and reported in DAG Instruction Selection pass,
    the same place where the state for -EHsc (synchronous exceptions) is
    done.

    -- If the first instruction in a reported block scope can trap, a Nop is
    injected before this instruction. This nop is needed to accommodate LLVM
    Windows EH implementation, in which the address in IPToState table is
    offset by +1. (note the purpose of that is to ensure the return address
    of a call is in the same scope as the call address.

    -- The handler for catch(...) for -EHa must handle HW exception. So it
    is 'adjective' flag is reset (it cannot be IsStdDotDot (0x40) that
only
    catches C++ exceptions).


From: Ten Tzen <tentzen at microsoft.com<mailto:tentzen at
microsoft.com>>
Sent: Friday, April 3, 2020 9:43 PM
To: rnk at google.com<mailto:rnk at google.com>
Cc: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>; Aaron
Smith <aaron.smith at microsoft.com<mailto:aaron.smith at
microsoft.com>>
Subject: RE: [EXTERNAL] Re: [llvm-dev] [RFC] [Windows SEH] Local_Unwind (Jumping
out of a _finally) and -EHa (Hardware Exception Handling)


Hi, Reid,
Nice to finally meet you😊.

Thank you for reading through the doc and providing insightful feedbacks.
Yes I definitely can separate these two features if it’s more convenient for
everyone.

For now, the local_unwind specific changes can be separated and reviewed between
these two commits:
      git diff 9b48ea90f4c9ae7ef030719d6c0b49b00861cdde
06c81a4b6262445432a4166627b87bf595f5291b

the -EHa changes can be read :
     git diff e943329ba00772f96fbc1fe5dec836cfd0707a38
9b48ea90f4c9ae7ef030719d6c0b49b00861cdde

My reply inline below in [Ten] lines.

--Ten

From: Reid Kleckner <rnk at google.com<mailto:rnk at google.com>>
Sent: Friday, April 3, 2020 3:36 PM
To: Ten Tzen <tentzen at microsoft.com<mailto:tentzen at
microsoft.com>>
Cc: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>; Aaron
Smith <aaron.smith at microsoft.com<mailto:aaron.smith at
microsoft.com>>
Subject: [EXTERNAL] Re: [llvm-dev] [RFC] [Windows SEH] Local_Unwind (Jumping out
of a _finally) and -EHa (Hardware Exception Handling)

UHi Ten,

Thanks for the writeup and implementation, nice to meet you.

I wonder if it would be best to try to discuss the features separately. My view
is that catching hardware exceptions (/EHa) is critical functionality, but
it's not clear to me if local unwind is truly worth implementing. Having
looked at the code briefly, it seemed like a large portion of the complexity
comes from local unwind. Today, clang crashes on this small example that jumps
out of a __finally block, but the intention was to reject the code and avoid
implementing the functionality. Clang does, in fact, emit a warning:
$ clang -c t.cpp
t.cpp:7:7: warning: jump out of __finally block has undefined behavior
[-Wjump-seh-finally]
      goto lu1;
      ^
Local unwind, in my view, is the user saying, "I wrote __finally, but
actually I decided I wanted to catch the exception, so let's transfer to
normal control flow now." It seems to me that the user already has a way to
express this: __except. I know the mapping isn't trivial and it's not
exactly the same, but it seems feasible to rewrite most uses of local unwind
this way.

[Ten] Right, I agree that to some degree a local_unwind can be viewed as another
type of _except handler in the middle of unwinding. And true that  some usage
patterns can be worked around by rewriting SEH hierarchy. But I believe the work
can be substantial and risky, especially in an OS Kernel.  Furthermore, to
broaden the interpretation, local_unwind can also serve as  a _filter (or even
rethrow-like handler in C++ EH), and the target block is the final handler.  See
the multi-local-unwind  example in the doc.

Can you estimate the prevalence of local unwind? What percent of __finally
blocks in your experience use non-local control flow? I see a lot of value in
supporting catching hardware exceptions, but if we can avoid carrying over the
complexity of this local unwind feature, it seems to me that future generations
of compiler engineers will thank us.

[Ten] I don’t have this data in hand. But what I know is that local_unwind is an
essential feature to build Windows Kernel.  One most important SEH test (the
infamous xcpt4u.c) is composed of 88 tests; among them there are 25
jumping-out-of-finally occurrences.  Of course this does not translate to a
percentage of local_unwind, but it does show us the significance of this feature
to Windows. FYI Passing xcpt4u.c is the very first fundamental requirement
before building Windows Kernel.

---

Regarding trap / non-call / hardware exception handling, I guess I am a bit more
blase about precisely modeling the control flow. As Eli mentions, we already
have setjmp, and we already don't model it. Users file bugs about problems
with setjmp, and we essentially close them as "wontfix" and tell them
to put more "volatile" on the problem until it stops hurting.

One thing that I am very concerned about is the implications for basic block
layout. Right now, machine basic block layout is very free-handed. Today,
CodeGen puts labels around every potentially-throwing call, does block layout
without considering try regions, and then collapses adjacent label regions with
the same landingpad during AsmPrinting. For MSVC C++ EH, state number stores and
the ip2state table achieve the same goal.

[Ten] Yes, I saw that (pretty nice implementation actually).  This design and
implementation completely inherits current mechanism except that now it’s
allowed to report EH state ranges that only contain memory/computation
instructions  (for obvious reason). I’m not sure which part of that concerns
you.

