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wasm: fix C realloc and optimize it a bit #4898

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@aykevl aykevl commented May 21, 2025

  • Do not use make([]byte, ...) to allocate, instead allocate a slice of pointers. This makes sure the precise GC will scan the contents of the allocation, since C could very well put pointers in there.
  • Simplify the map to use the pointer as the key and the size as the value, instead of storing the slices directly in the map.

I didn't really test this apart from running ./testdata/cgo. I'm not entirely sure how to properly test this. But if we start using libc_realloc for cabi_realloc I think @ydnar has some test cases on hand?

@aykevl aykevl requested a review from ydnar May 21, 2025 10:37
@aykevl aykevl mentioned this pull request May 21, 2025
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Size difference with the dev branch:

Binary size difference 
not the same command!
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    go: downloading tinygo.org/x/tinyfont v0.3.0
not the same command!
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    go: downloading golang.org/x/exp v0.0.0-20241204233417-43b7b7cde48d
drivers/sizes-dev.txt has more commands than drivers/sizes-pr.txt
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 263720  263728      8   0.00%   46744   46744      0   0.00% tinygo build -size short -o ./build/test.hex -target=pyportal ./examples/ili9341/slideshow
  69220   69228      8   0.01%    6196    6196      0   0.00% tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/mcp2515/main.go
  63216   63224      8   0.01%    5948    5948      0   0.00% tinygo build -size short -o ./build/test.hex -target=feather-nrf52840 ./examples/is31fl3731/main.go
  76656   76664      8   0.01%    6336    6336      0   0.00% tinygo build -size short -o ./build/test.hex -target=feather-m0 ./examples/dht/main.go
  71292   71300      8   0.01%    6340    6340      0   0.00% tinygo build -size short -o ./build/test.hex -target=xiao ./examples/pcf8563/time/
  74080   74088      8   0.01%   10756   10756      0   0.00% tinygo build -size short -o ./build/test.hex -target=feather-m4 ./examples/sdcard/console/
  62264   62272      8   0.01%    8228    8228      0   0.00% tinygo build -size short -o ./build/test.hex -target=feather-m4 ./examples/i2csoft/adt7410/
  82956   82964      8   0.01%    6592    6592      0   0.00% tinygo build -size short -o ./build/test.hex -target=nucleo-wl55jc ./examples/lora/lorawan/atcmd/
  65592   65600      8   0.01%    6252    6252      0   0.00% tinygo build -size short -o ./build/test.hex -target=arduino-nano33 ./examples/ndir/main_ndir.go
3307130 3307230    100   0.00%  702270  702270      0   0.00%

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ydnar commented May 21, 2025

The Component Model never calls free on these allocations. We'd need some way to assert ownership of them so the GC can free them later.

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Looks like real test failure here https://github.com/tinygo-org/tinygo/actions/runs/15159995695/job/42637182590#step:17:28

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ydnar commented May 21, 2025

The Component Model never calls free on these allocations. We'd need some way to assert ownership of them so the GC can free them later.

It looks like we can have the generated code call runtime.libc_free on any allocated pointers it knows about, since libc_free doesn’t actually free the memory. I think I can wire this up into package cm with a small TinyGo-specific code path.

@aykevl would it help to have libc_free call some inner function release or something, that explicitly doesn’t free the memory, doing what it does now, allowing the GC to clean it up later?

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aykevl commented May 21, 2025

Huh, this is weird. This PR increases binary size by a large amount for wasm. For example, testdata/alias.go went from 17kb to 49kb. I don't know why this PR causes such a large increase, but I am pretty sure that we wouldn't have this problem if malloc and free weren't exported by default on wasm.

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aykevl commented May 21, 2025

The Component Model never calls free on these allocations. We'd need some way to assert ownership of them so the GC can free them later.

@ydnar so the host calls cabi_realloc (implicitly claiming ownership of the memory) and then at some point in the future transfers ownership to the guest/Go size? Is that correct? (Or at least the spec says something like "we won't use the memory after this point"?)

