Stabilize fn_align: #[repr(align(N))] on functions and -Zmin-function-alignment

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -475,10 +475,10 @@ pub(crate) fn llfn_attrs_from_instance<'ll, 'tcx>(
         let allocated_pointer = AttributeKind::AllocatedPointer.create_attr(cx.llcx);
         attributes::apply_to_llfn(llfn, AttributePlace::Argument(0), &[allocated_pointer]);
     }
-    // function alignment can be set globally with the `-Zmin-function-alignment=<n>` flag;
+    // function alignment can be set globally with the `-Cmin-function-alignment=<n>` flag;
     // the alignment from a `#[repr(align(<n>))]` is used if it specifies a higher alignment.
     if let Some(align) =
-        Ord::max(cx.tcx.sess.opts.unstable_opts.min_function_alignment, codegen_fn_attrs.alignment)
+        Ord::max(cx.tcx.sess.opts.cg.min_function_alignment, codegen_fn_attrs.alignment)
     {
         llvm::set_alignment(llfn, align);
     }

compiler/rustc_codegen_ssa/src/mir/naked_asm.rs

@@ -121,10 +121,10 @@ fn prefix_and_suffix<'tcx>(
     let attrs = tcx.codegen_fn_attrs(instance.def_id());
     let link_section = attrs.link_section.map(|symbol| symbol.as_str().to_string());
 
-    // function alignment can be set globally with the `-Zmin-function-alignment=<n>` flag;
+    // Function alignment can be set globally with the `-Cmin-function-alignment=<n>` flag;
     // the alignment from a `#[repr(align(<n>))]` is used if it specifies a higher alignment.
     // if no alignment is specified, an alignment of 4 bytes is used.
-    let min_function_alignment = tcx.sess.opts.unstable_opts.min_function_alignment;
+    let min_function_alignment = tcx.sess.opts.cg.min_function_alignment;
     let align_bytes =
         Ord::max(min_function_alignment, attrs.alignment).map(|a| a.bytes()).unwrap_or(4);
 

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -875,10 +875,10 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         if let Some(fn_val) = self.get_fn_alloc(id) {
             let align = match fn_val {
                 FnVal::Instance(instance) => {
-                    // Function alignment can be set globally with the `-Zmin-function-alignment=<n>` flag;
+                    // Function alignment can be set globally with the `-Cmin-function-alignment=<n>` flag;
                     // the alignment from a `#[repr(align(<n>))]` is used if it specifies a higher alignment.
                     let fn_align = self.tcx.codegen_fn_attrs(instance.def_id()).alignment;
-                    let global_align = self.tcx.sess.opts.unstable_opts.min_function_alignment;
+                    let global_align = self.tcx.sess.opts.cg.min_function_alignment;
 
                     Ord::max(global_align, fn_align).unwrap_or(Align::ONE)
                 }

compiler/rustc_feature/src/accepted.rs

@@ -214,6 +214,8 @@ declare_features! (
     (accepted, f16c_target_feature, "1.68.0", Some(44839)),
     /// Allows field shorthands (`x` meaning `x: x`) in struct literal expressions.
     (accepted, field_init_shorthand, "1.17.0", Some(37340)),
+    /// Allows using `#[repr(align(...))]` on function items
+    (accepted, fn_align, "CURRENT_RUSTC_VERSION", Some(82232)),
     /// Allows `#[must_use]` on functions, and introduces must-use operators (RFC 1940).
     (accepted, fn_must_use, "1.27.0", Some(43302)),
     /// Allows capturing variables in scope using format_args!

compiler/rustc_feature/src/unstable.rs

@@ -496,8 +496,6 @@ declare_features! (
     (unstable, ffi_pure, "1.45.0", Some(58329)),
     /// Controlling the behavior of fmt::Debug
     (unstable, fmt_debug, "1.82.0", Some(129709)),
-    /// Allows using `#[repr(align(...))]` on function items
-    (unstable, fn_align, "1.53.0", Some(82232)),
     /// Support delegating implementation of functions to other already implemented functions.
     (incomplete, fn_delegation, "1.76.0", Some(118212)),
     /// Allows impls for the Freeze trait.

