cmd/internal/obj/wasm: correct return PC for frameless wasmexport wrappers

For a wasmexport wrapper, we generate a call to the actual
exported Go function, and use the wrapper function's PC 1 as the
(fake) return address. This address is not used for returning,
which is handled by the Wasm call stack. It is used for stack
unwinding, and PC 1 makes it past the prologue and therefore has
the right SP delta. But if the function has no arguments and
results, the wrapper is frameless, with no prologue, and PC 1
doesn't exist. This causes the unwinder to fail. In this case, we
put PC 0, which also has the correct SP delta (0).

Fixes #69584.

Change-Id: Ic047a6e62100db540b5099cc5a56a1d0f16d58b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/624000
Reviewed-by: David Chase <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: cherrymui

lib/wasm/wasm_exec.js

@@ -217,6 +217,7 @@
 			}
 
 			const testCallExport = (a, b) => {
+				this._inst.exports.testExport0();
 				return this._inst.exports.testExport(a, b);
 			}
 

src/cmd/internal/obj/wasm/wasmobj.go

@@ -974,6 +974,10 @@ func genWasmExportWrapper(s *obj.LSym, appendp func(p *obj.Prog, as obj.As, args
 		Sym:    s, // PC_F
 		Offset: 1, // PC_B=1, past the prologue, so we have the right SP delta
 	}
+	if framesize == 0 {
+		// Frameless function, no prologue.
+		retAddr.Offset = 0
+	}
 	p = appendp(p, AI64Const, retAddr)
 	p = appendp(p, AI64Store, constAddr(0))
 	// Set PC_B parameter to function entry
@@ -1014,11 +1018,13 @@ func genWasmExportWrapper(s *obj.LSym, appendp func(p *obj.Prog, as obj.As, args
 	}
 
 	// Epilogue. Cannot use ARET as we don't follow Go calling convention.
-	// SP += framesize
-	p = appendp(p, AGet, regAddr(REG_SP))
-	p = appendp(p, AI32Const, constAddr(framesize))
-	p = appendp(p, AI32Add)
-	p = appendp(p, ASet, regAddr(REG_SP))
+	if framesize > 0 {
+		// SP += framesize
+		p = appendp(p, AGet, regAddr(REG_SP))
+		p = appendp(p, AI32Const, constAddr(framesize))
+		p = appendp(p, AI32Add)
+		p = appendp(p, ASet, regAddr(REG_SP))
+	}
 	p = appendp(p, AReturn)
 }
 

src/syscall/js/js_test.go

@@ -75,6 +75,11 @@ func testExport(a int32, b int64) int64 {
 	return <-ch + <-ch
 }
 
+//go:wasmexport testExport0
+func testExport0() { // no arg or result (see issue 69584)
+	runtime.GC()
+}
+
 var testExportCalled bool
 
 func growStack(n int64) {