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Merged
merged 1 commit into from
Nov 12, 2025
Merged

machine: make sure DMA buffers do not escape unnecessarily #4930

merged 1 commit into from
Nov 12, 2025

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@aykevl
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@aykevl aykevl commented Jun 20, 2025 •
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Writing the pointer of a buffer to memory-mapped I/O will normally cause it to escape, which forces the compiler to heap-allocate the buffer. But we do know how long the value stays alive, so we can tell the compiler to keep it alive exactly until it is not needed anymore - and tell it to not treat the pointer-to-uintptr cast as escaping.

This PR tries to solve the same problem as #4889 but in a different way. In fact, see #4889 (comment) where I first came up with this idea. Basically, with this PR, the allocation inside legacy.ReadRegister (but probably not legacy.WriteRegister!) should be fixed. See: tinygo-org/drivers#766.

It's perhaps not the cleanest solution, but it seems to work in my tests.

@ysoldak can you take a look?

@dgryski @eliasnaur you may also be interested in this PR.

@aykevl aykevl requested a review from ysoldak June 20, 2025 08:10
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github-actions bot commented Jun 20, 2025 •
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Size difference with the dev branch:

Binary size difference 
not the same command!
    tinygo build -size short -o ./build/test.hex -target=feather-rp2040 ./examples/adafruit4650
    go: downloading tinygo.org/x/tinyfont v0.3.0
not the same command!
    tinygo build -size short -o ./build/test.hex -target=pico ./examples/tmc5160/main.go
    go: downloading golang.org/x/exp v0.0.0-20241204233417-43b7b7cde48d
not the same command!
    tinygo build -size short -o ./build/test.hex -target=nano-rp2040 -stack-size 8kb ./examples/net/websocket/dial/
    go: downloading golang.org/x/net v0.33.0
not the same command!
    tinygo build -size short -o ./build/test.hex -target=nano-rp2040 -stack-size 8kb ./examples/net/mqttclient/natiu/
    go: downloading github.com/soypat/natiu-mqtt v0.5.1
not the same command!
    tinygo build -size short -o ./build/test.hex -target=wioterminal -stack-size 8kb ./examples/net/mqttclient/paho/
    go: downloading github.com/eclipse/paho.mqtt.golang v1.2.0
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eliasnaur
eliasnaur reviewed Jun 20, 2025
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src/machine/machine.go
Comment on lines +69 to +94
// Convert the pointer to a uintptr, to be used for memory I/O (DMA for
// example). It also means the pointer is "gone" as far as the compiler is
// concerned, and a GC cycle might deallocate the object. To prevent this from
// happening, also call keepAliveNoEscape at a point after the address isn't
// accessed anymore by the hardware.
// The only exception is if the pointer is accessed later in a volatile way
// (volatile read/write), which also forces the value to stay alive until that
// point.
//
// This function is treated specially by the compiler to mark the 'ptr'
// parameter as not escaping.
//
// TODO: this function should eventually be replaced with the proposed ptrtoaddr
// instruction in LLVM. See:
// https://discourse.llvm.org/t/clarifiying-the-semantics-of-ptrtoint/83987/10
// https://github.com/llvm/llvm-project/pull/139357
func unsafeNoEscape(ptr unsafe.Pointer) uintptr {
return uintptr(ptr)
}

// Make sure the given pointer stays alive until this point. This is similar to
// runtime.KeepAlive, with the difference that it won't let the pointer escape.
// This is typically used together with unsafeNoEscape.
//
// This is a compiler intrinsic.
func keepAliveNoEscape(ptr unsafe.Pointer)
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This is, as you say, hacky and won't help external drivers that don't have access to the unexported unsafeNoEscape and keepAliveNoEscape.

Can't runtime.KeepAlive be made to not move the allocation to the heap in TinyGo? If so, that leaves unsafeNoEscape. I suggest the more drastical solution of accepting #4889 that relaxes uintptr(unsafe.Pointer(&x)) to not escape. My reasoning is that for something like DMA, you need something like runtime.KeepAlive to keep the pointer alive anyway (it's not enough for the pointer to be kept alive at the point of conversion to uintptr).

That way, all code can use the same tricks to avoid allocations.

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@aykevl aykevl Jun 20, 2025 •
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This is, as you say, hacky and won't help external drivers that don't have access to the unexported unsafeNoEscape and keepAliveNoEscape.

I intentionally kept them unexported. Drivers (like the tinygo.org/x/drivers/* packages) won't need it as long as they don't access these hardware registers directly - which is the vast majority. If there are drivers which really need these functions, we can export these functions.

Can't runtime.KeepAlive be made to not move the allocation to the heap in TinyGo?

Yes, but that won't help the machine package due to a circular dependency (machine -> runtime -> machine).

I suggest the more drastical solution of accepting #4889 that relaxes uintptr(unsafe.Pointer(&x)) to not escape.

The only way to implement that at the moment is to let the compiler do something exactly like unsafeNoEscape: insert a call at the point of the cast. I much prefer to keep this explicit and keep the default (escaping) behavior for these casts. In the future there might be ptrtoaddr but not today.

My reasoning is that for something like DMA, you need something like runtime.KeepAlive to keep the pointer alive anyway (it's not enough for the pointer to be kept alive at the point of conversion to uintptr).

Yeah that's the goal of this PR.

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@ysoldak ysoldak Nov 12, 2025 •
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In the future there might be ptrtoaddr but not today.