compiler/rustc_interface/src/tests.rs

@@ -624,6 +624,7 @@ fn test_codegen_options_tracking_hash() {
     tracked!(llvm_args, vec![String::from("1"), String::from("2")]);
     tracked!(lto, LtoCli::Fat);
     tracked!(metadata, vec![String::from("A"), String::from("B")]);
+    tracked!(min_function_alignment, Some(Align::EIGHT));
     tracked!(no_prepopulate_passes, true);
     tracked!(no_redzone, Some(true));
     tracked!(no_vectorize_loops, true);
@@ -815,7 +816,6 @@ fn test_unstable_options_tracking_hash() {
     tracked!(location_detail, LocationDetail { file: true, line: false, column: false });
     tracked!(maximal_hir_to_mir_coverage, true);
     tracked!(merge_functions, Some(MergeFunctions::Disabled));
-    tracked!(min_function_alignment, Some(Align::EIGHT));
     tracked!(mir_emit_retag, true);
     tracked!(mir_enable_passes, vec![("DestProp".to_string(), false)]);
     tracked!(mir_opt_level, Some(4));

compiler/rustc_passes/messages.ftl

@@ -629,9 +629,6 @@ passes_remove_fields =
      *[other] fields
     }
 
-passes_repr_align_function =
-    `repr(align)` attributes on functions are unstable
-
 passes_repr_align_greater_than_target_max =
     alignment must not be greater than `isize::MAX` bytes
     .note = `isize::MAX` is {$size} for the current target

compiler/rustc_passes/src/check_attr.rs

@@ -1975,17 +1975,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                 ReprAttr::ReprAlign(align) => {
                     match target {
                         Target::Struct | Target::Union | Target::Enum => {}
-                        Target::Fn | Target::Method(_) => {
-                            if !self.tcx.features().fn_align() {
-                                feature_err(
-                                    &self.tcx.sess,
-                                    sym::fn_align,
-                                    *repr_span,
-                                    fluent::passes_repr_align_function,
-                                )
-                                .emit();
-                            }
-                        }
+                        Target::Fn | Target::Method(_) => {}
                         _ => {
                             self.dcx().emit_err(
                                 errors::AttrApplication::StructEnumFunctionMethodUnion {
@@ -2049,15 +2039,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                     if item.is_some() {
                         match target {
                             Target::Struct | Target::Union | Target::Enum => continue,
-                            Target::Fn | Target::Method(_) => {
-                                feature_err(
-                                    &self.tcx.sess,
-                                    sym::fn_align,
-                                    *repr_span,
-                                    fluent::passes_repr_align_function,
-                                )
-                                .emit();
-                            }
+                            Target::Fn | Target::Method(_) => continue,
                             _ => {
                                 self.dcx().emit_err(
                                     errors::AttrApplication::StructEnumFunctionMethodUnion {

compiler/rustc_session/src/options.rs

@@ -2004,6 +2004,8 @@ options! {
         "perform LLVM link-time optimizations"),
     metadata: Vec<String> = (Vec::new(), parse_list, [TRACKED],
         "metadata to mangle symbol names with"),
+    min_function_alignment: Option<Align> = (None, parse_align, [TRACKED],
+        "align all functions to at least this many bytes. Must be a power of 2"),
     no_prepopulate_passes: bool = (false, parse_no_value, [TRACKED],
         "give an empty list of passes to the pass manager"),
     no_redzone: Option<bool> = (None, parse_opt_bool, [TRACKED],
@@ -2309,8 +2311,6 @@ options! {
         "gather metadata statistics (default: no)"),
     metrics_dir: Option<PathBuf> = (None, parse_opt_pathbuf, [UNTRACKED],
         "the directory metrics emitted by rustc are dumped into (implicitly enables default set of metrics)"),
-    min_function_alignment: Option<Align> = (None, parse_align, [TRACKED],
-        "align all functions to at least this many bytes. Must be a power of 2"),
     mir_emit_retag: bool = (false, parse_bool, [TRACKED],
         "emit Retagging MIR statements, interpreted e.g., by miri; implies -Zmir-opt-level=0 \
         (default: no)"),

src/doc/rustc/src/codegen-options/index.md

@@ -331,6 +331,28 @@ opt-level=0`](#opt-level)). That is:
 
 See also [linker-plugin-lto](#linker-plugin-lto) for cross-language LTO.
 