I think we need rules about how LLVM is allowed to transform the following code:
void foo(volatile int *pv) {
  __try {
    if (cond()) {
      ++*pv;
      __builtin_unreachable();
    }
  } __except(1) { }
  __try {
    if (cond()) {
      ++*pv;
      __builtin_unreachable();
    }
  } __except(1) { }
}

In this case, the *pv operation may throw, but I believe it would be semantics
preserving to merge the two identical if-then blocks. The call.setup proposal I
sent not long ago runs into the same issue. I have written a patch to tail merge
such similar blocks, but I have not landed it:
https://reviews.llvm.org/D29428<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Freviews.llvm.org%2FD29428&data=02%7C01%7Ctentzen%40microsoft.com%7C334ed759562941d3f2ba08d7e194d0d6%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637225901600863316&sdata=C2rPxOIKLoLFbFgpRlT%2F4E4aaazpphHhTiL3HNF%2BI9Y%3D&reserved=0>
Even though it's not yet landed, I think we need to know if the transform is
valid. If it is, then we need to do more than volatilize the try region to make
EHa work.

[Ten] The merging should not happen.  Per C-standard, a volatile must be read
(or write) ONCE and only once (as long as it’s naturally aligned and can be
accessed in one operation by HW).  So merging two volatiles violates the
standard.  I’m sure it’s currently well-protected in LLVM today.

For a long time I've wanted regions of some kind in LLVM IR, and this use
case has made me want to pick it up again. However, assuming that you want to
land support for hardware exceptions without some kind of generalized region
support in the IR, I think we do need to do something about these blocks ending
in unreachable in __try regions. The simplest thing that could possibly work is
to make clang end the try region before unreachable. This would mean ending the
block and adding `invoke void @seh_try_end` after every unreachable. It would be
redundant for noreturn calls, since those will already have an unwind edge,
ensuring they remain in the try region.

[Ten] it’s interesting you mentioned this “blocks ending in unreachable in __try
regions" here.  With these two features supported, two remaining bugs in my
ToDo list are; one setjmp() and one nested EH throw.  The second one seems
caused by a _try_block ended with an unreachable.   Yes, this is on my list.
Will discuss with you guys further when I look into it.

---

Another interesting aspect of /EHa is how it affects C++ destructor cleanups. I
am personally comfortable with the requirement that LLVM avoid moving around
volatile instructions in __try blocks. LLVM is already required to leave
volatile operations in order. But I *am* concerned about C++ destructor scopes,
which are much more frequent than __try. As you have described it, clang would
invoke eha_scope_begin() / eha_scope_end() around the object lifetime, but are
you proposing to volatilize all memory operations in the region? If not, I see
nothing that would prevent LLVM from moving potentially faulting operations in
or out of this scope. We cannot require passes to look for non-local EH regions
before doing code motion. Would that be acceptable behavior? It could lead to
some strange behavior, where a load is sunk to the point of use outside the
cleanup region, but maybe users don't care about this in practice.

[Ten] No, memory operations in C++ need not be volatilized.  The order of
exception in C++ code does not matter for -EHa.  Potential trap instructions are
free to move in/out of any EH region.  The only criteria is that when a HW
exception is caught and handled, local live objects must be dtored gracefully,
the same manner as C++ Synchronous exception.  By reporting the EH state of
those trap instructions, this is automatically done in LLVM today.

---

To summarize, my feedback would be:
1. Focus on __try and hardware exceptions first, the value proposition is clear
and large. In particular, something has to be done about unreachable. Clang
should already thread other abnormal control flow through the region exit.
2. Please gather some data on prevalence of local unwind to motivate the feature
3. Please elaborate on the design for /EHa C++ destructor cleanups and code
motion

I hope that helps, and I'm sorry if I'm slow to respond, this is a
tricky problem, and it's not my first priority.

Reid

On Wed, Apr 1, 2020 at 8:22 AM Ten Tzen via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hi, all,

The intend of this thread is to complete the support for Windows SEH.
Currently there are two major missing features:  Jumping out of a _finally and
Hardware exception handling.

The document below is my proposed design and implementation to fully support SEH
on LLVM.
I have completely implemented this design on a branch in repo:
https://github.com/tentzen/llvm-project<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Ftentzen%2Fllvm-project&data=02%7C01%7Ctentzen%40microsoft.com%7C334ed759562941d3f2ba08d7e194d0d6%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637225901600863316&sdata=bDssTTeo13Fw1qheHB%2BX%2FG1rcJty0l%2FYFeDUdtZYpO4%3D&reserved=0>.
It now passes MSVC’s in-house SEH suite.

Sorry for this long write-up.  For better readability, please read it on
https://github.com/tentzen/llvm-project/wiki<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Ftentzen%2Fllvm-project%2Fwiki&data=02%7C01%7Ctentzen%40microsoft.com%7C334ed759562941d3f2ba08d7e194d0d6%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637225901600873310&sdata=YdaHyRNh4JigYkzQQPx7COVJXYz2WTjaqr2E18RHo6Y%3D&reserved=0>

Special thanks to Joseph Tremoulet for his earlier comments and suggestions.

Note: I just subscribed llvm-dev, probably not in the list yet.  So please reply
with my email address (tentzen at microsoft.com<mailto:tentzen at
microsoft.com>) explicitly in To-list.
Thanks,

--Ten
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20200416/94fcd202/attachment-0001.html>