Can't compile SingleCoreAllocator.swift with "-no-allocations"

mkbrwr · March 16, 2025, 4:42pm

I'm trying to build my project with as little C as possible, but one the requirements of allocating Embedded Swift is "free" and "posix_memalign" functions. Here's the allocator code I'm trying to use:

gist.github.com

https://gist.github.com/phausler/3af5dbd45950c5a7e869c2c0bec803f7

SingleCoreAllocator.swift

// This hasnt been robustly tested for all alignments but is a simple malloc heap implementation that 
// is roughly based upon the K&R reference examples; it has a few caveats - namely it is not suitable
// for systems with more than one core and provides no thread local affinities nor does it have any 
// locking to prevent concurrent access. However it is roughly suitable for implementing enough to
// heap allocate reference types in Swift.

struct SingleCoreAllocator {
  static let shared = SingleCoreAllocator()

  struct State {

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Embedded Vision Document says: "Interestingly, it’s possible to write that implementation in “non-allocating” Baremetal Swift."

But when compiling SingleCoreAllocator.swift with -no-allocations

"$SWIFTC" -target armv7em-none-none-eabi -wmo -enable-experimental-feature Embedded -no-allocations -c SingleCoreAllocator.swift -o $OUTPUT_DIR/SingleCoreAllocator.o

I get the followin errors

<unknown>:0: error: cannot use allocating type '_ContiguousArrayStorage<UInt>' in -no-allocations mode
in function specialized _copyCollectionToContiguousArray<A>(_:)
<unknown>:0: error: cannot use allocating type '__SwiftNativeNSArrayWithContiguousStorage' in -no-allocations mode
in function __SwiftNativeNSArrayWithContiguousStorage.__allocating_init()

To debug the issue I compile SingleCoreAllocator.swift without -no-allocations and I can see the symbols mentioned in error in output of nm:

...
000027a8 T $ss28__ContiguousArrayStorageBaseC012_doNotCallMeD0AByt_tcfC
000027a8 T $ss28__ContiguousArrayStorageBaseC16canStoreElements13ofDynamicTypeSbypXp_tF
000027a4 T $ss28__ContiguousArrayStorageBaseCABycfC
000027a8 T $ss28__ContiguousArrayStorageBaseCfD
...
000027b8 T $ss41__SwiftNativeNSArrayWithContiguousStorageCABycfC
000027d0 T $ss41__SwiftNativeNSArrayWithContiguousStorageCfD
00000000 D $ss41__SwiftNativeNSArrayWithContiguousStorageCN

But these symbols don't exist in output of objdump -D. And for addresses there's something like this

...
 400:	000027a4 	andeq	r2, r0, r4, lsr #15
 404:	00000001 	andeq	r0, r0, r1
 408:	000027a8 	andeq	r2, r0, r8, lsr #15
 40c:	00000001 	andeq	r0, r0, r1
 410:	000027a8 	andeq	r2, r0, r8, lsr #15
 414:	00000001 	andeq	r0, r0, r1
 418:	000027a8 	andeq	r2, r0, r8, lsr #15
... 
 420:	000027b8 			@ <UNDEFINED> instruction: 0x000027b8
 424:	00000001 	andeq	r0, r0, r1
 428:	000027d0 	ldrdeq	r2, [r0], -r0
 42c:	00000001 	andeq	r0, r0, r1
...

To me it looks like this two functions should have been stripped from the .o file, but weren't.

I saw in this answer that -no-allocations is aspirational. Is it not implemented yet ? Or am I doing something wrong ? Thanks!

kubamracek · March 17, 2025, 9:50pm

I think it's a bug in the compiler. Can you file the repro steps as a Github issue?

I agree with your analysis. Also I can reproduce the problem. Interestingly, if I build for a 64-bit target (-target aarch64-none-none-elf), the problem disappears for some reason.