+## min-function-alignment
+
+The `-Cmin-function-alignment=<align>` flag specifies the minimum alignment of functions for which code is generated.
+The `align` value must be a power of 2, other values are rejected.
+
+Note that `-Zbuild-std` (or similar) is required to apply this minimum alignment to standard library functions.
+By default, these functions come precompiled and their alignments won't respect the `min-function-alignment` flag.
+
+This flag is equivalent to:
+
+- `-fmin-function-alignment` for [GCC](https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-fmin-function-alignment_003dn)
+- `-falign-functions` for [Clang](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang1-falign-functions)
+
+The specified alignment is a minimum. A higher alignment can be specified for specific functions by annotating the function with a `#[repr(align(<align>))]` attribute.
+The attribute's value is ignored when it is lower than the value passed to `min-function-alignment`.
+
+There are two additional edge cases for this flag:
+
+- targets have a minimum alignment for functions (e.g. on x86_64 the lowest that LLVM generates is 16 bytes).
+    A `min-function-alignment` value lower than the target's minimum has no effect.
+- the maximum alignment supported by rust (and LLVM) is `2^29`. Trying to set a higher value results in an error.
+
 ## metadata
 
 This option allows you to control the metadata used for symbol mangling. This

src/tools/miri/tests/pass/fn_align.rs

@@ -1,5 +1,4 @@
-//@compile-flags: -Zmin-function-alignment=8
-#![feature(fn_align)]
+//@compile-flags: -Cmin-function-alignment=8
 
 // When a function uses `repr(align(N))`, the function address should be a multiple of `N`.
 
@@ -16,6 +15,6 @@ fn main() {
     assert!((foo as usize).is_multiple_of(256));
     assert!((bar as usize).is_multiple_of(16));
 
-    // The maximum of `repr(align(N))` and `-Zmin-function-alignment=N` is used.
+    // The maximum of `repr(align(N))` and `-Cmin-function-alignment=N` is used.
     assert!((baz as usize).is_multiple_of(8));
 }

tests/assembly/naked-functions/aix.rs

@@ -9,7 +9,7 @@
 //@[aix] needs-llvm-components: powerpc
 
 #![crate_type = "lib"]
-#![feature(no_core, asm_experimental_arch, f128, linkage, fn_align)]
+#![feature(no_core, asm_experimental_arch, f128, linkage)]
 #![no_core]
 
 // tests that naked functions work for the `powerpc64-ibm-aix` target.

tests/assembly/naked-functions/wasm32.rs

@@ -9,7 +9,7 @@
 //@ [wasm32-wasip1] needs-llvm-components: webassembly
 
 #![crate_type = "lib"]
-#![feature(no_core, asm_experimental_arch, f128, linkage, fn_align)]
+#![feature(no_core, asm_experimental_arch, f128, linkage)]
 #![no_core]
 
 extern crate minicore;

tests/codegen/align-fn.rs

@@ -1,7 +1,6 @@
 //@ compile-flags: -C no-prepopulate-passes -Z mir-opt-level=0
 
 #![crate_type = "lib"]
-#![feature(fn_align)]
 
 // CHECK: align 16
 #[no_mangle]

tests/codegen/min-function-alignment.rs

@@ -1,10 +1,9 @@
 //@ revisions: align16 align1024
 //@ compile-flags: -C no-prepopulate-passes -Z mir-opt-level=0
-//@ [align16] compile-flags: -Zmin-function-alignment=16
-//@ [align1024] compile-flags: -Zmin-function-alignment=1024
+//@ [align16] compile-flags: -Cmin-function-alignment=16
+//@ [align1024] compile-flags: -Cmin-function-alignment=1024
 
 #![crate_type = "lib"]
-#![feature(fn_align)]
 
 // functions without explicit alignment use the global minimum
 //
@@ -33,7 +32,7 @@ pub fn higher_align() {}
 // cold functions follow the same rules as other functions
 //
 // in GCC, the `-falign-functions` does not apply to cold functions, but
-// `-Zmin-function-alignment` applies to all functions.
+// `-Cmin-function-alignment` applies to all functions.
 //
 // CHECK-LABEL: @no_explicit_align_cold
 // align16: align 16

tests/codegen/naked-fn/aligned.rs

@@ -3,7 +3,6 @@
 //@ ignore-arm no "ret" mnemonic
 
 #![crate_type = "lib"]
-#![feature(fn_align)]
 use std::arch::naked_asm;
 
 // CHECK: .balign 16

tests/codegen/naked-fn/min-function-alignment.rs

@@ -1,8 +1,7 @@
-//@ compile-flags: -C no-prepopulate-passes -Copt-level=0 -Zmin-function-alignment=16
+//@ compile-flags: -C no-prepopulate-passes -Copt-level=0 -Cmin-function-alignment=16
 //@ needs-asm-support
 //@ ignore-arm no "ret" mnemonic
 
