proposal: function and method arguments and results should be passed in registers #18597

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dr2chase opened this Issue Jan 10, 2017 · 58 comments

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@dr2chase
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dr2chase commented Jan 10, 2017 edited

Performance would be generally somewhat improved if arguments-to and results-from function and method calls used registers instead of the stack. Projected improvements based on a limited prototype are in the range of 5-10%.

The running CL for the prototype: https://go-review.googlesource.com/c/28832/
The prototype, because it is a prototype, uses a pragma that should be unnecessary in the final design, though helpful during development.

(This is a placeholder bug for a design doc.)

problems/tradeoffs noted below, through 2017-01-19 (#18597 (comment))

This will reduce the usefulness of panic tracebacks because it will confuse/corrupt the per-frame argument information there. This was already a problem with SSA and register allocation and spilling, this will make it worse. Perhaps only results should be returned in registers.

Does this provide enough performance boost to justify "breaking" (how much?) existing assembly language?

Given that this breaks existing assembly language, why aren't we also doing a more thorough revision of the calling conventions to include callee-saves registers?

This should use the per-platform ABIs, to make interaction with native code go more smoothly.

Because this preserves the same memory layout of the existing calling, it will consume more stack space than it otherwise might, if that had been optimized out.

And the responses to above, roughly:

In compiler work, 5% (as a geometric mean across benchmarks) is a big deal.

Panic tracebacks are a problem; we will work on that. One possible mitigation is to use DWARF information to make tracebacks much better in general, including properly named and interpreted primitive values. A simpler mitigation for targeted debugging could be an annotation indicating that a function should be compiled to store arguments back to the stack (old style) to ensure that particular function's frame was adequately informative. This is also going to be an issue for improved inlining because regarded at a low level intermediate frames will disappear from backtraces.

The scope of required assembly-language changes is expected to be quite small; from Go-side function declarations the compiler ought to be able to create shims for the assembly to use around function entry, function exit, and surrounding assembly-language CALLs to named functions. The remaining case is assembly-language CALL via a pointer, and these are rare, especially outside the runtime. Thus, the need to bundle changes because they are disruptive is reduced, because the changes aren't that disruptive.

Incorporating callee-saves registers introduces a garbage-collector interaction that is not necessarily insurmountable, but other garbage-collected languages (e.g., Haskell) have been sufficiently intimidated by it that they elected not to use callee-saves registers. Because the assembler can also modify stack frames to include callee-save spill areas and introduce entry/exit shims to save/restore callee-save registers, this appears to have lower impact than initially estimated, and thus we have reduced need to bundle changes. In addition, if we stake out potential callee-save registers early developers can look ahead and adapt their code before it is required.

If each platform's ABI were used instead of this adaptation of the existing calling conventions, the assembly-language impact would be larger, the garbage-collector interactions would be larger, and either best-treatment for Go's multivalue returns would suffer or the cgo story would be sprinkled with asterisks and gotchas. As an alternative (a different way of obtaining a better story for cgo), would we consider a special annotation for cgo-related functions to indicate that exactly the platform calling conventions were used, with no compromises for Go performance?

@dr2chase dr2chase added this to the Go1.9Maybe milestone Jan 10, 2017
@dr2chase dr2chase self-assigned this Jan 10, 2017
@cespare cespare added the Proposal label Jan 10, 2017
@dsnet dsnet modified the milestone: Proposal, Go1.9Maybe Jan 10, 2017
@gopherbot

CL https://golang.org/cl/35054 mentions this issue.

@davecheney
Contributor
davecheney commented Jan 11, 2017 edited

5% for amd64 doesn't feel like it justifies the cost of breaking all the assembly written to date.

@minux
Member
minux commented Jan 11, 2017
@davecheney
Contributor

I agree with Minux, optimising function calls for 10% speedup vs inlining and all the code generation benefits that unlocks doesn't feel like a good investment.

@josharian
Contributor

I'm still thinking about the proposal, but I will note that for asm functions with go prototypes and normal stack usage, we can do automated rewrites. Which is also a reminder that the scope for this proposal should include updating vet's asmdecl check.

@minux
Member
minux commented Jan 11, 2017
@randall77
Contributor

@minux Yes, we will have all the information necessary to print args as we do now. I suspect it just isn't implemented yet.

