iface_patch

iface_patch

Optional: patch a Go 1.25.x, 1.26.x, or 1.27.x source tree so -tags goplus.ifacefuncval can replace icall stubs with MakeFunc funcvals. reflectx stores an untagged *makeFuncImpl (GC-scannable) and sets tflagIfaceFuncval (1<<6). The patched runtime tags itab.Fun (makeFuncImpl*|1) at itabInit / methodReceiver.

Supported GOARCH values: wasm, arm64, amd64, 386 (any GOOS that uses that backend).

The tag is explicit (not a ToolTag). Without it, the patched compiler emits the same interface-call sequence as unmodified Go.

reflectx does not check whether the compiler can unwrap tagged Fun. An unpatched gc still compiles with the tag; interface method calls then trap at runtime (uninitialized element / invalid PC).

Patch and rebuild

Install:

go install github.com/goplus/reflectx/cmd/iface_patch@latest

iface_patch only edits source. Rebuild with make.bash in that tree (go install cmd/asm cmd/compile cmd/go is not enough):

iface_patch /path/to/go
iface_patch -check /path/to/go
cd /path/to/go/src && ./make.bash   # Windows: make.bat
export GOROOT=/path/to/go
export PATH="$GOROOT/bin:$PATH"
go version   # from $GOROOT

Re-running on an already patched tree is a no-op. Supported versions are the directories under _data/ (iface_patch -h lists them).

Patching is transactional. Before writing, iface_patch snapshots every target file. If any patch step fails, it restores all files changed by that run. After writing, it runs a complete check pass; validation failure also triggers rollback. A separate rollback failed message reports any file that could not be restored and requires manual inspection.

go clean -cache can clear the build cache; it is not required.

Do not mix this GOROOT with another go on PATH.

Run tests

Native (linux/darwin amd64, arm64, or 386)

export GOROOT=/path/to/go
export PATH="$GOROOT/bin:$PATH"
go test -tags goplus.ifacefuncval -v .

make.bash installs stdlib without -ifacefuncval. The tagged go test rebuilds fmt/runtime into GOCACHE so their interface calls unwrap.

wasip1

The host cannot run a wasip1 binary. Install Wasmtime and pass -exec wasmtime:

export GOROOT=/path/to/go
export PATH="$GOROOT/bin:$PATH"
GOOS=wasip1 GOARCH=wasm go test -exec wasmtime -tags goplus.ifacefuncval -v .

This does not run the tests:

GOOS=wasip1 GOARCH=wasm go test -tags goplus.ifacefuncval .

GOFLAGS

export GOFLAGS='-tags=goplus.ifacefuncval'
go test -v .                                          # native
GOOS=wasip1 GOARCH=wasm go test -exec wasmtime -v .    # wasip1

If GOFLAGS already has -tags, merge them:

export GOFLAGS='-tags=netgo,goplus.ifacefuncval'

Disable by omitting the tag. GOFLAGS cannot subtract a tag (-tags=-goplus.ifacefuncval is not supported).

GOFLAGS=-gcflags=-ifacefuncval is not enough: that only unwraps calls. Without the build tag, reflectx still compiles the icall path.

What the patch changes

Unwrap runs only when objabi.EnableIfaceFuncval is set (-ifacefuncval). Untagged builds must match stock gc.

A tagged itab.Fun is makeFuncImpl* | 1. Code PCs and heap pointers are even, so bit 0 is free. The unwrap sets CTXT to the untagged pointer and calls the first word (makeFuncStub).

_data/

Matching uses VERSION as the directory name:

1. Exact _data/go1.N.x/ if that directory exists
2. Else _data/go1.N/ (go1.25.14 → go1.25)

Current series dirs and the trees they were taken from:

Go fragments use //go:build ignore. Assembly CALLFN old/new pairs are plain .s files used as exact text replacements.

To support another series, copy _data/go1.27/ to _data/go1.28/. To pin a patch release that diverges, copy to _data/go1.27.2/ (exact match wins). Adjust the snippets until iface_patch matches that tree.