wasm: report the generic context to the GC stack walk

src/coreclr/jit/codegencommon.cpp

@@ -5740,9 +5740,11 @@ void CodeGen::genFnProlog()
 
     genZeroInitFrame(untrLclHi, untrLclLo, initReg, &initRegZeroed);
 
-#ifndef TARGET_WASM // TODO-WASM: enable as needed.
+    // Save the generic context arg in the prolog so GetParamTypeArg can report it.
     genReportGenericContextArg(initReg, &initRegZeroed);
 
+#ifndef TARGET_WASM // TODO-WASM: enable as needed.
+
 #ifdef JIT32_GCENCODER
     // Initialize the LocalAllocSP slot if there is localloc in the function.
     if (m_compiler->lvaLocAllocSPvar != BAD_VAR_NUM)

src/coreclr/jit/codegenwasm.cpp

@@ -352,6 +352,37 @@ void CodeGen::genHomeRegisterParamsOutsideProlog()
     }
 }
 
+//------------------------------------------------------------------------
+// genReportGenericContextArg: spill the generic context (or "this") into its
+// GC-reportable frame slot in the prolog so the GC stack walk can locate it.
+// initReg / pInitRegZeroed are unused on wasm.
+//
+void CodeGen::genReportGenericContextArg(regNumber initReg, bool* pInitRegZeroed)
+{
+    assert(m_compiler->compGeneratingProlog);
+
+    const bool reportArg  = m_compiler->lvaReportParamTypeArg();
+    const bool reportThis = m_compiler->lvaKeepAliveAndReportThis();
+    if (!reportArg && !reportThis)
+    {
+        return;
+    }
+
+    const unsigned contextArg = reportArg ? m_compiler->info.compTypeCtxtArg : m_compiler->info.compThisArg;
+    noway_assert(contextArg != BAD_VAR_NUM);
+
+    // The context arg is still in its incoming register at this point in the prolog.
+    const ABIPassingInformation& abiInfo = m_compiler->lvaGetParameterABIInfo(contextArg);
+    assert(abiInfo.HasExactlyOneRegisterSegment());
+    const regNumber reg = abiInfo.Segment(0).GetRegister();
+
+    // [FP + lvaCachedGenericContextArgOffset()] = contextArg
+    emitter* emit = GetEmitter();
+    emit->emitIns_I(INS_local_get, EA_PTRSIZE, GetFramePointerRegIndex());
+    emit->emitIns_I(INS_local_get, EA_PTRSIZE, WasmRegToIndex(reg));
+    emit->emitIns_I(ins_Store(TYP_I_IMPL), EA_PTRSIZE, m_compiler->lvaCachedGenericContextArgOffset());
+}
+
 void CodeGen::genFnEpilog(BasicBlock* block)
 {
 #ifdef DEBUG