[MIR] non-zeroing drop #33622

arielb1 · May 13, 2016

This enables non-zeroing drop through stack flags for MIR.

Fixes #30380.
Fixes #5016.

rust-highfive · May 13, 2016

r? @nrc

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arielb1 · May 13, 2016

r? @nikomatsakis

solson · May 13, 2016

After this MIR pass, will all MIR drops be unconditional? (That is, any conditional drops will be guarded by explicit conditional branching in the MIR.)

arielb1 · May 13, 2016

@solson

Right.

arielb1 · May 14, 2016

Passes make check. I should do something about @pnkfelix's warning-ridden code, but otherwise it's ready.

durka · May 14, 2016

cc #5016

This is awesome! Do we have benchmarks? Will functions like std::ptr::read_and_drop go away or can they be refitted to set the drop flag instead of filling?

arielb1 · May 14, 2016

Performance numbers:

stage1 MIR librustc (old):

21:26:53 O: time: 17.426; rss: 1510MB   translation
21:26:53 O: time: 0.003; rss: 1510MB    assert dep graph
21:26:53 O: time: 0.000; rss: 1510MB    serialize dep graph
21:27:00 O:   time: 6.139; rss: 922MB   llvm function passes [0]
21:29:08 O:   time: 128.802; rss: 1004MB        llvm module passes [0]
21:29:45 O:   time: 35.232; rss: 1009MB codegen passes [0]
21:29:46 O:   time: 0.010; rss: 228MB   codegen passes [0]
21:29:46 O: time: 172.367; rss: 212MB   LLVM passes

stage2 MIR librustc:

21:46:37 O: time: 25.428; rss: 1319MB   translation
21:46:37 O: time: 0.003; rss: 1319MB    assert dep graph
21:46:37 O: time: 0.000; rss: 1319MB    serialize dep graph
21:46:43 O:   time: 5.261; rss: 1131MB  llvm function passes [0]
21:48:16 O:   time: 92.960; rss: 1133MB llvm module passes [0]
21:48:46 O:   time: 29.626; rss: 1137MB codegen passes [0]
21:48:47 O:   time: 0.005; rss: 1070MB  codegen passes [0]
21:48:47 O: time: 130.045; rss: 1051MB  LLVM passes

arielb1 · May 14, 2016

stage 3 MIR librustc (there was some perf issue with stage2 apparently).

22:19:57 O: time: 17.890; rss: 1263MB   translation
22:19:57 O: time: 0.003; rss: 1263MB    assert dep graph
22:19:57 O: time: 0.000; rss: 1263MB    serialize dep graph
22:20:02 O:   time: 4.530; rss: 782MB   llvm function passes [0]
22:21:25 O:   time: 82.637; rss: 847MB  llvm module passes [0]
22:21:49 O:   time: 22.951; rss: 862MB  codegen passes [0]
22:21:49 O:   time: 0.008; rss: 221MB   codegen passes [0]
22:21:49 O: time: 111.719; rss: 221MB   LLVM passes

Non -Z orbit:

time: 18.584; rss: 1219MB       translation
time: 0.003; rss: 1219MB        assert dep graph
time: 0.000; rss: 1219MB        serialize dep graph
  time: 3.910; rss: 711MB       llvm function passes [0]
  time: 89.009; rss: 814MB      llvm module passes [0]
  time: 23.172; rss: 811MB      codegen passes [0]
  time: 0.008; rss: 221MB       codegen passes [0]
time: 117.952; rss: 220MB       LLVM passes

bors · May 14, 2016

☔️ The latest upstream changes (presumably #33632) made this pull request unmergeable. Please resolve the merge conflicts.

arielb1 · May 14, 2016

perf after rebase:

16:14:04 O: time: 17.806; rss: 1254MB   translation
16:14:04 O: time: 0.003; rss: 1254MB    assert dep graph
16:14:04 O: time: 0.000; rss: 1254MB    serialize dep graph
16:14:08 O:   time: 4.457; rss: 783MB   llvm function passes [0]
16:15:32 O:   time: 83.976; rss: 858MB  llvm module passes [0]
16:15:55 O:   time: 22.197; rss: 854MB  codegen passes [0]
16:15:56 O:   time: 0.007; rss: 229MB   codegen passes [0]
16:15:56 O: time: 112.204; rss: 213MB   LLVM passes
16:15:56 O:   time: 0.340; rss: 213MB   running linker
16:15:57 O: time: 1.180; rss: 213MB     linking

I get the feeling that SimpifyCfg is not doing its work.

dotdash · May 15, 2016

@arielb1 What makes you think that? IOW, what do you expect it to do that doesn't happen?

arielb1 · May 15, 2016

Just that there are no perf improvements over its merge. Maybe LLVM just does this by itself.

pnkfelix · May 16, 2016

@nikomatsakis @arielb1 I was under the impression that I would be given a chance to put up a properly revised PR of the data flow fixes (cleaning up the warnings and the commit history).

