Fix Issue 21097 - prevent stack allocation when destroying large aggregates #3467

JohanEngelen · 2021-05-10T21:07:21Z

This fixes destroy and core.internal.lifetime.emplaceInitializer for large aggregates by preventing stack allocation for LDC and DMD.
For DMD, it also fixes a codegen issue for large static array of structs where destroy/emplaceInitializer would generate a string of mov instructions proportional to the size of the array.

Intentionally did not add unittests for this issue here, but instead added a runnable test case to the testsuite, to prevent a very large unittest binary. Test is added in separate DMD PR: dlang/dmd#12509.
Tests have been added to this PR.

dlang-bot · 2021-05-10T21:07:25Z

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21097	regression	[REG2.083] Stack exhaustion upon large struct .destroy

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "stable + druntime#3467"

src/core/internal/lifetime.d

JohanEngelen · 2021-05-11T21:45:06Z

Adding @nogc to emplaceInitializer ran into this Phobos unittest error:

std/internal/test/dummyrange.d(40): Error: array literal in `@nogc` function `core.internal.lifetime.emplaceInitializerImpl!(S).emplaceInitializerImpl!(S).emplaceInitializerImpl` may cause a GC allocation
../druntime/import/core/internal/lifetime.d(101): Error: template instance `core.internal.lifetime.emplaceInitializerImpl!(S).emplaceInitializerImpl!(S)` error instantiating
../druntime/import/core/internal/lifetime.d(56):        instantiated from here: `emplaceInitializer!(S)`
std/array.d(132):        instantiated from here: `emplaceRef!(DummyRange!(ReturnBy.Reference, Length.Yes, RangeType.Random, uint[]), DummyRange!(ReturnBy.Reference, Length.Yes, RangeType.Random, uint[]), DummyRange!(ReturnBy.Reference, Length.Yes, RangeType.Random, uint[]))`
std/range/package.d(9581):        instantiated from here: `array!(Result!())`
std/array.d(135): Error: forward reference to inferred return type of function call `function () @trusted => cast(E[])result()`
std/range/package.d(9581): Error: `pure` function `std.range.__unittest_L9550_C20` cannot call impure function `std.array.array!(Result!()).array`
std/range/package.d(9581): Error: template instance `std.array.array!(Result!())` error instantiating
std/internal/test/dummyrange.d(40): Error: array literal in `@nogc` function `core.internal.lifetime.emplaceInitializerImpl!(S).emplaceInitializerImpl!(S).emplaceInitializerImpl` may cause a GC allocation
../druntime/import/core/internal/lifetime.d(101): Error: template instance `core.internal.lifetime.emplaceInitializerImpl!(S).emplaceInitializerImpl!(S)` error instantiating
../druntime/import/core/internal/lifetime.d(56):        instantiated from here: `emplaceInitializer!(S)`
std/array.d(132):        instantiated from here: `emplaceRef!(DummyRange!(ReturnBy.Value, Length.Yes, RangeType.Random, uint[]), DummyRange!(ReturnBy.Value, Length.Yes, RangeType.Random, uint[]), DummyRange!(ReturnBy.Value, Length.Yes, RangeType.Random, uint[]))`
std/range/package.d(9581):        instantiated from here: `array!(Result!())`
std/array.d(135): Error: forward reference to inferred return type of function call `function () @trusted => cast(E[])result()`
std/range/package.d(9581): Error: `pure` function `std.range.__unittest_L9550_C20` cannot call impure function `std.array.array!(Result!()).array`
std/range/package.d(9581): Error: template instance `std.array.array!(Result!())` error instantiating

The problematic line is chunk = T.init; (if commented-out, the Phobus unittest compile error is gone).
I don't know how to fix it. The original function also did not have the @nogc attribute, so I've removed for the improved function in this PR too.

src/core/internal/lifetime.d

MoonlightSentinel · 2021-05-13T21:16:08Z

src/core/internal/lifetime.d

-            alias Unshared = T;
+    // Avoid stack allocation by hacking to get to the init symbol.
+    pragma(mangle, "_D" ~ T.mangleof[1..$] ~ "6__initZ")
+    __gshared extern immutable typeof(T.init) initializer;


This is only used in the else block

Indeed.
I guess your comment is to static-if this away, but I would have to replicate the full static-if chain, and it does not harm to have it here.

You could move it into the else block?

it needs to be a global variable (and what i remember is that making it static did not work)

It's just a fwd decl, so no harm done if unused.

That's weird but should not block this PR.

It's just a fwd decl, so no harm done if unused.

