purego: support RegisterFunc and typed callbacks on Windows 386

[nekohasekai]

What issue is this addressing?

There is no existing issue. This expands the Windows/386 support currently described in the platform support table.

What type of issue is this addressing?

feature

What this PR does | solves

Windows/386 previously only officially supported SyscallN and NewCallback. The partial RegisterFunc path also treated 64-bit values as a single uintptr, only observed EAX for integer results, and did not retrieve floating-point results from x87.

This change adds the Windows/386 ABI handling needed for typed RegisterFunc/RegisterLibFunc calls and typed callbacks:

• lays out integer, pointer, bool, string, float32, and float64 arguments on the 32-bit stack;
• splits 64-bit arguments into little-endian low/high slots and rebuilds integer results from EDX:EAX;
• saves direct floating-point results from x87 ST(0) and restores callback floating-point results there;
• decodes 64-bit and floating-point callback arguments, supports 64-bit callback results, and preserves both stdcall and cdecl stack cleanup;
• passes up to the declared 32 stack slots instead of the old 15-slot fallback;
• supports by-value struct arguments and returns for direct calls, matching the existing Windows limitation that structs are not supported in callbacks; and
• adds Windows/386 ABI, x87 edge-case, stack-limit, concurrency, reentrancy, native-thread callback, and callback-table coverage.

The Windows CI jobs now use an explicit MSYS2 MINGW32 GNU toolchain, verify that it can build a 32-bit DLL, and run both Cgo and non-Cgo Windows/386 suites (including unoptimized builds) on Go 1.25, 1.26, and 1.27rc1.

Validation:

• go test ./...
• Windows/386 tests in a real 32-bit Windows process with LLVM-MinGW
• full GitHub Actions matrix, including all three windows-latest jobs: https://github.com/SagerNet/purego/actions/runs/29236606685

[TotallyGamerJet]

Can you create an issue to track this change?

[hajimehoshi]

Update README.md too

[nekohasekai]

The README is already updated in this PR — the platform table now lists Windows 386 with footnote 2 (structs supported as arguments/return values in calls, but not in callbacks), replacing the previous footnotes 3 and 6. Let me know if you'd like it documented differently.

[hajimehoshi]

Ah sorry, I missed that

[Copilot]

Pull request overview

Extends PureGo’s Windows/386 support beyond SyscallN/NewCallback by implementing the ABI handling needed for typed RegisterFunc/RegisterLibFunc calls, typed callbacks (including cdecl/stdcall cleanup), 64-bit scalar splitting/reconstruction, and x87 float returns, and adds dedicated Windows/386 ABI and callback test coverage plus CI updates.

Changes:

• Adds Windows/386 callback trampoline/dispatch path and x87 return handling to support typed callbacks and float/int64 returns.
• Adds Windows/386 struct-by-value support for direct calls and updates existing struct tests accordingly.
• Introduces comprehensive Windows/386 ABI tests and updates CI to exercise Windows/386 (cgo + non-cgo, optimized + unoptimized).

Reviewed changes

Copilot reviewed 18 out of 19 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
zcallback_386.s	Expands callback trampoline table build coverage to include windows/386.
windows_386_test.go	Adds Windows/386 ABI + callback + concurrency/reentrancy + x87 edge-case tests.
wincallback.go	Updates generated assembly build constraints to include windows/386.
testdata/structtest/struct_test.c	Fixes register-sum computation to be width-safe across architectures.
testdata/abitest/abi_test.c	Adds Windows/386-specific ABI/callback test helpers implemented in C.
syscall_windows.go	Routes NewCallback through an internal newCallback to enable custom windows/386 behavior.
syscall_32bit.go	Adds floatReturn control field used by the 386 bridge to capture x87 results safely.
sys_windows_386.s	Implements windows/386 callback trampoline part that restores integer/x87 return conventions.
sys_arm.s	Updates struct layout commentary to reflect renamed field.
sys_386.s	Enables syscall bridge on windows/386 and adds guarded x87 return capture logic.
struct_windows_386.go	Implements windows/386 struct argument packing and struct return reconstruction.
struct_test.go	Enables struct tests on windows/386 and fixes type-width expectations.
struct_other.go	Excludes windows/386 from “no-struct-support” fallback implementation.
README.md	Updates Windows/386 support tier notes to reflect added struct support for direct calls.
func.go	Implements windows/386 slot accounting, 64-bit return reconstruction, and x87 float-return modes.
func_test.go	Removes windows/386 skips now that CI/tooling is expected to support building test DLLs.
callback_windows_other.go	Keeps stdlib-backed callbacks on non-386 Windows.
callback_windows_386.go	Adds typed callback decoding/encoding for windows/386 using a stdlib-compatible bridge.
.github/workflows/test.yml	Adds MSYS2-based Windows/386 toolchain setup and windows/386 test runs.

Files not reviewed (1)

• wincallback.go: Generated file

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