comments and some cleanup

src/coreclr/jit/codegencommon.cpp

@@ -1517,11 +1517,15 @@ void CodeGen::genExitCode(BasicBlock* block)
     if (compiler->getNeedsGSSecurityCookie())
     {
 #ifndef JIT32_GCENCODER
-        // The call to CORINFO_HELP_FAIL_FAST in GS cookie check makes an impression
-        // of trashing all calee-trashed registers.
-        // Instead of ensuring that these registers are live across the check
-        // we just claim that the whole thing is not GC-interruptible.
+        // At this point the gc info that we track in codegen is often incorrect,
+        // as it could be missing return registers or arg registers (in a case of tail call).
+        // GS cookie check will emit a call and that will pass our GC info to emit and potentially mess things up.
+        // While we could infer returns/args and force them to be live and it seems to work in JIT32_GCENCODER case,
+        // it appears to be nontrivial in more general case.
+        // So, instead, we just claim that the whole thing is not GC-interruptible.
         // Effectively this starts the epilog a few instructions earlier.
+        //
+        // CONSIDER: is that a good place to be that codegen loses track of returns/args at this point?
         GetEmitter()->emitDisableGC();
 #endif
         genEmitGSCookieCheck(jmpEpilog);
@@ -4706,14 +4710,13 @@ void CodeGen::genReserveProlog(BasicBlock* block)
 
 void CodeGen::genReserveEpilog(BasicBlock* block)
 {
-    regMaskTP gcrefRegsArg = gcInfo.gcRegGCrefSetCur;
-    regMaskTP byrefRegsArg = gcInfo.gcRegByrefSetCur;
-
     JITDUMP("Reserving epilog IG for block " FMT_BB "\n", block->bbNum);
 
     assert(block != nullptr);
-    const VARSET_TP& gcrefVarsArg(GetEmitter()->emitThisGCrefVars);
-    GetEmitter()->emitCreatePlaceholderIG(IGPT_EPILOG, block, gcrefVarsArg, gcrefRegsArg, byrefRegsArg,
+    // We pass empty GC info, because epilog is always an extend IG and will ignore what we pass.
+    // Besides, at this point the GC info that we track in CodeGen is often incorrect.
+    // See comments in genExitCode for more info.
+    GetEmitter()->emitCreatePlaceholderIG(IGPT_EPILOG, block, VarSetOps::MakeEmpty(compiler), 0, 0,
                                           block->IsLast());
 }
 

src/coreclr/jit/codegenxarch.cpp

@@ -97,10 +97,10 @@ void CodeGen::genEmitGSCookieCheck(bool pushReg)
     noway_assert(compiler->gsGlobalSecurityCookieAddr || compiler->gsGlobalSecurityCookieVal);
 
 #ifdef JIT32_GCENCODER
-    // Make sure that EAX is reported as live GC-ref so that any GC that kicks in while
-    // executing GS cookie check will not collect the object pointed to by EAX.
     if (!pushReg)
     {
+        // Make sure that EAX is reported as live GC-ref so that any GC that kicks in while
+        // executing GS cookie check will not collect the object pointed to by EAX.
         if (compiler->compMethodReturnsRetBufAddr())
         {
             // This is for returning in an implicit RetBuf.
@@ -123,6 +123,28 @@ void CodeGen::genEmitGSCookieCheck(bool pushReg)
             }
         }
     }
+    else
+    {
+        // 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;
+    }
 #else
     assert(GetEmitter()->emitGCDisabled());
 #endif