Not for LDC:

FAILED: runtime/objects-unittest-debug/std/experimental/allocator/building_blocks/affix_allocator.o cd /home/runner/work/ldc/ldc/runtime/phobos && /home/runner/work/ldc/ldc/bin/ldc2 -c --output-o -conf= -w -de -dip1000 -preview=dtorfields -g -link-defaultlib-debug -d-debug -d-version=StdUnittest -unittest -linkonce-templates -relocation-model=pic -I/home/runner/work/ldc/ldc/runtime/druntime/src -I/home/runner/work/ldc/ldc/runtime/phobos -of=/home/runner/work/ldc/ldc/runtime/objects-unittest-debug/std/experimental/allocator/building_blocks/affix_allocator.o std/experimental/allocator/building_blocks/affix_allocator.d /home/runner/work/ldc/ldc/runtime/druntime/src/core/internal/lifetime.d(99): Error: Global variable type does not match previous declaration with same mangled name: `_D6__initZ` /home/runner/work/ldc/ldc/runtime/druntime/src/core/internal/lifetime.d(99): Previous IR type: i64, const, non-thread-local /home/runner/work/ldc/ldc/runtime/druntime/src/core/internal/lifetime.d(99): New IR type: i32, const, non-thread-local

JohanEngelen · 2021-05-15T10:56:16Z

ping

MoonlightSentinel · 2021-05-15T14:52:31Z

Your test is not executed by make, you'll need to add the directory to this list.

JohanEngelen · 2021-05-15T18:26:52Z

Thanks @MoonlightSentinel . Added it to the makefile (really, this test framework we have here is so horrible...), and squashed all fixup commits.

JohanEngelen · 2021-05-15T18:29:09Z

~~And rebased to target stable branch.~~
Rebased back to master, because stable branch testing is broken.

JohanEngelen · 2021-05-15T21:51:21Z

OK to rebase onto stable and merge?

MoonlightSentinel · 2021-05-16T14:56:36Z

Rebased back to master, because stable branch testing is broken.

What was the failure?

OK to rebase onto stable and merge?

Yes as this fixes a regression.

MoonlightSentinel · 2021-05-16T14:57:10Z

src/core/internal/lifetime.d

+            import core.stdc.string : memset;
+            memset(cast(void*) &chunk, 0, T.sizeof);
+        }
+        else static if (T.sizeof <= 16 && !hasElaborateAssign!T && __traits(compiles, (){ T chunk; chunk = T.init; }))


Out of curiousity, why 16 instead of a larger value?

this value is indeed pretty arbitrary. It's to limit the stack space used.

…alizer for large aggregates by preventing stack allocation

JohanEngelen · 2021-05-18T21:10:17Z

Rebased onto stable, PR targets stable now.

thewilsonator · 2021-05-18T23:41:32Z

Build fails because CI checks out phobos master instead of phobos stable. I'm going to tentatively force merge this.

JohanEngelen requested review from MartinNowak and schveiguy as code owners May 10, 2021 21:07

dlang-bot added the Bug Fix Include reference to corresponding bugzilla issue label May 10, 2021

JohanEngelen mentioned this pull request May 10, 2021

Add tests for the druntime fix for destroy on large aggregates (issue 21097) dlang/dmd#12509

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JohanEngelen force-pushed the largeaggregates branch from 5e12dec to c8fd138 Compare May 10, 2021 21:15

JohanEngelen mentioned this pull request May 10, 2021

Don't stack allocate in core.lifetime.emplaceInitializer ldc-developers/ldc#3531

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JohanEngelen commented May 10, 2021

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JohanEngelen force-pushed the largeaggregates branch 3 times, most recently from 885cf5a to 9f6dc63 Compare May 10, 2021 21:55

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JohanEngelen force-pushed the largeaggregates branch from 9f6dc63 to efd561e Compare May 11, 2021 12:44

JohanEngelen requested review from andralex and wilzbach as code owners May 11, 2021 12:44

nordlow reviewed May 13, 2021

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12345swordy approved these changes May 13, 2021

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JohanEngelen force-pushed the largeaggregates branch from fa60672 to 32e0aaa Compare May 15, 2021 18:24

JohanEngelen force-pushed the largeaggregates branch from 32e0aaa to 21d863c Compare May 15, 2021 18:28

JohanEngelen changed the base branch from master to stable May 15, 2021 18:28

JohanEngelen changed the base branch from stable to master May 15, 2021 18:37

JohanEngelen force-pushed the largeaggregates branch from 21d863c to 1320e41 Compare May 15, 2021 18:38

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Fix Issue 21097 - Fix destroy and core.internal.lifetime.emplaceIniti…

5981d49

…alizer for large aggregates by preventing stack allocation

JohanEngelen force-pushed the largeaggregates branch from 1320e41 to 5981d49 Compare May 18, 2021 21:09

JohanEngelen changed the base branch from master to stable May 18, 2021 21:09

MoonlightSentinel added the auto-merge label May 18, 2021

thewilsonator merged commit 7068155 into dlang:stable May 18, 2021

JohanEngelen deleted the largeaggregates branch May 19, 2021 07:40

kinke mentioned this pull request May 29, 2021

[stable] core.internal.lifetime: Fix recent regressions for emplaceInitializer() #3484

Merged

kinke mentioned this pull request Jun 27, 2021

Hacky init symbol declaration can cause undefined symbols with -linkonce-templates ldc-developers/ldc#3773

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Fix Issue 21097 - prevent stack allocation when destroying large aggregates #3467

Fix Issue 21097 - prevent stack allocation when destroying large aggregates #3467

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Fix Issue 21097 - prevent stack allocation when destroying large aggregates #3467

Fix Issue 21097 - prevent stack allocation when destroying large aggregates #3467

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Testing this PR locally

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