@minux
Member
minux commented Jan 11, 2017
@randall77
Contributor

The values enter in registers but they will be spilled to the arg slots if they are live at any call. So all live arguments will still be correct. Even modified live arguments should work (except for a few weird corner cases where multiple values derived from the same input are live).

Dead arguments are not correct even today. What you see in tracebacks is stale. They may get more wrong with this implementation, so it may be worth marking/not displaying dead args.

Things will get trickier if we allow callee-saved registers. We're thinking about it, but it isn't in the design doc yet.

@minux
Member
minux commented Jan 11, 2017
@randall77
Contributor

When we pass an argument in a register, the corresponding stack slot is reserved but not initialized, so there's no memory traffic for it. Only if the callee needs to spill it will that slot actually get initialized.

@dr2chase
Contributor

I'm not sure how "full" the design docs are.
I've got a CL up, and here's a more readable version of it that I'll try to keep up to date: https://gist.github.com/dr2chase/5a1107998024c76de22e122ed836562d

(Repeating some of that gist) I reviewed a bunch of ABIs, and the combination of

  • need someplace to spill if the stack must grow, and we have no frame yet
  • callee-saves makes tracing stacks for GC a good deal more painful; the impact on assembly language is also a good deal larger.
  • there's no memory traffic unless there's a spill, and then we must spill somewhere
  • most of the existing ABIs (all but Arm64) are unfriendly to multiple return values in registers
  • assurances from debugger/dump-inspection people (Delve and Backtrace.io) that they use DWARF, if we take the time to get it right

caused me to to decide to try something other than standard ABI(s). The new goal was to minimize change/cost while still getting plausible benefits, so I decided to keep as much of the existing design as possible.

The main loss in reserving stack space is increased stack size; we only spill if we must, but if we must, we spill to the "old" location.

As far as backtraces go, we need to improve our DWARF output for debuggers, and I think we will, and then use that to produce readable backtraces. That should get this as right as it can be (up to variables not being live) and would be more generally accessible than binary data.

So actually turning this on may be gated by DWARF improvements.

@minux
Member
minux commented Jan 11, 2017
@bcmills
Member
bcmills commented Jan 12, 2017

if we can align our callee-saved registers with the platform ABI, cgo callbacks will be faster

Not to mention more reliable. A lot of the signal trampolines in the runtime could be a whole lot simpler if they didn't have to deal with the ABI mismatch between signal handlers (which must use the platform ABI) and the runtime's Go functions. (I'm still not entirely convinced that there aren't more calling-convention bugs lurking.)

@minux
Member
minux commented Jan 12, 2017
@ianlancetaylor
Contributor

At each call, have the PCDATA for that call record, for each callee-saved register, whether it holds 1) a pointer; 2) a non-pointer; 3) whatever it held on function entry. Also at each call, record the list of callee-saved registers saved from the caller, and where they are.

Now, on stack unwind, you can see the list of callee-saved registers. Then you look to the caller to see whether the value is a pointer or not. Of course you may have to look at the caller's caller, etc., but since the number of callee-saved registers is small and fixed it's easy to keep track of what you need to do as you walk up the stack.

@philhofer
Contributor

5% for amd64 doesn't feel like it justifies the cost of breaking all the assembly written to date.

Much like SSA, I expect this change will have a much more meaningful effect on minority architectures.

Those fancy Intel chips can resolve push/pop pairs halfway through the instruction pipeline (without even emitting a uop!), while a load from the stack on a Cortex A57 has a 4-cycle result latency..

@minux
Member
minux commented Jan 12, 2017
@philhofer
Contributor

Result latency doesn't matter much on a out-of-order core because L1D on
Intel
chips has similar latency.

It especially doesn't matter if you have shadow registers, but AFAIK none of the arm/arm64 designs have 'em.

I suspect the benefit could be higher, but for non-leaf functions we still
need to
spill the argument registers to stack, which negates most of the benefit
for large
and non-leaf functions.

You can't just move them to callee-saves if they're still live? Is that for GC, or a requirement for traceback?

@minux
Member
minux commented Jan 12, 2017
@philhofer
Contributor

In the current ABI, every register is caller-save. My counterproposal is
introducing callee-save
registers but keep arguments passing on stack to preserve the current
traceback behavior.

