[llvm-dev] Support token type in struct for landingpad

Chen Li via llvm-dev
2015-12-05T05:33:51Z

> On Dec 4, 2015, at 1:27 PM, Joseph Tremoulet <jotrem at
microsoft.com> wrote:
>
> > I dont have a concrete design right now and I am happy to take any
other ideas
>
> Three ideas come to mind, none of which are perfect:
>
> 1) I'm tempted to say that now that we have token type, landingpad
should generally produce a token, the pointer should be extracted with the
@llvm.eh.exceptionpointer intrinsic instead of an extractvalue, and the selector
should likewise be extracted with a new @llvm.eh.exceptionselector intrinsic
instead of extractvalue (and personalities that communicate other things via
their landingpads would need to add similar intrinsics to extract them, like the
@llvm.eh.exceptioncode intrinsic that SEH uses).  But that would require
updating all the front-ends generating landingpads, and be awkward for any
target personality routines that literally do pass a struct to the landing pad
(are there any?), and so probably just reflects my bias coming from dealing with
a personality using catchpads/cleanuppads instead of landingpads.
>
> 2) Since you're not actually using the landingpad's exception
selector nor, if I understand " This is enough to support the gc.statepoint
work " correctly, its exception pointer, it's possible that
TLI.getExceptionPointerRegister and TLI.getExceptionSelectorRegister should be
returning NoRegister for your personality.  That would require modifying the
EHPersonality enum and corresponding string matching in
Analysis/EHPersonalities.h to recognize your personality, but I think that would
be fine (it highlights a potential scaling issue if we add lots of targets that
each need this, but that's a somewhat independent and pre-existing issue,
and in reality I doubt you'd be opening a floodgate here).

Yes, we don’t neither the exception selector nor the exception pointer. I’ve
taken a similar approach as #2 you suggested, but instead of modifying the
personality, I directly add a check in visitLandingPad to see if landingpad is
token type. If so, don’t create DAG node for selector and exception pointer even
if TLI.getExceptionPointerRegister and TLI.getExceptionSelectorRegister are not
zero or NoRegister. I think this works correctly and I’ve passed all the tests I
have. I plan to check this in as the first step, but I’d like to actually
support selector and exception pointer extraction for landingpad of token type.
Personally I like #1 you suggested, but I am also afraid that requires updating
all front-ends generating landingpads. I think #4 might be the most practical
approach that I could think of.