I obviously can't stop people from merging this PR as is, but it would be a little irritating to me if that happened

pnkfelix · May 16, 2016

(In any case, I'll try to soon put up a separate PR with just the data flow fixes, that will include the relevant changes contributed by @arielb1 from this PR, and link to this PR when i do. hopefully that will happen in the next eight hours. I recommend waiting until that happens before initiating formal code review, at least if you plan to go commit by commit)

arielb1 · May 16, 2016

@pnkfelix

I just wanted to open my code for review. Will rebase on top of yours.

dotdash · May 16, 2016

@arielb1 Yeah, LLVM does this just fine, the point of having it as a MIR pass is that we a) generate a little less bad code for the unoptimized case, and b) can possibly do more optimizations that are local to a single basic block instead of having to look across block boundaries.

dotdash · May 16, 2016

dotdash reviewed May 16, 2016

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dotdash · May 16, 2016

src/librustc/mir/repr.rs

+    /// Drop the Lvalue and write an operand over it
+    DropAndReplace {
+        location: Lvalue<'tcx>,
+        value: Operand<'tcx>,


For consistency, using value and replacement might be a better idea here. Having value mean different things for Drop and DropAndReplace seems confusing.

I'll just use location for both.

arielb1 · May 16, 2016

@dotdash

c) humans can actually read the generated MIR.

That's 3 good reasons.

I might just add a span for temporaries. For debugging and readability, you know.

pnkfelix · May 16, 2016

@arielb1

okay, thanks. (Sorry for any cranky old man attitude in my response; it was 5 in the morning and I (unlike niko) don't operate well at that hour at this point in my life.)

I have posted the dataflow fixes PR at #33667 ; I tried to extract the relevant portions of the things you fixes in the dataflow into that PR, while leaving out the stuff that was solely for your non-zeroing drop work. So while it probably won't be a 100% trivial rebase, it shouldn't be too onerous, I hope.

arielb1 · May 17, 2016

rebased on top of #33667

nikomatsakis · May 18, 2016

nikomatsakis reviewed May 18, 2016

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nikomatsakis · May 18, 2016

src/librustc/mir/repr.rs

+            DropAndReplace { target, unwind: Some(unwind), .. } |
+            Drop { target, unwind: Some(unwind), .. } =>
+                vec![target, unwind].into_cow(),
+            DropAndReplace { ref target, .. } | Drop { ref target, .. } =>


Nit: can you write unwind: None here to make this more robust against accidental re-ordering? (and more obvious, I missed the lines above for a while)

nikomatsakis · May 18, 2016

nikomatsakis reviewed May 18, 2016

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nikomatsakis · May 18, 2016

src/librustc/mir/repr.rs

            Drop { ref mut target, unwind: Some(ref mut unwind), .. } => vec![target, unwind],
+            DropAndReplace { ref mut target, .. } |
            Drop { ref mut target, .. } => vec![target]


Nit: also here

nikomatsakis · May 18, 2016

nikomatsakis reviewed May 18, 2016

View changes

nikomatsakis · May 18, 2016

src/librustc_borrowck/borrowck/mir/dataflow/mod.rs

+            repr::TerminatorKind::Drop { ref target, location: _, unwind: Some(ref unwind) } |
+            repr::TerminatorKind::DropAndReplace {
+                ref target, value: _, location: _, unwind: Some(ref unwind)
+            } => {
                self.propagate_bits_into_entry_set_for(in_out, changed, target);
                self.propagate_bits_into_entry_set_for(in_out, changed, unwind);


I guess I'll see how this works out; but I'm a bit surprised that we don't wind up with some careful logic around the unwind path. In particular it might be that location is uninitialized before (and only initialized on the success path), no?

location is initialized in both paths - that's the point of #30380.

arielb1 · May 18, 2016

fixed nit

bors · Jun 4, 2016

⌛️ Testing commit 4248269 with merge 788af3d...

bors · Jun 4, 2016

💔 Test failed - auto-win-msvc-64-opt-mir

arielb1 · Jun 4, 2016

Looks like an LLVM bug. See gist: https://gist.github.com/arielb1/8717eeaf5ac80fc4125e118729b5cb0c

cc @eddyb @dotdash

arielb1 · Jun 4, 2016

Blocked on LLVM bug https://llvm.org/bugs/show_bug.cgi?id=28005

eddyb · Jun 4, 2016

@arielb1 I'm not sure we can afford to wait for that to get fixed. Any chance we can work around it?