Ah. I guess I assumed, incorrectly, that this proposal included making some registers callee-save.

@cherrymui
Contributor

Therefore, it seems registered parameters will mostly help small leaf
functions. And if that's true, I imagine inlining those functions will
actually provide even more speedup because then the compiler can optimize
across function call boundaries.

Inlining doesn't help on function pointer calls.

@bcmills
Member
bcmills commented Jan 12, 2017 edited

@dr2chase

there's no memory traffic unless there's a spill, and then we must spill somewhere

That's not strictly true. Empty stack slots still decrease cache locality by forcing the actual in-use stack across a larger number of cache lines. There's no extra memory traffic between the CPU and cache unless there's a spill, but reserving stack slots may well increase memory traffic on the bus.

most of the existing ABIs (all but Arm64) are unfriendly to multiple return values in registers

Not true. It would be trivial to extend the SysV AMD64 ABI for multiple return values, for example: it already has two return registers (rax and rdx), and it's easy enough to define extensions of the ABI that pass additional Go return values in other caller-save registers.

For example, we could do something like:

rax: varargs count; 1st return
rbx: callee-saved
rcx: 4th argument; 3rd return
rdx: 3rd argument; 2nd return
rsp: stack pointer
rbp: frame pointer
rsi: 2nd argument
rdi: 1st argument
r8: 5th argument; 4th return
r9: 6th argument; 5th return
r11: temporary register
r12-r14: callee-saved
r15: GOT base pointer
@mundaym
Member
mundaym commented Jan 13, 2017 edited

Am I understanding correctly that this proposal is calling for structs to always be unpacked into registers before a call? Has any thought been given to passing structs as read-only references? I think this is how most (all?) of the ELF ABIs handle structs, particularly larger ones.

This way the callee gets to decide whether it needs to create a local copy of anything, avoiding copying in many cases. It is also presumably fairly common for only a subset of struct fields to be accessed so unpacking all or part of the struct may be unecessarily expensive (particularly if the struct has boolean fields). Obviously the reference would have to be to something on the stack (copied there, if necessary, by the caller) or to read-only memory.

For Go in particular it seems like this would be a nice fit because the only way to get const-like behaviour is to pass structs by value and internally passing them by reference instead would potentially make that idiom a lot cheaper.

Arrays and maybe slices could also be handled the same way.

@minux
Member
minux commented Jan 13, 2017
@josharian
Contributor

It might be interesting to gather empirical data about the distribution of pointer vs non-pointer arguments. Or perhaps more precisely, GC-relevant vs non-GC-relevant arguments, since there are some pointers that GC doesn't care about.

Ian's suggestion for tracking pointer vs non-pointer vs inbound in PCDATA is nice and clean and flexible, but even simpler and cheaper would be to simply partition into fixed sets of GC-able registers and non-GC-able registers.

@dr2chase
Contributor

I don't know if I made this point adequately well, but the estimated performance gains are based on benchmarks where X% of calls are hand-enabled (tied to function) to pass arguments in registers, spills and all. Rick helped me with VTune to figure out the smallest set of functions to opt-in to get a meaningful percentage of calls covered. The 5 and 10% improvement estimates are a conservative scale-up of the observed improvements to 100% coverage of calls.

I'm in the process of attempting to get both -gcflags=-l=4 and the current experiment working simultaneously so I can estimate their combined benefit in the same way; there are bugs in the interaction.

The current version of the proposal does call for structs to be unpacked field-by-field. This is influenced by a desire to get strings, slices, and interfaces to land entirely within registers, combined with lack of support for selecting a "field" from a register in the current backend and some worries about how this might work on amd64p32. I don't think that "field selection" from a register is something that we've implemented (yet).

@minux
Member
minux commented Jan 13, 2017
@minux
Member
minux commented Jan 13, 2017
@josharian
Contributor

we need to gather the types of hot values that live across a function call, not types of function input arguments.

Right, thanks.

@bcmills
Member
bcmills commented Jan 13, 2017

@josharian

even simpler and cheaper would be to simply partition into fixed sets of GC-able registers and non-GC-able registers.

That isn't obvious to me. Partitioning registers adds complexity to the register allocator, while PCDATA tracking adds complexity to the GC. Either approach seems straightforward, but both require additional code.