thanks,
chen


>
> 3) Maybe the default should be switched, so that
TLI.getExceptionPointerRegister and TLI.getExceptionSelectorRegister
returnNoRegister for EHPersonality::Unknown, and only return actual registers
for personalities they recognized.  This would require any targets using
landingpads with exception pointers / exception selectors to update their code
and add themselves to Analysis/EHPersonalities.h, similar to how #2 would
require adding your personality, so it seems more disruptive if conceptually a
touch cleaner than #2.
>
> 4) Explicitly checking for token type in visitLandingPad as you suggest
sounds okay to me as a pragmatic approach, too.
>
> I'd probably lean toward #2 as being the least disruptive and most
explicit/straightforward about the personality's expectations, but I'm
curious what others think.
>
> Thanks
> -Joseph
>   <>
> From: Chen Li [mailto:meloli87 at gmail.com]
> Sent: Thursday, December 3, 2015 4:06 PM
> To: David Majnemer <david.majnemer at gmail.com>
> Cc: Igor Laevsky <igor at azulsystems.com>; llvm-dev <llvm-dev at
lists.llvm.org>; Joseph Tremoulet <jotrem at microsoft.com>
> Subject: Re: Support token type in struct for landingpad
>
> Hi David and Joseph,
>
> I’ve just added landingpad with token type locally and changed gc.relocate
to work in the following way:
>
>   %0 = invoke token (i64, i32, void (i64 addrspace(1)*)*, i32, i32, ...)
@llvm.experimental.gc.statepoint.p0f_isVoidp1i64f(i64 0, i32 0, void (i64
addrspace(1)*)* @some_call, i32 1, i32 0, i64 addrspace(1)* %obj, i32 0, i32 5,
i32 0, i32 -1, i32 0, i32 0, i32 0, i64 addrspace(1)* %obj, i64 addrspace(1)*
%obj1)
>           to label %invoke_safepoint_normal_dest unwind label
%exceptional_return
>
> invoke_safepoint_normal_dest:
>   ...
>
> exceptional_return:
>   %landing_pad = landingpad token
>           cleanup
>   %obj.relocated1 = call coldcc i64 addrspace(1)*
@llvm.experimental.gc.relocate.p1i64(token %landing_pad, i32 13, i32 13)
>   %obj1.relocated1 = call coldcc i64 addrspace(1)*
@llvm.experimental.gc.relocate.p1i64(token %landing_pad, i32 14, i32 14)
>   ret i64 addrspace(1)* %obj1.relocated1
>
>
> Now gc.statepoint return a token type instead of i32 type, and gc.relocate
also takes a token type as its first argument (the first argument should either
be the corresponding gc.statepoint for call statepoint or invoke statepoint on
the normal path, or a reference that could help find the corresponding
gc.statepoint on the unwind the path). And since landingpad produces a token
type here as well, it can be passed as the reference to the gc.relocate’s first
argument.
>
> To make this work, I have changed two parts of the code. First is how
gc.relocate looks up for its corresponding gc.statepoint on the unwind path. It
used to use the extracted selector value to find the landingpad and then use the
landingpad to find the invoke instruction, which is the gc.statepoint. Now, it
can use the landingpad directly to find the invoke instruction. The second part
is to make landingpad work with token type. In LLVM’s front end (passes before
SelectionDAG), there is no restrictions on what type a landingpad should have
(there are test cases in LLVM that has landingpad of i8 or i32 type). However,
in SelectionDAGBuilder::visitLandingPad, it is enforced that landingpad must be
two-valued (type of { i8*, i32 }), in which way it can handle the exception
pointer and selector value inside it. As the first step, I’d like to just add a
check to see if the landingpad is of token type, and if so stop it and don’t
bother to create the DAG nodes for the exception pointer and selector value
(same as what happens during SjLj exceptions). This is enough to support the
gc.statepoint work but will not support for C++ style exception handling with
gc.statepoint. As for follow-up work, I’d like to add some support to extract
selector value from token landingpad. I think we could either do it explicitly
in IR (maybe add a intrinsic call extract.selector or something similar) or
implicitly during SelectionDAG (in visitLandingPad, check if it’s token type,
and if so add an additional transform to extract the exception pointer and
selector value from the token). I dont have a concrete design right now and I am
happy to take any other ideas. My plan is to get the first step checked in and
incrementally work on the follow-up work. Does that sound a reasonable approach
to you guys?
>
> thanks,
> chen
>
>
>
> On Dec 2, 2015, at 9:47 AM, Chen Li <meloli87 at gmail.com
<mailto:meloli87 at gmail.com>> wrote:
>
>
> On Dec 1, 2015, at 11:14 PM, David Majnemer <david.majnemer at gmail.com
<mailto:david.majnemer at gmail.com>> wrote:
