Rollup merge of #105578 - erikdesjardins:addrspacecast, r=bjorn3

Fix transmutes between pointers in different address spaces (e.g. fn ptrs on AVR)

Currently, this causes a verifier error (https://godbolt.org/z/YYohed4bj), since it uses `bitcast`, which can't convert between address spaces.

Uncovered due to #105545 (comment)

r? `@bjorn3`

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -1802,15 +1802,20 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         match (src.layout.abi, dst.layout.abi) {
             (abi::Abi::Scalar(src_scalar), abi::Abi::Scalar(dst_scalar)) => {
                 // HACK(eddyb) LLVM doesn't like `bitcast`s between pointers and non-pointers.
-                if (src_scalar.primitive() == abi::Pointer)
-                    == (dst_scalar.primitive() == abi::Pointer)
-                {
+                let src_is_ptr = src_scalar.primitive() == abi::Pointer;
+                let dst_is_ptr = dst_scalar.primitive() == abi::Pointer;
+                if src_is_ptr == dst_is_ptr {
                     assert_eq!(src.layout.size, dst.layout.size);
 
                     // NOTE(eddyb) the `from_immediate` and `to_immediate_scalar`
                     // conversions allow handling `bool`s the same as `u8`s.
                     let src = bx.from_immediate(src.immediate());
-                    let src_as_dst = bx.bitcast(src, bx.backend_type(dst.layout));
+                    // LLVM also doesn't like `bitcast`s between pointers in different address spaces.
+                    let src_as_dst = if src_is_ptr {
+                        bx.pointercast(src, bx.backend_type(dst.layout))
+                    } else {
+                        bx.bitcast(src, bx.backend_type(dst.layout))
+                    };
                     Immediate(bx.to_immediate_scalar(src_as_dst, dst_scalar)).store(bx, dst);
                     return;
                 }

src/test/codegen/avr/avr-func-addrspace.rs

@@ -9,7 +9,7 @@
 // It also validates that functions can be called through function pointers
 // through traits.
 
-#![feature(no_core, lang_items, unboxed_closures, arbitrary_self_types)]
+#![feature(no_core, lang_items, intrinsics, unboxed_closures, arbitrary_self_types)]
 #![crate_type = "lib"]
 #![no_core]
 
@@ -49,6 +49,10 @@ pub trait Fn<Args: Tuple>: FnOnce<Args> {
     extern "rust-call" fn call(&self, args: Args) -> Self::Output;
 }
 
+extern "rust-intrinsic" {
+    pub fn transmute<Src, Dst>(src: Src) -> Dst;
+}
+
 pub static mut STORAGE_FOO: fn(&usize, &mut u32) -> Result<(), ()> = arbitrary_black_box;
 pub static mut STORAGE_BAR: u32 = 12;
 
@@ -87,3 +91,21 @@ pub extern "C" fn test() {
         STORAGE_FOO(&1, &mut buf);
     }
 }
+
+// Validate that we can codegen transmutes between data ptrs and fn ptrs.
+
+// CHECK: define{{.+}}{{void \(\) addrspace\(1\)\*|ptr addrspace\(1\)}} @transmute_data_ptr_to_fn({{\{\}\*|ptr}}{{.*}} %x)
+#[no_mangle]
+pub unsafe fn transmute_data_ptr_to_fn(x: *const ()) -> fn() {
+    // It doesn't matter precisely how this is codegenned (through memory or an addrspacecast),
+    // as long as it doesn't cause a verifier error by using `bitcast`.
+    transmute(x)
+}
+
+// CHECK: define{{.+}}{{\{\}\*|ptr}} @transmute_fn_ptr_to_data({{void \(\) addrspace\(1\)\*|ptr addrspace\(1\)}}{{.*}} %x)
+#[no_mangle]
+pub unsafe fn transmute_fn_ptr_to_data(x: fn()) -> *const () {
+    // It doesn't matter precisely how this is codegenned (through memory or an addrspacecast),
+    // as long as it doesn't cause a verifier error by using `bitcast`.
+    transmute(x)
+}