cmd/compile: add basic wasmexport support

This CL adds a compiler directive go:wasmexport, which applies to
a Go function and makes it an exported function of the Wasm module
being built, so it can be called directly from the host. As
proposed in #65199, parameter and result types are limited to
32-bit and 64-bit integers and floats, and there can be at most
one result.

As the Go and Wasm calling conventions are different, for a
wasmexport function we generate a wrapper function does the ABI
conversion at compile time.

Currently this CL only adds basic support. In particular,
- it only supports executable mode, i.e. the Go wasm module calls
  into the host via wasmimport, which then calls back to Go via
  wasmexport. Library (c-shared) mode is not implemented yet.
- only supports wasip1, not js.
- if the exported function unwinds stacks (goroutine switch, stack
growth, etc.), it probably doesn't work.

TODO: support stack unwinding, c-shared mode, js.

For #65199.

Change-Id: Id1777c2d44f7d51942c1caed3173c0a82f120cc4
Reviewed-on: https://go-review.googlesource.com/c/go/+/603055
Reviewed-by: Than McIntosh <thanm@golang.org>
Reviewed-by: Randy Reddig <randy.reddig@fastly.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/src/cmd/link/internal/ld/deadcode.go b/src/cmd/link/internal/ld/deadcode.go
index 3d547259a1..a1378fc02c 100644
--- a/src/cmd/link/internal/ld/deadcode.go
+++ b/src/cmd/link/internal/ld/deadcode.go
@@ -116,6 +116,13 @@ func (d *deadcodePass) init() {
 		}
 		d.mark(s, 0)
 	}
+	// So are wasmexports.
+	for _, s := range d.ldr.WasmExports {
+		if d.ctxt.Debugvlog > 1 {
+			d.ctxt.Logf("deadcode start wasmexport: %s<%d>\n", d.ldr.SymName(s), d.ldr.SymVersion(s))
+		}
+		d.mark(s, 0)
+	}
 
 	d.mapinitnoop = d.ldr.Lookup("runtime.mapinitnoop", abiInternalVer)
 	if d.mapinitnoop == 0 {