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Set by AlexDaniel on 12 June 2018.
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nine Actually a rather interesting question is also, how we get back to JITed code after an invoke. 15:09
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timotimo something something trampoline something far jump? 16:24
nine Ok, the JIT bytecode prologue sets tc->jit_return_address to the address of a stack variable. MVM_frame_invoke calls MVM_jit_code_trampoline which sets *tc->jit_return_address = code->exit_label; That exit_label is the ->exit: in 16:35
Called a global label in dynasm lingo 16:36
I just don't get it. I can't really find how we invoke something in the first place and not how we get back. I can't find anything about what happens after the MVM_frame_invoke which just sets up the interpreter's state so it would start invoking the target. But how do we get from the JIT code to the interpreter? 16:54
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nine The code gen for invocation does generate a reentry_label, but I can't find a reference to that anywhere 16:56
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nine The stack variable I mentioned is rsp-0x8. The stack grows top-down, i.e. rsp-0x8 is actuall the next stack slot that _will_ be used. It's where the `call` to MVM_frame_invoke_code (or friend) will place the current instruction pointer, i.e. the return address. 17:29
This means that the call to MVM_jit_code_trampoline will actually overwrite the return address of the MVM_frame_invoke_code call itself. This is how we end up in the JIT code's epilogue, which returns to the interpreter. 17:30
The interpreter will then go on to perform the actual invocation (which may itself just be an sp_jit_enter op) 17:31
Before storing the JIT code's epilogue's address into *tc->jit_return_address (i.e. the ret address), MVM_jit_code_trampoline will move the current value to tc->cur_frame->jit_entry_label which is later used by sp_jit_enter. I.e. jit_entry_label is a moving target that gets updated whenever we interrupt the JIT code's control flow for an invocation 17:37
So it's actuall a jit_(re-)entry_label 17:38
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nine Putting all of that together I arrive at just a few lines of assembly that I generate after the call to MVM_nativecall_invoke which check if tc->cur_frame->spesh_cand is NULL and if yes jump to ->exit, so we get back to the interpreter. 18:26
Alas, I seem to arrive in the wrong place in the bytecode still
Of course! I also need to call MVM_jit_trampoline(tc) before calling MVM_nativecall_invoke, to record the current position. And with that it works!! 18:39
All tests successful. with MVM_JIT_EXPR_DISABLE=1 MVM_SPESH_BLOCKING=1 MVM_SPESH_NODELAY=1. Now what's still missing is the same for the expr jit (the call to MVM_jit_trampoline and the deopt check) and then all of that for nativeinvoke_o 18:45
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nine Actually the tests succeed even with the expr JIT turned on. It bails before reaching the nativecallinvoke anywy 19:01
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nwc10 nine: is that only "good luck" on the part of the tests? - ie is it possible to write other code that would not bail, and hence crash in the way you assumed that it would? 19:40
(I think that this is the first useful question I can ask all evening. The rest of the detail is beyond me)
Geth MoarVM: c799de4218 | (Stefan Seifert)++ | 5 files
Fix relocatability of modules using NativeCall

There are cases where we actually don't want the library's path to get serialized into the bytecode file, e.g. when building a module into a Staging repository for packaging. The previous attempt to solve this failed because we cannot keep the NativeCall site's state in sync with HLL function's state due to repossession issues. The latter were the whole reason for serializing ... (7 more lines)
MoarVM: d7b6855d35 | (Stefan Seifert)++ | 13 files
Fix segfault caused by deopt all in NativeCall callback

A callback from a native function starts a new runloop. When a deopt all happens in that runloop, the outer runloop did not jump back to the unoptimized bytecode. An sp_getspeshslot op would then cause a segfault because tc->cur_frame->effective_spesh_slots was NULLed by the deopt.
... (6 more lines)
nine nwc10: I'm not sure. Code that's simple enough to make it through the expr JIT is more likely to get fully JIT compiled and go through nativeinvoke_o instead of the old generic nativecallinvoke that I worked on. 19:45
I pushed the fixes because that nested runloop segfault thing has been there forever, so even just fixing it in some cases is an improvement already 19:46
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japhb nine: I'm seeing new warnings I didn't see before. My compiler is warning that your new backup variables might get clobbered by longjmp or vfork. I'm assuming this is not surprising to you, but just in case it is .... 20:23
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nine japhb: yeah, I'm aware. Hope someone else knows what to do about it though... 21:02
dogbert17 nine: I get plenty of test failures when running with a reduced nursery 21:06
more or less all files in t/spec/S01-perl-5-integration fails with a SEGV when the nursery size is reduced. Could possibly be something wrong on my system. 21:08
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dogbert17 #0 0x00007fb5fe296575 in MVM_serialization_demand_object (tc=0x1edbb10, sc=0x7fb5fed00640, idx=47) at src/6model/serialization.c:2848 21:14
2848 MVM_reentrantmutex_unlock(tc, (MVMReentrantMutex *)sc->body->mutex);
nine dogbert17: how small? 21:22
dogbert17: seems to run fine on 512 bytes here 21:23
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dogbert17 nine: it works after I wiped my rakudo install, sorry for the noise 22:15
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