-#![feature(fn_align)]
 #![crate_type = "lib"]
 
 // functions without explicit alignment use the global minimum
@@ -33,7 +32,7 @@ pub extern "C" fn naked_higher_align() {
 // cold functions follow the same rules as other functions
 //
 // in GCC, the `-falign-functions` does not apply to cold functions, but
-// `-Zmin-function-alignment` applies to all functions.
+// `-Cmin-function-alignment` applies to all functions.
 //
 // CHECK: .balign 16
 #[no_mangle]

tests/ui/asm/naked-with-invalid-repr-attr.rs

@@ -1,5 +1,4 @@
 //@ needs-asm-support
-#![feature(fn_align)]
 #![crate_type = "lib"]
 use std::arch::naked_asm;
 

tests/ui/asm/naked-with-invalid-repr-attr.stderr

@@ -1,5 +1,5 @@
 error[E0517]: attribute should be applied to a struct, enum, or union
-  --> $DIR/naked-with-invalid-repr-attr.rs:6:8
+  --> $DIR/naked-with-invalid-repr-attr.rs:5:8
    |
 LL |   #[repr(C)]
    |          ^
@@ -11,7 +11,7 @@ LL | | }
    | |_- not a struct, enum, or union
 
 error[E0517]: attribute should be applied to a struct, enum, or union
-  --> $DIR/naked-with-invalid-repr-attr.rs:14:8
+  --> $DIR/naked-with-invalid-repr-attr.rs:13:8
    |
 LL |   #[repr(transparent)]
    |          ^^^^^^^^^^^
@@ -23,7 +23,7 @@ LL | | }
    | |_- not a struct, enum, or union
 
 error[E0517]: attribute should be applied to a struct, enum, or union
-  --> $DIR/naked-with-invalid-repr-attr.rs:22:19
+  --> $DIR/naked-with-invalid-repr-attr.rs:21:19
    |
 LL |   #[repr(align(16), C)]
    |                     ^
@@ -35,7 +35,7 @@ LL | | }
    | |_- not a struct, enum, or union
 
 error[E0517]: attribute should be applied to a struct, enum, or union
-  --> $DIR/naked-with-invalid-repr-attr.rs:31:8
+  --> $DIR/naked-with-invalid-repr-attr.rs:30:8
    |
 LL |   #[repr(C, packed)]
    |          ^
@@ -48,7 +48,7 @@ LL | | }
    | |_- not a struct, enum, or union
 
 error[E0517]: attribute should be applied to a struct or union
-  --> $DIR/naked-with-invalid-repr-attr.rs:31:11
+  --> $DIR/naked-with-invalid-repr-attr.rs:30:11
    |
 LL |   #[repr(C, packed)]
    |             ^^^^^^
@@ -61,7 +61,7 @@ LL | | }
    | |_- not a struct or union
 
 error[E0517]: attribute should be applied to an enum
-  --> $DIR/naked-with-invalid-repr-attr.rs:41:8
+  --> $DIR/naked-with-invalid-repr-attr.rs:40:8
    |
 LL |   #[repr(u8)]
    |          ^^

tests/ui/attributes/malformed-fn-align.rs

@@ -1,7 +1,9 @@
-#![feature(fn_align)]
 #![crate_type = "lib"]
 
 trait MyTrait {
     #[repr(align)] //~ ERROR invalid `repr(align)` attribute: `align` needs an argument
-    fn myfun();
+    fn myfun1();
+
+    #[repr(align(1, 2))] //~ ERROR incorrect `repr(align)` attribute format: `align` takes exactly one argument in parentheses
+    fn myfun2();
 }

tests/ui/attributes/malformed-fn-align.stderr

@@ -1,9 +1,16 @@
 error[E0589]: invalid `repr(align)` attribute: `align` needs an argument
-  --> $DIR/malformed-fn-align.rs:5:12
+  --> $DIR/malformed-fn-align.rs:4:12
    |
 LL |     #[repr(align)]
    |            ^^^^^ help: supply an argument here: `align(...)`
 
-error: aborting due to 1 previous error
+error[E0693]: incorrect `repr(align)` attribute format: `align` takes exactly one argument in parentheses
+  --> $DIR/malformed-fn-align.rs:7:12
+   |
+LL |     #[repr(align(1, 2))]
+   |            ^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0589`.
+Some errors have detailed explanations: E0589, E0693.
+For more information about an error, try `rustc --explain E0589`.