Index: clang/lib/CodeGen/CGBuiltin.cpp
===================================================================
--- clang/lib/CodeGen/CGBuiltin.cpp
+++ clang/lib/CodeGen/CGBuiltin.cpp
@@ -3514,6 +3514,12 @@
     return RValue::get(Result);
   }
 
+  // An alloca will always return a pointer to the alloca (stack) address
+  // space. This address space need not be the same as the AST / Language
+  // default (e.g. in C / C++ auto vars are in the generic address space). At
+  // the AST level this is handled within CreateTempAlloca et al., but for the
+  // builtin / dynamic alloca we have to handle it here. We use an explicit cast
+  // instead of passing an AS to CreateAlloca so as to not inhibit optimisation.
   case Builtin::BIalloca:
   case Builtin::BI_alloca:
   case Builtin::BI__builtin_alloca_uninitialized:
@@ -3529,6 +3535,8 @@
     AI->setAlignment(SuitableAlignmentInBytes);
     if (BuiltinID != Builtin::BI__builtin_alloca_uninitialized)
       initializeAlloca(*this, AI, Size, SuitableAlignmentInBytes);
+    if (getASTAllocaAddressSpace() != LangAS::Default)
+      return RValue::get(Builder.CreateAddrSpaceCast(AI, CGM.Int8PtrTy));
     return RValue::get(AI);
   }
 
@@ -3544,6 +3552,8 @@
     AI->setAlignment(AlignmentInBytes);
     if (BuiltinID != Builtin::BI__builtin_alloca_with_align_uninitialized)
       initializeAlloca(*this, AI, Size, AlignmentInBytes);
+    if (getASTAllocaAddressSpace() != LangAS::Default)
+      return RValue::get(Builder.CreateAddrSpaceCast(AI, CGM.Int8PtrTy));
     return RValue::get(AI);
   }
 
Index: clang/test/CodeGen/dynamic-alloca-with-address-space.c
===================================================================
--- /dev/null
+++ clang/test/CodeGen/dynamic-alloca-with-address-space.c
@@ -0,0 +1,28 @@
+// RUN: %clang_cc1 -triple amdgcn-amd-amdhsa -emit-llvm %s -o - | FileCheck %s
+
+void allocas(unsigned long n) {
+    char *a = (char *)__builtin_alloca(n);
+    char *uninitialized_a = (char *)__builtin_alloca_uninitialized(n);
+    char *aligned_a = (char *)__builtin_alloca_with_align(n, 8);
+    char *aligned_uninitialized_a = (char *)__builtin_alloca_with_align_uninitialized(n, 8);
+}
+
+// CHECK: @allocas(
+// CHECK: store i64 %n, ptr %n.addr.ascast, align 8
+// CHECK: %0 = load i64, ptr %n.addr.ascast, align 8
+// CHECK: %1 = alloca i8, i64 %0, align 8, addrspace(5)
+// CHECK: %2 = addrspacecast ptr addrspace(5) %1 to ptr
+// CHECK: store ptr %2, ptr %a.ascast, align 8
+// CHECK: %3 = load i64, ptr %n.addr.ascast, align 8
+// CHECK: %4 = alloca i8, i64 %3, align 8, addrspace(5)
+// CHECK: %5 = addrspacecast ptr addrspace(5) %4 to ptr
+// CHECK: store ptr %5, ptr %uninitialized_a.ascast, align 8
+// CHECK: %6 = load i64, ptr %n.addr.ascast, align 8
+// CHECK: %7 = alloca i8, i64 %6, align 1, addrspace(5)
+// CHECK: %8 = addrspacecast ptr addrspace(5) %7 to ptr
+// CHECK: store ptr %8, ptr %aligned_a.ascast, align 8
+// CHECK: %9 = load i64, ptr %n.addr.ascast, align 8
+// CHECK: %10 = alloca i8, i64 %9, align 1, addrspace(5)
+// CHECK: %11 = addrspacecast ptr addrspace(5) %10 to ptr
+// CHECK: store ptr %11, ptr %aligned_uninitialized_a.ascast, align 8
+// CHECK: ret void