We may expect this function added in LLVM 22, seems like.
https://llvm.org/docs/ReleaseNotes.html#changes-to-the-llvm-ir

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ysoldak commented Jun 20, 2025

Any idea why it fails on Windows?

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ysoldak commented Jun 20, 2025

Do we have a test that proves this helps?
Asking because with tricks like tinygo-org/drivers#766 I was able to eliminate all I2C escapes, at least in my project.

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eliasnaur commented Jun 20, 2025

Do we have a test that proves this helps? Asking because with tricks like tinygo-org/drivers#766 I was able to eliminate all I2C escapes, at least in my project.

I agree with this sentiment. Why avoid escaping buffers that could surely be scratch space in some larger, already allocated, structure?

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eliasnaur commented Jul 10, 2025

What about drivers written on top of an interface, say drivers.I2C? AFAIK, Unless TinyGo can see through the interface indirection, any buffers passed to I2C.Tx will escape.

@aykevl aykevl force-pushed the machine-noescape-dma branch from 6e803f9 to a411cf7 Compare October 3, 2025 10:02
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aykevl commented Oct 3, 2025

Rebased, and added tests.

What about drivers written on top of an interface, say drivers.I2C? AFAIK, Unless TinyGo can see through the interface indirection, any buffers passed to I2C.Tx will escape.

Yeah, this is only a partial solution. Callers that use the drivers.I2C interface should probably still allocate a static buffer to be sure (in the driver object, for example). But I do think it fixes a few real world cases.

@aykevl aykevl force-pushed the machine-noescape-dma branch from a411cf7 to e0a3e4b Compare October 3, 2025 10:05
eliasnaur
eliasnaur reviewed Oct 3, 2025
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src/machine/machine_nrf528xx.go Outdated
// Make sure the w and r buffers stay alive until this point, so they won't
// be garbage collected while the buffers are used by the hardware.
if len(w) > 0 {
keepAliveNoEscape(unsafe.Pointer(&w[0]))
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Nit: why not use unsafe.SliceData and drop the len(w) > 0 check? Here and elsewhere.

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Because it's an old PR that I rebased.
I've updated the PR to use unsafe.SliceData instead where possible.

@aykevl aykevl force-pushed the machine-noescape-dma branch from e0a3e4b to 48185fc Compare October 3, 2025 11:00
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ysoldak commented Nov 12, 2025

I have a reproducer that confirms this solves unnecessary heap escapes, at least for nrf52 chip.

In the drivers repo, on 0.33.0 version, on release branch of tinygo, i.e. 0.39.0 (same results on dev branch).

$ tinygo build -target nano-33-ble -o build/test.hex -size=short -opt=z -print-allocs=drivers ./examples/lsm6dsox

~/tinygo-drivers/lsm6dsox/lsm6dsox.go:43:16: object allocated on the heap: escapes at line 43
~/tinygo-drivers/internal/legacy/i2clegacy.go:10:13: object allocated on the heap: escapes at line 13
~/tinygo-drivers/internal/legacy/i2clegacy.go:6:36: object allocated on the heap: escapes at line 6
   code    data     bss |   flash     ram
  62376    1580    6472 |   63956    8052

Interestingly, rc2040 targets do not have this problem:
$ tinygo build -target nano-rp2040 -o build/test.hex -size=short -opt=z -print-allocs=drivers ./examples/lsm6dsox

~/tinygo-drivers/lsm6dsox/lsm6dsox.go:43:16: object allocated on the heap: escapes at line 43
   code    data     bss |   flash     ram
  65840    1636    5240 |   67476    6876

After switching to to machine-noescape-dma branch and doing go install, I now have this:
$ tinygo build -target nano-33-ble -o build/test.hex -size=short -opt=z -print-allocs=drivers ./examples/lsm6dsox

~/tinygo-drivers/lsm6dsox/lsm6dsox.go:43:16: object allocated on the heap: escapes at line 43
   code    data     bss |   flash     ram
  62392    1580    6472 |   63972    8052

This change seems to be fixing escapes for short-living slices, I'm happy.
@deadprogram @soypat makes our live easier in "drivers".
We may, as @soypat says, want to implement some CI to check for such escapes even if this change to be merged, to monitor regressions.

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deadprogram commented Nov 12, 2025

I personally am in favor of this PR, mainly due to being able to at least catch some of these unintentional escapes now.

Also agree that avoiding them in the first place is the ideal, but this is at least another defensive step.

@soypat any opinions on this?

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soypat commented Nov 12, 2025

Seems like a neat compiler intrinsic to help out in these cases. Don't think it helps out too much in drivers package, but nice to have

@aykevl @deadprogram
machine: make sure DMA buffers do not escape unnecessarily
04a4e2a 
Writing the pointer of a buffer to memory-mapped I/O will normally cause
it to escape, which forces the compiler to heap-allocate the buffer. But
we do know how long the value stays alive, so we can tell the compiler
to keep it alive exactly until it is not needed anymore - and tell it to
not treat the pointer-to-uintptr cast as escaping.
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deadprogram commented Nov 12, 2025

Rebased against latest dev to make sure tests all pass.

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deadprogram commented Nov 12, 2025

All passing so now merging. Thank you everyone!

@deadprogram deadprogram merged commit b203314 into dev Nov 12, 2025
25 checks passed
@deadprogram deadprogram deleted the machine-noescape-dma branch November 12, 2025 17:07
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