Index: llvm/lib/Target/X86/X86WinAllocaExpander.cpp
===================================================================
--- llvm/lib/Target/X86/X86WinAllocaExpander.cpp
+++ llvm/lib/Target/X86/X86WinAllocaExpander.cpp
@@ -211,13 +211,13 @@
 
   bool Is64Bit = STI->is64Bit();
   assert(SlotSize == 4 || SlotSize == 8);
-  unsigned RegA = (SlotSize == 8) ? X86::RAX : X86::EAX;
 
   switch (L) {
-  case TouchAndSub:
+  case TouchAndSub: {
     assert(Amount >= SlotSize);
 
     // Use a push to touch the top of the stack.
+    unsigned RegA = Is64Bit ? X86::RAX : X86::EAX;
     BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::PUSH64r : X86::PUSH32r))
         .addReg(RegA, RegState::Undef);
     Amount -= SlotSize;
@@ -226,10 +226,12 @@
 
     // Fall through to make any remaining adjustment.
     LLVM_FALLTHROUGH;
+  }
   case Sub:
     assert(Amount > 0);
     if (Amount == SlotSize) {
       // Use push to save size.
+      unsigned RegA = Is64Bit ? X86::RAX : X86::EAX;
       BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::PUSH64r : X86::PUSH32r))
           .addReg(RegA, RegState::Undef);
     } else {
@@ -241,7 +243,11 @@
     break;
   case Probe:
     if (!NoStackArgProbe) {
-      // The probe lowering expects the amount in RAX/EAX.
+      // The probe lowering expects the amount in RAX/EAX. Caveat: On the x32
+      // ABI the alloca size will be in a 32-bit register, even though the
+      // target is 64-bit.
+      bool Is64BitAlloca = MI->getOpcode() == X86::WIN_ALLOCA_64;
+      unsigned RegA = Is64BitAlloca ? X86::RAX : X86::EAX;
       BuildMI(*MBB, MI, DL, TII->get(TargetOpcode::COPY), RegA)
           .addReg(MI->getOperand(0).getReg());
 
Index: llvm/test/CodeGen/X86/pr41477.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/X86/pr41477.ll
@@ -0,0 +1,23 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnux32 | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnux32"
+
+define void @test(i32 %size) nounwind #0 {
+; CHECK-LABEL: test:
+; CHECK:       # %bb.0: # %start
+; CHECK-NEXT:    pushq %rbp
+; CHECK-NEXT:    movl %esp, %ebp
+; CHECK-NEXT:    # kill: def $edi killed $edi def $rdi
+; CHECK-NEXT:    leal 15(%rdi), %eax
+; CHECK-NEXT:    andl $-16, %eax
+; CHECK-NEXT:    # kill: def $eax killed $eax def $rax
+; CHECK-NEXT:    callq __rust_probestack
+; CHECK-NEXT:    subq %rax, %rsp
+start:
+  %unsized_tmp = alloca i8, i32 %size
+  unreachable
+}
+
+attributes #0 = { "probe-stack"="__rust_probestack" }