@bcmills
Member
bcmills commented Jan 13, 2017 edited

@mundaym

It is also presumably fairly common for only a subset of struct fields to be accessed so unpacking all or part of the struct may be unecessarily expensive (particularly if the struct has boolean fields).

That assumes that you've constructed the struct somewhere in memory in the first place. For large and/or long-lived structs, that's a valid assumption — but for many small structs (given reasonably effective inlining) it is not.

FWIW, the AMD64 psABI passes structs of "up to four eightbytes" in registers.

@minux
Member
minux commented Jan 13, 2017
@josharian
Contributor

the induction pointer to its backing array (when saved in callee-saved registers) doesn't have to be labeled as pointer.

I think it would, since stack copying still needs to update it.

@dr2chase
Contributor

Returning values in registers is slightly more difficult when runtime shims are needed (e.g., reflectcall). On the call side, there's already a "natural" potential second entrypoint for all functions that do a stack size check (provided that the original spill-site memory layout is retained) that makes it relatively easy to implement go statements.

Notes:

  • "if the compiler is sure that the GC can find another pointer to a slice" -- and, also, if the slice's backing store is known not to reside on a stack, since stacks can relocate and grow.
  • The pointer-to-(large-)struct trick also requires that the struct not be mutable from multiple threads, else we can end up with nasty concurrency issues.

Do we need separate proposals for all the other great ideas put forth as alternatives?

@josharian
Contributor

Do we need separate proposals for all the other great ideas put forth as alternatives?

I say we continue the conversation here, and once it settles, someone--you, I imagine :)--digests all the input and comes back with an updated proposal taking it all into account.

@dr2chase
Contributor

Josh, it took a fair amount of work to obtain the credible-to-me 5 and 10% estimates for the existing proposal, and I took the time to do it because the potential impacts of doing this work are relatively large. Anything that doesn't have a large cost-to-other-people I think just gets put on a list of stuff to do, and we prioritize by roughly-expected bang-for-buck.

My view on this is that a 5-10% gain in general (geomean) compiled code performance is worthwhile (back in the price-workstations-by-benchmark-performance days, we'd have been giddily happy for such a gain, and that was back when pipelines were short and simple) and the possibility that this might have an annoying impact makes me want to do it sooner, rather than later, since the more users we have writing "interesting" code, the more concrete is poured around our feet. Low-impact changes we can do later.

I've been trying to get a feel for what the "actual problems" are with this proposal, where "hey, what about this other optimization" is not an actual problem. The ones that I see are:

  1. hex numbers in stack traces will get less informative, possibly even misleading;
  2. stacks will not shrink as much as we might wish because we're allocating spill slots that might not be necessary (that we hope are usually not necessary);
  3. treatment of structs containing lots of small fields might get worse, not better
  4. there's an opportunity here to change how (large) structs are passed that might be very beneficial, and that's not been thought about in any great detail.
  5. the assembly language migration story is not entirely satisfying; outbound calls through pointers remain a problem.
@bcmills
Member
bcmills commented Jan 13, 2017

I would add one more specific concern to your list:

  1. if the new convention isn't a superset of the platform ABI, we may lose the opportunity to simplify assembly functions. (The migration is likely a lot of work, and I doubt we'll do it twice.)
@josharian
Contributor

I'm not saying that you necessarily need to fundamentally alter the proposal. It might well be that the outcome is that the proposal gets an "Alternatives" section added explaining why the various alternative suggestions here were declined in favor of the current one.

I've been trying to get a feel for what the "actual problems" are with this proposal, where "hey, what about this other optimization" is not an actual problem.

It depends to what extent the "what about this other optimization" suggestions interact with the original proposal. At least at first blush, that interaction is non-trivial. And having potentially better optimizations available is an "actual problem", since the work required to implement, debug, and migrate to a new ABI will dwarf the (not inconsiderable) work it took to validate your 5-10% number. That says to me it is worth at least taking some of the alternatives seriously, and updating the proposal in light of them.

@mundaym
Member
mundaym commented Jan 13, 2017

@bcmills

if the new convention isn't a superset of the platform ABI, we may lose the opportunity to simplify assembly functions. (The migration is likely a lot of work, and I doubt we'll do it twice.)

As an aside: the s390x ELF ABI requires a 160-byte (!) caller-allocated save area, so I would rather not adhere strictly to it in Go due to the effect it would have on goroutine stack growth. Probably fine to keep the register allocations the same though.

