From 2de1151002758ad1f8900e28cf7715d602242125 Mon Sep 17 00:00:00 2001
From: Andy Ayers <andya@microsoft.com>
Date: Mon, 29 Jun 2026 13:21:26 -0700
Subject: [PATCH 1/2] wasm: report the generic context to the GC stack walk

Spill the generic context to its GC slot in the prolog, and populate the
frame pointer in TransitionFrame::UpdateRegDisplay so the stack walk can
locate it.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 src/coreclr/jit/codegencommon.cpp |  4 +++-
 src/coreclr/jit/codegenwasm.cpp   | 31 +++++++++++++++++++++++++++++++
 src/coreclr/vm/wasm/helpers.cpp   |  4 ++++
 3 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/src/coreclr/jit/codegencommon.cpp b/src/coreclr/jit/codegencommon.cpp
index 3e5f54cb2423f4..b93a07677c19d3 100644
--- a/src/coreclr/jit/codegencommon.cpp
+++ b/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)
diff --git a/src/coreclr/jit/codegenwasm.cpp b/src/coreclr/jit/codegenwasm.cpp
index 45b432c2a96537..2dc4f79bbeddf0 100644
--- a/src/coreclr/jit/codegenwasm.cpp
+++ b/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
diff --git a/src/coreclr/vm/wasm/helpers.cpp b/src/coreclr/vm/wasm/helpers.cpp
index 8abf0c736818f2..78050a8631d811 100644
--- a/src/coreclr/vm/wasm/helpers.cpp
+++ b/src/coreclr/vm/wasm/helpers.cpp
@@ -520,12 +520,16 @@ void FaultingExceptionFrame::UpdateRegDisplay_Impl(const PREGDISPLAY pRD, bool u
     PORTABILITY_ASSERT("FaultingExceptionFrame::UpdateRegDisplay_Impl is not implemented on wasm");
 }
 
+TADDR GetWasmFramePointerFromStackPointer(TADDR sp);
+
 void TransitionFrame::UpdateRegDisplay_Impl(const PREGDISPLAY pRD, bool updateFloats)
 {
     pRD->IsCallerContextValid = FALSE;
 
     pRD->pCurrentContext->InterpreterIP = GetReturnAddress();
     pRD->pCurrentContext->InterpreterSP = GetSP();
+    // Recover the frame pointer so GC-info readers can locate frame slots.
+    pRD->pCurrentContext->InterpreterFP = GetWasmFramePointerFromStackPointer(GetSP());
 
     SyncRegDisplayToCurrentContext(pRD);
 

From 185e153bfe8913e87b95963b4f4d63ffe9e50225 Mon Sep 17 00:00:00 2001
From: Andy Ayers <andya@microsoft.com>
Date: Mon, 29 Jun 2026 18:31:22 -0700
Subject: [PATCH 2/2] wasm: drop the runtime change, keep JIT-only

The TransitionFrame fp fix will go in a separate change.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 src/coreclr/vm/wasm/helpers.cpp | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/src/coreclr/vm/wasm/helpers.cpp b/src/coreclr/vm/wasm/helpers.cpp
index 78050a8631d811..8abf0c736818f2 100644
--- a/src/coreclr/vm/wasm/helpers.cpp
+++ b/src/coreclr/vm/wasm/helpers.cpp
@@ -520,16 +520,12 @@ void FaultingExceptionFrame::UpdateRegDisplay_Impl(const PREGDISPLAY pRD, bool u
     PORTABILITY_ASSERT("FaultingExceptionFrame::UpdateRegDisplay_Impl is not implemented on wasm");
 }
 
-TADDR GetWasmFramePointerFromStackPointer(TADDR sp);
-
 void TransitionFrame::UpdateRegDisplay_Impl(const PREGDISPLAY pRD, bool updateFloats)
 {
     pRD->IsCallerContextValid = FALSE;
 
     pRD->pCurrentContext->InterpreterIP = GetReturnAddress();
     pRD->pCurrentContext->InterpreterSP = GetSP();
-    // Recover the frame pointer so GC-info readers can locate frame slots.
-    pRD->pCurrentContext->InterpreterFP = GetWasmFramePointerFromStackPointer(GetSP());
 
     SyncRegDisplayToCurrentContext(pRD);