nagisa · Jun 4, 2016

I’m having a hard time reproducing the issue in the gist it with the Arch’s 3.8 build of LLVM, which suggests that perhaps our own fork of LLVM went wrong somewhere?

alexcrichton · Jun 5, 2016

Claimed to be fixed in llvm-mirror/llvm@8c4b617, I've backported it to our branch (rust-lang/llvm@80ad955), so @arielb1 wanna try updating and see if that fixes it?

arielb1 · Jun 5, 2016

@alexcrichton

I manually-applied the fix and am testing whether it works. That error is fixed, but there is a different error from a different pass (now I make it to the codegen passes).

alexcrichton · Jun 5, 2016

Ok, just lemme know if anything needs backporting! These MSVC bugs have really bitten me in the past in terms of taking quite a long time to track down, but once you have a minimal IR reproduction it's generally ready to get reported to LLVM.

arielb1 · Jun 5, 2016

rustc: /home/ariel/Rust/rust/src/llvm/include/llvm/ADT/DenseMap.h:1025: llvm::DenseMapIterator<KeyT, ValueT, KeyInfoT, Bucket, IsConst>::value_type* llvm::DenseMapIterator<KeyT, ValueT, KeyInfoT, Bucket, IsConst>::operator->() const [with KeyT = const llvm::MachineBasicBlock*; ValueT = int; KeyInfoT = llvm::DenseMapInfo<const llvm::MachineBasicBlock*>; Bucket = llvm::detail::DenseMapPair<const llvm::MachineBasicBlock*, int>; bool IsConst = false; llvm::DenseMapIterator<KeyT, ValueT, KeyInfoT, Bucket, IsConst>::pointer = llvm::detail::DenseMapPair<const llvm::MachineBasicBlock*, int>*; llvm::DenseMapIterator<KeyT, ValueT, KeyInfoT, Bucket, IsConst>::value_type = llvm::detail::DenseMapPair<const llvm::MachineBasicBlock*, int>]: Assertion `isHandleInSync() && "invalid iterator access!"' failed.

#0  0x00007f31bedbe478 in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:55
#1  0x00007f31bedbf8fa in __GI_abort () at abort.c:89
#2  0x00007f31bedb73a7 in __assert_fail_base (fmt=<optimized out>, 
    assertion=assertion@entry=0x7f31b8ca2fe0 "isHandleInSync() && \"invalid iterator access!\"", 
    file=file@entry=0x7f31b8ca2dc8 "/home/ariel/Rust/rust/src/llvm/include/llvm/ADT/DenseMap.h", line=line@entry=1025, 
    function=function@entry=0x7f31b9374540 <llvm::DenseMapIterator<llvm::MachineBasicBlock const*, int, llvm::DenseMapInfo<llvm::MachineBasicBlock const*>, llvm::detail::DenseMapPair<llvm::MachineBasicBlock const*, int>, false>::operator->() const::__PRETTY_FUNCTION__> "llvm::DenseMapIterator<KeyT, ValueT, KeyInfoT, Bucket, IsConst>::value_type* llvm::DenseMapIterator<KeyT, ValueT, KeyInfoT, Bucket, IsConst>::operator->() const [with KeyT = const llvm::MachineBasicBl"...) at assert.c:92
#3  0x00007f31bedb7452 in __GI___assert_fail (assertion=0x7f31b8ca2fe0 "isHandleInSync() && \"invalid iterator access!\"", 
    file=0x7f31b8ca2dc8 "/home/ariel/Rust/rust/src/llvm/include/llvm/ADT/DenseMap.h", line=1025, 
    function=0x7f31b9374540 <llvm::DenseMapIterator<llvm::MachineBasicBlock const*, int, llvm::DenseMapInfo<llvm::MachineBasicBlock const*>, llvm::detail::DenseMapPair<llvm::MachineBasicBlock const*, int>, false>::operator->() const::__PRETTY_FUNCTION__> "llvm::DenseMapIterator<KeyT, ValueT, KeyInfoT, Bucket, IsConst>::value_type* llvm::DenseMapIterator<KeyT, ValueT, KeyInfoT, Bucket, IsConst>::operator->() const [with KeyT = const llvm::MachineBasicBl"...) at assert.c:101
#4  0x00007f31b8251fdf in llvm::BranchFolder::SplitMBBAt(llvm::MachineBasicBlock&, llvm::MachineBasicBlock::bundle_iterator<llvm::MachineInstr, llvm::ilist_iterator<llvm::MachineInstr> >, llvm::BasicBlock const*) ()
   from /home/ariel/Rust/rust/build-debug-assertions/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustc_llvm-fe3cdf61.so
#5  0x00007f31b825274d in llvm::BranchFolder::CreateCommonTailOnlyBlock(llvm::MachineBasicBlock*&, llvm::MachineBasicBlock*, unsigned int, unsigned int&) () from /home/ariel/Rust/rust/build-debug-assertions/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustc_llvm-fe3cdf61.so
#6  0x00007f31b8252c30 in llvm::BranchFolder::TryTailMergeBlocks(llvm::MachineBasicBlock*, llvm::MachineBasicBlock*) ()
   from /home/ariel/Rust/rust/build-debug-assertions/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustc_llvm-fe3cdf61.so
#7  0x00007f31b8253cab in llvm::BranchFolder::TailMergeBlocks(llvm::MachineFunction&) [clone .part.237] ()
   from /home/ariel/Rust/rust/build-debug-assertions/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustc_llvm-fe3cdf61.so
#8  0x00007f31b825481c in llvm::BranchFolder::OptimizeFunction(llvm::MachineFunction&, llvm::TargetInstrInfo const*, llvm::TargetRegisterInfo const*, llvm::MachineModuleInfo*) ()
   from /home/ariel/Rust/rust/build-debug-assertions/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustc_llvm-fe3cdf61.so
#9  0x00007f31b8254bfa in (anonymous namespace)::BranchFolderPass::runOnMachineFunction(llvm::MachineFunction&) ()
   from /home/ariel/Rust/rust/build-debug-assertions/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustc_llvm-fe3cdf61.so
#10 0x00007f31b8bb80f3 in llvm::FPPassManager::runOnFunction(llvm::Function&) ()
   from /home/ariel/Rust/rust/build-debug-assertions/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustc_llvm-fe3cdf61.so
#11 0x00007f31b8bb84fb in llvm::FPPassManager::runOnModule(llvm::Module&) ()
   from /home/ariel/Rust/rust/build-debug-assertions/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustc_llvm-fe3cdf61.so
#12 0x00007f31b8bb7c3d in llvm::legacy::PassManagerImpl::run(llvm::Module&) ()

