Rollup merge of #135025 - Flakebi:alloca-addrspace, r=nikic

Cast allocas to default address space

Pointers for variables all need to be in the same address space for correct compilation. Therefore ensure that even if an `alloca` is created in a different address space, it is casted to the default address space before its value is used.

This is necessary for the amdgpu target and others where the default address space for `alloca`s is not 0.

For example the following code compiles incorrectly when not casting the address space to the default one:

```rust
fn f(p: *const i8 /* addrspace(0) */) -> *const i8 /* addrspace(0) */ {
    let local = 0i8; /* addrspace(5) */
    let res = if cond { p } else { &raw const local };
    res
}
```

results in

```llvm
    %local = alloca addrspace(5) i8
    %res = alloca addrspace(5) ptr

if:
    ; Store 64-bit flat pointer
    store ptr %p, ptr addrspace(5) %res

else:
    ; Store 32-bit scratch pointer
    store ptr addrspace(5) %local, ptr addrspace(5) %res

ret:
    ; Load and return 64-bit flat pointer
    %res.load = load ptr, ptr addrspace(5) %res
    ret ptr %res.load
```

For amdgpu, `addrspace(0)` are 64-bit pointers, `addrspace(5)` are 32-bit pointers.
The above code may store a 32-bit pointer and read it back as a 64-bit pointer, which is obviously wrong and cannot work. Instead, we need to `addrspacecast %local to ptr addrspace(0)`, then we store and load the correct type.

Tracking issue: #135024

compiler/rustc_codegen_llvm/src/builder.rs

@@ -543,7 +543,8 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         unsafe {
             let alloca = llvm::LLVMBuildAlloca(bx.llbuilder, ty, UNNAMED);
             llvm::LLVMSetAlignment(alloca, align.bytes() as c_uint);
-            alloca
+            // Cast to default addrspace if necessary
+            llvm::LLVMBuildPointerCast(bx.llbuilder, alloca, self.cx().type_ptr(), UNNAMED)
         }
     }
 
@@ -552,7 +553,8 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
             let alloca =
                 llvm::LLVMBuildArrayAlloca(self.llbuilder, self.cx().type_i8(), size, UNNAMED);
             llvm::LLVMSetAlignment(alloca, align.bytes() as c_uint);
-            alloca
+            // Cast to default addrspace if necessary
+            llvm::LLVMBuildPointerCast(self.llbuilder, alloca, self.cx().type_ptr(), UNNAMED)
         }
     }