>
> While we support 'opaque' types nested within struct types, they
are not exactly battle tested:
>
> $ cat t.ll
> %opaque_ty = type opaque
>
> %struct_ty = type { i32, %opaque_ty }
>
> define %struct_ty @f(%struct_ty* %p) {
>   %load = load %struct_ty, %struct_ty* %p
>   ret %struct_ty %load
> }
>
> $ opt -O2 t.ll -S
> lib/IR/DataLayout.cpp:623: unsigned int
llvm::DataLayout::getAlignment(llvm::Type *, bool) const: Assertion
`Ty->isSized() && "Cannot getTypeInfo() on a type that is
unsized!"' failed.
>
> Thanks David! I’ve actually hacked to add token type into struct type and
ended up with the same failure as above. I will take a look at the catchpad and
cleanuppad code, and create a patch to add token landingpad and have you review
it.
>
> thanks,
> chen
>
>
> As a practical matter, I fear nesting 'token' types within struct
types will have similar issues.  Beyond that, the design philosophy behind
'token' is that it is incredibly opaque and permitting it to nest inside
a struct creates scenarios where one might try to GEP to the end of the field
right before the token field in an attempt to examine or manipulate the token.
>
> Your other recommendation, having landingpad produce a token, is quite
similar to how we've designed catchpad and cleanuppad.  I think that
direction will be quite nice.
>
> On Tue, Dec 1, 2015 at 8:07 PM, Chen Li <meloli87 at gmail.com
<mailto:meloli87 at gmail.com>> wrote:
> Hi David,
>
> Sorry to bother you, but I would like to get some suggestions on your
recent work of token type.
>
> I’m currently working on changing gc.statepoint to return a token type
instead of a i32 type. The reason is that with the current implementation,
gc.statepoint could potentially be fed into PHI nodes, and break
RewriteStatepointsForGC pass later. Using token type would help us to avoid
this. I have most of the code work but got a problem when gc.statepint is an
InvokeInst and has an unwind path.
>
> Currently, gc.statepoint of InvokeInst works as below (the code snippet is
from test/CodeGen/X86/statepoint-invoke.ll):
>
>   %0 = invoke i32 (i64, i32, void (i64 addrspace(1)*)*, i32, i32, ...)
@llvm.experimental.gc.statepoint.p0f_isVoidp1i64f(i64 0, i32 0, void (i64
addrspace(1)*)* @some_call, i32 1, i32 0, i64 addrspace(1)* %obj, i32 0, i32 5,
i32 0, i32 -1, i32 0, i32 0, i32 0, i64 addrspace(1)* %obj, i64 addrspace(1)*
%obj1)
>           to label %invoke_safepoint_normal_dest unwind label
%exceptional_return
>
> invoke_safepoint_normal_dest:
>   …
>
> exceptional_return:
>   %landing_pad = landingpad { i8*, i32 }
>           cleanup
>   %relocate_token = extractvalue { i8*, i32 } %landing_pad, 1
>   %obj.relocated1 = call coldcc i64 addrspace(1)*
@llvm.experimental.gc.relocate.p1i64(i32 % relocate_token, i32 13, i32 13)
>   %obj1.relocated1 = call coldcc i64 addrspace(1)*
@llvm.experimental.gc.relocate.p1i64(i32 % relocate_token, i32 14, i32 14)
>   ret i64 addrspace(1)* %obj1.relocated1
>
>
> Each gc.relocate needs to take its corresponding gc.statepoint as its first
argument. However, on the unwind path, there is no way to get gc.statepoint
directly because the return value of the InvokeInst is undefined there. In this
scenario, we tie gc.relocate to the landingpad, and use the landingpad to find
its unique predecessor to get the corresponding gc.statepoint. We pick the
selector value from the landingpad to feed into gc.relocate just because it has
the same type (i32) as gc.statepoint's return type. The actual value of the
selector doesn’t really matter because gc.relocate only uses it as a reference
to find gc.statepoint and not consume it during lowering.
>
> However, this will no longer work if we change gc.statepoint's return
type to token type. To make it work, I could see two potential approaches. 1)
add support of token type inside struct type so that we can have a landingpad
with result type of { i8*, token }, or 2) add support of landingpad with a token
result type. Approach 1 seems to be easier since all the other parts of
statepoint handling does not need to be changed at all, and having a selector of
token type also seems reasonable (furthermore, we don’t ever need to extract
selector value to do exception handling in our code base so I think only
supporting token type in struct should be enough for us). Approach 2 requires to
modify the way how gc.relocate looks up for its corresponding gc.statepoint
through landingpad, but shouldn’t be hard either.
>
> Does either of the approaches sound reasonable to you? Other ideas are also
welcomed :)
>
> Thank you very much!
>
> Best,
> Chen

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