diff --git a/llvm/lib/Transforms/IPO/ArgumentPromotion.cpp b/llvm/lib/Transforms/IPO/ArgumentPromotion.cpp
--- a/llvm/lib/Transforms/IPO/ArgumentPromotion.cpp
+++ b/llvm/lib/Transforms/IPO/ArgumentPromotion.cpp
@@ -971,6 +971,12 @@
       ArgsToPromote.insert(PtrArg);
   }
 
+  // Don't promote the first argument to x86 thiscall functions.
+  if (F->getCallingConv() == CallingConv::X86_ThisCall) {
+    assert(!F->arg_empty());
+    ArgsToPromote.erase(F->arg_begin());
+  }
+
   // No promotable pointer arguments.
   if (ArgsToPromote.empty() && ByValArgsToTransform.empty())
     return nullptr;
diff --git a/llvm/test/Transforms/ArgumentPromotion/X86/thiscall.ll b/llvm/test/Transforms/ArgumentPromotion/X86/thiscall.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/ArgumentPromotion/X86/thiscall.ll
@@ -0,0 +1,35 @@
+; PR41658 happened because we remove parameters from an x86_thiscallcc function
+; and then try to pass an inalloca in a register. This test verifies that we
+; don't do that anymore.
+;
+; RUN: opt -S -argpromotion %s | FileCheck %s
+; CHECK: define internal x86_thiscallcc void @internalfun(%struct.a* %this, <{ %struct.a
+
+target datalayout = "e-m:x-p:32:32-i64:64-f80:32-n8:16:32-a:0:32-S32"
+target triple = "i386-pc-windows-msvc19.11.0"
+
+%struct.a = type { i8 }
+
+define internal x86_thiscallcc void @internalfun(%struct.a* %this, <{ %struct.a }>* inalloca) {
+entry:
+  %a = getelementptr inbounds <{ %struct.a }>, <{ %struct.a }>* %0, i32 0, i32 0
+  %argmem = alloca inalloca <{ %struct.a }>, align 4
+  %1 = getelementptr inbounds <{ %struct.a }>, <{ %struct.a }>* %argmem, i32 0, i32 0
+  %call = call x86_thiscallcc %struct.a* @copy_ctor(%struct.a* %1, %struct.a* dereferenceable(1) %a)
+  call void @ext(<{ %struct.a }>* inalloca %argmem)
+  ret void
+}
+
+; This is here to ensure @internalfun is live.
+define void @exportedfun(%struct.a* %a) {
+  %inalloca.save = tail call i8* @llvm.stacksave()
+  %argmem = alloca inalloca <{ %struct.a }>, align 4
+  call x86_thiscallcc void @internalfun(%struct.a* %a, <{ %struct.a }>* inalloca %argmem)
+  call void @llvm.stackrestore(i8* %inalloca.save)
+  ret void
+}
+
+declare x86_thiscallcc %struct.a* @copy_ctor(%struct.a* returned, %struct.a* dereferenceable(1))
+declare void @ext(<{ %struct.a }>* inalloca)
+declare i8* @llvm.stacksave()
+declare void @llvm.stackrestore(i8*)