@randall77
Contributor

For callee-saved pointer registers, we need callers to do some work. In particular, they need to make sure to zero any such registers which are dead before a call. Otherwise, callees will dutifully pass the values in those registers on to the GC which will then preserve the (maybe otherwise collectible) objects they point to. That's another cost that must be borne by callee-saved pointer registers.

Maybe that cost goes away with Ian's strategy, I'm not sure.

@randall77
Contributor

I agree that it's important to get this right so we only have to do it once. I don't want to force people to rewrite their assembly twice. To the extent that we can do meaningful experiments to gauge the effect of ABI-modifying optimizations (e.g. callee-saved registers), we should do that.

@minux
Member
minux commented Jan 14, 2017
@dr2chase
Contributor

The goal of the proposal is that assembly-language meddling be minimized
(readable proposal here https://gist.github.com/dr2chase/5a1107998024c76de22e122ed836562d ).
Assumption is that there will be a new tag set for adapted assembly language, the assembler will look for that tag, and if it does not see it then the code will automatically be made to work, provided that it lacks indirect CALLs (and perhaps it will error out for unadapted code containing indirect CALL).

For call-free assembly language, the prologue/epilogue is modified to call adapters emitted by the compiler to handle loads/stores from the argument/result portion of the stack.

For calls to named functions, one option is to surround the call with calls to compiler-generated wrappers. Doing this for every function leads to a lot of mostly-unused extra boilerplate to be discarded by the linker; a slightly more complex example requires some back-and-forth with the compiler to ensure that these function-type-specific adapters are generated on demand.

And indirect calls in Go asm are rare, certainly within Google where we can (and did) search for them.

So I'd like to suggest that separating these args-in-registers and callee-saves is not necessarily "doing it twice", though it would be for last-cycle performance-critical assembly.

Adding callee-saves seems to guarantee a lot more meddling in existing assembly language, unless we come up with some clever plan. If the clever plan were a tool, the tool must notice writes to callee-save registers, modify stack frame to add spill area for those registers, include FUNCDATA to indicate presence of spill. How does the tool know where in the assembly frame to put the spills?

Alternately, we could choose the callee-saves registers in this turn of the crank and strongly suggest that anyone rewriting their assembly language now avoid using them, or preemptively add the save/restore code, so that when we do start using them for callee-saves in the future there is not a roadblocking need to repair code in a hurry.

Another earlier proposal by Keith appears to get some of the benefits of callee-save for direct calls with less impact on assembly language; there, the compiler records which registers a function does not clobber, records this in the export data, and for each direct call instruction its unclobbered registers can be left unspilled. There's still interaction with GC, and this does nothing for indirect calls, but it has reduced source code impact.

For helping Cgo, might we be better off conforming exactly to the platform ABI for such calls, and only for those calls?

@minux
Member
minux commented Jan 17, 2017
@aclements
Member

My main concern for the current proposal is still argument display in tracebacks.

@minux, can you expand on the problem you see with tracebacks? As I understand it, in the current proposal, any "controlled" traceback (where the goroutine is at a safe-point) works exactly like it does right now because the arguments will all be spilled to exactly the same places on the stack (unless, perhaps, their last use is before the first call?). Tracebacks caused by signals (nil pointer, divide-by-zero) may be a problem for printing the arguments of the inner-most frame, though all of the other frames will be fine. Is there some other problem you see?

Maybe we need a more elaborate way to communicate argument locations to the runtime for traceback. I think this is going to be more of a problem for inlining than for register arguments; we currently have no hope of recovering arguments for inlined frames (so, at least for now, we're not; perfect is the enemy of the good and all that.)

@minux
Member
minux commented Jan 17, 2017
@aclements
Member

This unnecessary spilling will have performance costs (and not to mention the stack usage is the same as before).

Backing up a step, are you concerned about tracebacks with the current proposal (which does potentially unnecessary spilling, partly to support finding arguments for things like tracebacks), or with an alternate proposal that does less spilling?

@bcmills
Member
bcmills commented Jan 18, 2017

Adding callee-saves seems to guarantee a lot more meddling in existing assembly language, unless we come up with some clever plan. If the clever plan were a tool, the tool must notice writes to callee-save registers, modify stack frame to add spill area for those registers, include FUNCDATA to indicate presence of spill. How does the tool know where in the assembly frame to put the spills?

