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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 
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  13408   13408      0   0.00%    5388    5388      0   0.00% tinygo build -size short -o ./build/test.hex -target=macropad-rp2040 ./examples/sh1106/macropad_spi
  10156   10156      0   0.00%    5824    5824      0   0.00% tinygo build -size short -o ./build/test.hex -target=macropad-rp2040 ./examples/encoders/quadrature-interrupt
  68576   68576      0   0.00%    6860    6860      0   0.00% tinygo build -size short -o ./build/test.uf2 -target=pico ./examples/mcp9808/main.go
  44192   44192      0   0.00%    7144    7144      0   0.00% tinygo build -size short -o ./build/test.uf2 -target=pico ./examples/tmc2209/main.go
  15092   15092      0   0.00%    5348    5348      0   0.00% tinygo build -size short -o ./build/test.hex -target=pico ./examples/tmc5160/main.go
  12472   12472      0   0.00%    4544    4544      0   0.00% tinygo build -size short -o ./build/test.uf2 -target=nicenano ./examples/sharpmem/main.go
  60640   60640      0   0.00%    5968    5968      0   0.00% tinygo build -size short -o ./build/test.hex -target=feather-nrf52840 ./examples/max6675/main.go
  14748   14748      0   0.00%    5408    5408      0   0.00% tinygo build -size short -o ./build/test.hex -target=pico ./examples/ens160/main.go
  88224   88224      0   0.00%    7260    7260      0   0.00% tinygo build -size short -o ./build/test.hex -target=challenger-rp2040 ./examples/net/ntpclient/
 347056  347056      0   0.00%   16764   16764      0   0.00% tinygo build -size short -o ./build/test.hex -target=pyportal -stack-size 8kb ./examples/net/http-get/
 119928  119928      0   0.00%    8020    8020      0   0.00% tinygo build -size short -o ./build/test.hex -target=arduino-nano33 -stack-size 8kb ./examples/net/tcpclient/
 308248  308248      0   0.00%   15924   15924      0   0.00% tinygo build -size short -o ./build/test.hex -target=nano-rp2040 -stack-size 8kb ./examples/net/websocket/dial/
 103568  103568      0   0.00%   10000   10000      0   0.00% tinygo build -size short -o ./build/test.hex -target=metro-m4-airlift -stack-size 8kb ./examples/net/socket/
 388420  388420      0   0.00%   18772   18772      0   0.00% tinygo build -size short -o ./build/test.hex -target=matrixportal-m4 -stack-size 8kb ./examples/net/webstatic/
 166364  166364      0   0.00%    9992    9992      0   0.00% tinygo build -size short -o ./build/test.hex -target=arduino-mkrwifi1010 -stack-size 8kb ./examples/net/tlsclient/
 155780  155780      0   0.00%    8812    8812      0   0.00% tinygo build -size short -o ./build/test.hex -target=nano-rp2040 -stack-size 8kb ./examples/net/mqttclient/natiu/
 116732  116732      0   0.00%   13296   13296      0   0.00% tinygo build -size short -o ./build/test.hex -target=wioterminal -stack-size 8kb ./examples/net/webclient/
 337980  337980      0   0.00%   21816   21816      0   0.00% tinygo build -size short -o ./build/test.hex -target=wioterminal -stack-size 8kb ./examples/net/webserver/
 338128  338128      0   0.00%   21740   21740      0   0.00% tinygo build -size short -o ./build/test.hex -target=wioterminal -stack-size 8kb ./examples/net/mqttclient/paho/
 174588  174588      0   0.00%   13840   13840      0   0.00% tinygo build -size short -o ./build/test.hex -target=elecrow-rp2040 -stack-size 8kb ./examples/net/tlsclient/
 120264  120264      0   0.00%   11864   11864      0   0.00% tinygo build -size short -o ./build/test.hex -target=elecrow-rp2350 -stack-size 8kb ./examples/net/ntpclient/
6254988 6254988      0   0.00%  963434  963434      0   0.00%

@ydnar
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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
                                }

@aykevl
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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 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

@deadprogram
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@aykevl any chance of getting back to this PR soon? I think it is kinda needed...

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aykevl commented Oct 3, 2025

Rebased since it's been a while and I want to see why CI fails (logs were expired).

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aykevl commented Oct 3, 2025

Removed the first commit that was causing problems.

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aykevl commented Oct 3, 2025

Actually no I think it is needed.

@aykevl aykevl marked this pull request as draft October 3, 2025 09:00
@aykevl
wasm: fix C realloc and optimize it a bit
cdb0a71 
  - 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 marked this pull request as ready for review October 3, 2025 09:21
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aykevl commented Oct 3, 2025

Ok, this should do it for now. Not pretty, but fixing the interp issue is more work. We can do that later.

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@dgryski @ydnar anyone else WASM oriented please take a look.

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@aykevl @ydnar @deadprogram @dgryski