stabilize -Cmin-function-aligmnemt

Author: folkertdev

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -491,10 +491,10 @@ 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

@@ -131,10 +131,10 @@ 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

@@ -877,10 +877,10 @@ 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_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);
@@ -818,7 +819,6 @@ 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_session/src/options.rs

@@ -2022,6 +2022,8 @@ 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],
@@ -2335,8 +2337,6 @@ 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

@@ -346,6 +346,28 @@ 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 `#[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/doc/unstable-book/src/compiler-flags/min-function-alignment.md

@@ -1,4 +1,4 @@
-//@compile-flags: -Zmin-function-alignment=8
+//@compile-flags: -Cmin-function-alignment=8
 
 // When a function uses `align(N)`, the function address should be a multiple of `N`.
 

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

@@ -1,7 +1,7 @@
 //@ 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"]
 
@@ -32,7 +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/min-function-alignment.rs

@@ -1,4 +1,4 @@
-//@ 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
 
@@ -32,7 +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]