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ydnar commented May 21, 2025

Transfer is implicit. There is not a specific call for each pointer that transfers ownership.

The component model does specify a post-call mechanism for exported functions to allow the host to free allocs.

This is where we can call libc_free.

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ydnar commented May 22, 2025

Huh, this is weird. This PR increases binary size by a large amount for wasm. For example, testdata/alias.go went from 17kb to 49kb. I don't know why this PR causes such a large increase, but I am pretty sure that we wouldn't have this problem if malloc and free weren't exported by default on wasm.

Can this split the implementation from the //export?

@hb9cwp hb9cwp mentioned this pull request May 23, 2025
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ydnar commented May 25, 2025

@aykevl for this to support the Component Model, I think we need a function like libc_free that:

  1. Removes the pointer from the allocs map.
  2. Does not panic if it’s an unknown pointer.

Can it be something like runtime.Release(ptr unsafe.Pointer) unsafe.Pointer that returns nil if the pointer was unknown?

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dgryski commented May 27, 2025

I'll try to help out here.

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It would be very good to get this fix, or some version of it, merged in time for the next release... 😸

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I think this PR will pass CI with this change:

diff --git a/builder/sizes_test.go b/builder/sizes_test.go
index 931e9970..e4d5396a 100644
--- a/builder/sizes_test.go
+++ b/builder/sizes_test.go
@@ -81,8 +81,9 @@ func TestSizeFull(t *testing.T) {
                "wasip1",
        }
 
-       libMatch := regexp.MustCompile(`^C [a-z -]+$`) // example: "C interrupt vector"
-       pkgMatch := regexp.MustCompile(`^[a-z/]+$`)    // example: "internal/task"
+       libMatch := regexp.MustCompile(`^C [a-z -]+$`)      // example: "C interrupt vector"
+       goMatch := regexp.MustCompile(`^Go [a-z -]+$`)      // example: "Go interface method"
+       pkgMatch := regexp.MustCompile(`^[a-z][a-z/0-9]+$`) // example: "internal/task"
 
        for _, target := range tests {
                target := target
@@ -106,6 +107,9 @@ func TestSizeFull(t *testing.T) {
                                if libMatch.MatchString(pkg) {
                                        continue
                                }
+                               if goMatch.MatchString(pkg) {
+                                       continue
+                               }
                                if pkgMatch.MatchString(pkg) {
                                        continue
                                }

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aykevl commented Jun 11, 2025

@deadprogram it would, but that's not the problem. The problem is that it increases binary size for wasm substantially. I'll need to investigate why that is.

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The problem is that it increases binary size for wasm substantially. I'll need to investigate why that is.

Thanks for the clarification.

aykevl added 2 commits June 13, 2025 12:47
@aykevl
compiler: treat unsafe.Pointer inside map keys like any other pointer
b611cbc 
Pointers are pointers, but unsafe.Pointer was not treated as such and
forced a map lookup using reflect. This commit fixes that.
@aykevl
wasm: fix C realloc and optimize it a bit
63c39af 
  - Do not use make([]byte, ...) to allocate, instead call the allocator
    directly with a nil (undefined) layout. This makes sure the precise
    GC will scan the contents of the allocation, since C could very well
    put pointers in there.
  - Simplify the map to use the pointer as the key and the size as the
    value, instead of storing the slices directly in the map.
@aykevl aykevl force-pushed the fix-wasm-realloc branch from 8a49a78 to 63c39af Compare June 13, 2025 10:47
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aykevl commented Jun 13, 2025

I think I fixed the issue. The problem was that unsafe.Pointer was not treated like all the other pointers and forced map lookups using reflection. This fix might also optimize other code, we'll see.

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aykevl commented Jun 13, 2025

Also, I optimized the code slightly by calling alloc directly instead of doing the weird round-up-and-divide to allocate a slice of pointers. The code is simpler now.

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@aykevl still error in this PR failing CI:

panic: cannot convert pointer value to byte

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@aykevl @ydnar @deadprogram @dgryski