We don't need to notice writes to callee-save registers: we can spill them unconditionally. (If that's a performance problem, the trivial workaround is to rewrite the assembly function to use the new calling convention.)

We would need to do the spills at the point where we change calling conventions: when we go to call across conventions, we spill all the callee-saved registers, then copy all of the arguments to the stack, call the assembly function, copy return-parameters to registers, restore callee-saved registers, and we're done.

IIUC, that would only require FUNCDATA to indicate the spills at the call sites, for which we would generate the same FUNCDATA as for any other spill (indicating that those registers do not contain GC roots after the spill occurs).

Alternately, we could choose the callee-saves registers in this turn of the crank and strongly suggest that anyone rewriting their assembly language now avoid using them, or preemptively add the save/restore code, so that when we do start using them for callee-saves in the future there is not a roadblocking need to repair code in a hurry.

I like that idea. We're already trending a bit in that direction (see https://go-review.googlesource.com/#/c/35068/).

For helping Cgo, might we be better off conforming exactly to the platform ABI for such calls, and only for those calls?

Which calls do you mean, exactly? There are (unfortunately) several routes by which Go functions can end up being called from C ABI functions: at least explicit cgo calls and signal handlers. At any rate, I do think that (at least for platform ABIs that don't have crazy stack bloat) we should try to conform to the platform ABI.

@minux
Member
minux commented Jan 18, 2017
@minux
Member
minux commented Jan 18, 2017
@bcmills
Member
bcmills commented Jan 18, 2017 edited

Thinking about tracebacks. At any given point in the program, we know whether an argument is still valid or not; we shouldn't show invalid arguments in tracebacks, but we may have some arguments unavailable.

The most interesting function arguments are in two categories. One is "things that could have been GC'd but weren't yet", which favors passing in registers without spilling to preserve tracebacks. The other is "values which were passed further down the stack". The latter could be traced and displayed via FUNCDATA annotations using a more general version of @ianlancetaylor's suggested approach: if we know where the contents of the registers (and stack slots) came from, we can reconstruct the arguments that are still around because they were passed as arguments (or stored to local variables) further down the call chain.

Does DWARF already support that kind of value-propagation annotation?

@dr2chase
Contributor

Can you clarify "things that could have been GC'd but weren't yet" ?

Do you mean pointers that are functionally dead, and if a GC has occurred might now point to reused memory, or something else? That's a bad idea unless there is some indication (to debugger and/or traceback) that the pointer is stale and that examining its referent might lead to confusion.

@bcmills
Member
bcmills commented Jan 19, 2017

Do you mean pointers that are functionally dead, and if a GC has occurred might now point to reused memory, or something else?

I mean pointers for which:

  • the last remaining reference is a function argument and
  • the execution of the corresponding function call has passed the last use of that argument.

If function arguments are passed on the stack (or spilled for the lifetime of the function), then collecting those pointers requires extra work. On the other hand, if function arguments are passed in registers (and only spilled if they have live references after the spill), then collecting those pointers is automatic — and preserving them for display in tracebacks requires extra work.

@minux
Member
minux commented Jan 19, 2017
@randall77
Contributor

We're already in a state where what is displayed in the traceback is context-dependent.

func f(x *int) {
   x = new(int)
   g()
   .. use x ..
}

If there is a panic during the call to g, the value displayed in the stack traceback for x's argument slot will show the newly allocated pointer, not the original value of x at the start of the function.

The reason this happens is that the newly allocated pointer is spilled to the argument slot for x. We used to do this to ensure that the old value x pointed to could be garbage collected during the call to g. Now that (as of go 1.8) we use our precise liveness information for arguments (and have runtime.KeepAlive available to override if necessary), we could revisit this decision. It would cost some stack space, but probably not much.

In any case, all that is pretty tangential to the proposal at hand. Not making tracebacks worse is certainly worth discussing here. Making them better should be a separate proposal.

@minux
Member
minux commented Jan 19, 2017
@randall77
Contributor

That the proposals conflict is ok. And we can discuss those conflicts here. But without a concrete proposal about how you'd like to make tracebacks better it's hard to see exactly what those conflicts would be.

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