Partial re-revert of dotnet#104336. Only JIT fixes are included.

Author: VSadov

src/coreclr/jit/codegencommon.cpp

@@ -1517,29 +1517,6 @@ void CodeGen::genExitCode(BasicBlock* block)
     if (compiler->getNeedsGSSecurityCookie())
     {
         genEmitGSCookieCheck(jmpEpilog);
-
-        if (jmpEpilog)
-        {
-            // Dev10 642944 -
-            // The GS cookie check created a temp label that has no live
-            // incoming GC registers, we need to fix that
-
-            unsigned   varNum;
-            LclVarDsc* varDsc;
-
-            /* Figure out which register parameters hold pointers */
-
-            for (varNum = 0, varDsc = compiler->lvaTable; varNum < compiler->lvaCount && varDsc->lvIsRegArg;
-                 varNum++, varDsc++)
-            {
-                noway_assert(varDsc->lvIsParam);
-
-                gcInfo.gcMarkRegPtrVal(varDsc->GetArgReg(), varDsc->TypeGet());
-            }
-
-            GetEmitter()->emitThisGCrefRegs = GetEmitter()->emitInitGCrefRegs = gcInfo.gcRegGCrefSetCur;
-            GetEmitter()->emitThisByrefRegs = GetEmitter()->emitInitByrefRegs = gcInfo.gcRegByrefSetCur;
-        }
     }
 
     genReserveEpilog(block);

src/coreclr/jit/emit.cpp

@@ -10427,9 +10427,9 @@ regMaskTP emitter::emitGetGCRegsKilledByNoGCCall(CorInfoHelpFunc helper)
 //    of the last instruction in the region makes GC safe again.
 //    In particular - once the IP is on the first instruction, but not executed it yet,
 //    it is still safe to do GC.
-//    The only special case is when NoGC region is used for prologs/epilogs.
-//    In such case the GC info could be incorrect until the prolog completes and epilogs
-//    may have unwindability restrictions, so the first instruction cannot have GC.
+//    The only special case is when NoGC region is used for prologs.
+//    In such case the GC info could be incorrect until the prolog completes, so the first
+//    instruction cannot have GC.
 
 void emitter::emitDisableGC()
 {

src/coreclr/jit/emitinl.h

@@ -594,8 +594,7 @@ bool emitter::emitGenNoGCLst(Callback& cb)
             emitter::instrDesc* id = emitFirstInstrDesc(ig->igData);
             assert(id != nullptr);
             assert(id->idCodeSize() > 0);
-            if (!cb(ig->igFuncIdx, ig->igOffs, ig->igSize, id->idCodeSize(),
-                    ig->igFlags & (IGF_FUNCLET_PROLOG | IGF_FUNCLET_EPILOG | IGF_EPILOG)))
+            if (!cb(ig->igFuncIdx, ig->igOffs, ig->igSize, id->idCodeSize(), ig->igFlags & (IGF_FUNCLET_PROLOG)))
             {
                 return false;
             }

src/coreclr/jit/gcencode.cpp

@@ -4027,8 +4027,7 @@ class InterruptibleRangeReporter
     // Report everything between the previous region and the current
     // region as interruptible.
 
-    bool operator()(
-        unsigned igFuncIdx, unsigned igOffs, unsigned igSize, unsigned firstInstrSize, bool isInPrologOrEpilog)
+    bool operator()(unsigned igFuncIdx, unsigned igOffs, unsigned igSize, unsigned firstInstrSize, bool isInProlog)
     {
         if (igOffs < m_uninterruptibleEnd)
         {
@@ -4042,9 +4041,9 @@ class InterruptibleRangeReporter
         if (igOffs > m_uninterruptibleEnd)
         {
             // Once the first instruction in IG executes, we cannot have GC.
-            // But it is ok to have GC while the IP is on the first instruction, unless we are in prolog/epilog.
+            // But it is ok to have GC while the IP is on the first instruction, unless we are in prolog.
             unsigned interruptibleEnd = igOffs;
-            if (!isInPrologOrEpilog)
+            if (!isInProlog)
             {
                 interruptibleEnd += firstInstrSize;
             }

src/coreclr/nativeaot/Runtime/thread.cpp

@@ -675,10 +675,16 @@ void Thread::HijackCallback(NATIVE_CONTEXT* pThreadContext, void* pThreadToHijac
     if (runtime->IsConservativeStackReportingEnabled() ||
         codeManager->IsSafePoint(pvAddress))
     {
+        // IsUnwindable is precise on arm64, but can give false negatives on other architectures.
+        // (when IP is on the first instruction of an epilog, we still can unwind,
+        // but we can tell if the instruction is the first only if we can navigate instructions backwards and check)
+        // The preciseness of IsUnwindable is tracked in https://github.com/dotnet/runtime/issues/101932
+#if defined(TARGET_ARM64)
         // we may not be able to unwind in some locations, such as epilogs.
         // such locations should not contain safe points.
         // when scanning conservatively we do not need to unwind
         ASSERT(codeManager->IsUnwindable(pvAddress) || runtime->IsConservativeStackReportingEnabled());
+#endif
 
         // if we are not given a thread to hijack
         // perform in-line wait on the current thread