From 69b95158e03df204457f57ca4b00fc942fdeb2c9 Mon Sep 17 00:00:00 2001
From: Jakob Botsch Nielsen <jakob.botsch.nielsen@gmail.com>
Date: Mon, 1 Sep 2025 15:04:57 +0200
Subject: [PATCH] JIT: Disallow insertions into SIMD registers for argument
 passing

The backend would produce illegal IR for this case (like SIMD16 <- long
bitcasts).
---
 src/coreclr/jit/lower.cpp | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/coreclr/jit/lower.cpp b/src/coreclr/jit/lower.cpp
index f8c3a7bc0975e3..2c02c16f04f0e3 100644
--- a/src/coreclr/jit/lower.cpp
+++ b/src/coreclr/jit/lower.cpp
@@ -5177,6 +5177,14 @@ bool Lowering::IsFieldListCompatibleWithRegisters(GenTreeFieldList*   fieldList,
                 return false;
             }
 
+            // All SIMDs insertions are not yet supported
+            if (varTypeIsSIMD(regType) && ((fieldStart != regStart) || (fieldEnd != regEnd)))
+            {
+                JITDUMP("it is not; field [%06u] requires an insertion into SIMD register %u\n",
+                        Compiler::dspTreeID(use->GetNode()), i);
+                return false;
+            }
+
             use = use->GetNext();
         } while (use != nullptr);
     }