Calling opt directly works fine, however.

alexcrichton · Jun 5, 2016

Oh that's actually a red herring, it'll pass fine on the Windows bots. If you cross from Linux to Windows, you'll get that error, though. That's fixed in upstream LLVM we just haven't backported the fix yet.

arielb1 · Jun 5, 2016

@bors r=nikomatsakis

bors · Jun 5, 2016

📌 Commit 063f882 has been approved by nikomatsakis

bors · Jun 5, 2016

⌛️ Testing commit 063f882 with merge f97c411...

bors · Jun 5, 2016

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-32cross-opt, auto-linux-64-cargotest, auto-linux-64-cross-armhf, auto-linux-64-cross-armsf, auto-linux-64-cross-freebsd, auto-linux-64-cross-netbsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-cross-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild

rust-highfive assigned nrc May 13, 2016

rust-highfive assigned nikomatsakis and unassigned nrc May 13, 2016

arielb1 changed the title ~~[WIP] [MIR] Non-zeroing drop~~ [MIR] non-zeroing drop May 14, 2016

arielb1 force-pushed the arielb1:elaborate-drops branch from b3e4b52 to 470ffbf May 14, 2016

arielb1 force-pushed the arielb1:elaborate-drops branch from 325c3b7 to 129c4b5 May 17, 2016

bors added a commit that referenced this pull request Jun 4, 2016

Auto merge of #33622 - arielb1:elaborate-drops, r=nikomatsakis
[MIR] non-zeroing drop This enables non-zeroing drop through stack flags for MIR. Fixes #30380. Fixes #5016.

788af3d

break critical edges only when needed
the *only* place where critical edges need to be broken is on Call instructions, so only break them there.

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4106ab2

arielb1 force-pushed the arielb1:elaborate-drops branch from c183dcb to 4106ab2 Jun 5, 2016

Update LLVM
Picks up the fix for PR28005

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063f882

bors added a commit that referenced this pull request Jun 5, 2016

Auto merge of #33622 - arielb1:elaborate-drops, r=nikomatsakis
[MIR] non-zeroing drop This enables non-zeroing drop through stack flags for MIR. Fixes #30380. Fixes #5016.

f97c411

bors merged commit 063f882 into rust-lang:master Jun 5, 2016
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[MIR] Implement overflow checking #33905

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[MIR] Handle call return values that need to be casted properly. #34054

eddyb referenced this pull request Jun 5, 2016
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Switch to MIR-based translation by default. #34096

bstrie referenced this pull request Jun 5, 2016
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C like SIMD intrinsics for Rust #1639

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Don't crash in item_path when an item lacks a name #34561

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Outdated info in nomicon on drop flags #38481

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