Stabilize -Cmin-function-alignment

compiler/rustc_codegen_cranelift/src/lib.rs

@@ -309,7 +309,7 @@ fn build_isa(sess: &Session, jit: bool) -> Arc<dyn TargetIsa + 'static> {
 
     flags_builder.set("enable_llvm_abi_extensions", "true").unwrap();
 
-    if let Some(align) = sess.opts.unstable_opts.min_function_alignment {
+    if let Some(align) = sess.opts.cg.min_function_alignment {
         flags_builder
             .set("log2_min_function_alignment", &align.bytes().ilog2().to_string())
             .unwrap();

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -450,7 +450,7 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
 
     // Apply the minimum function alignment here, so that individual backends don't have to.
     codegen_fn_attrs.alignment =
-        Ord::max(codegen_fn_attrs.alignment, tcx.sess.opts.unstable_opts.min_function_alignment);
+        Ord::max(codegen_fn_attrs.alignment, tcx.sess.opts.cg.min_function_alignment);
 
     let inline_span;
     (codegen_fn_attrs.inline, inline_span) = if let Some((inline_attr, span)) =

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

@@ -811,7 +811,7 @@ mod desc {
     pub(crate) const parse_wasm_c_abi: &str = "`spec`";
     pub(crate) const parse_mir_include_spans: &str =
         "either a boolean (`yes`, `no`, `on`, `off`, etc), or `nll` (default: `nll`)";
-    pub(crate) const parse_align: &str = "a number that is a power of 2 between 1 and 2^29";
+    pub(crate) const parse_align: &str = "a number that is a power of 2 between 1 and 8192";
 }
 
 pub mod parse {
@@ -1925,6 +1925,12 @@ pub mod parse {
             return false;
         }
 
+        // Limit the alignment to 8192 (i.e. 0x2000, or 1 << 13) bytes. It is the highest function
+        // alignment that works on all target platforms. COFF does not support higher alignments.
+        if bytes > 8192 {
+            return false;
+        }
+
         let Ok(align) = Align::from_bytes(bytes) else {
             return false;
         };
@@ -2014,6 +2020,8 @@ 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],
@@ -2327,8 +2335,6 @@ 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 this flag is `8192`. 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

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

tests/codegen/min-function-alignment.rs

@@ -1,7 +1,7 @@
 //@ revisions: align16 align1024
 //@ compile-flags: -C no-prepopulate-passes -Z mir-opt-level=0 -Clink-dead-code
-//@ [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)]
@@ -35,7 +35,7 @@ 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/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
 
@@ -33,7 +33,7 @@ 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/invalid-compile-flags/min-function-alignment.not-power-of-2.stderr

@@ -0,0 +1,2 @@
+error: incorrect value `3` for codegen option `min-function-alignment` - a number that is a power of 2 between 1 and 8192 was expected
+

tests/ui/invalid-compile-flags/min-function-alignment.rs

@@ -0,0 +1,6 @@
+//@ revisions: too-high not-power-of-2
+//
+//@ [too-high] compile-flags: -Cmin-function-alignment=16384
+//@ [not-power-of-2] compile-flags: -Cmin-function-alignment=3
+
+//~? ERROR a number that is a power of 2 between 1 and 8192 was expected

tests/ui/invalid-compile-flags/min-function-alignment.too-high.stderr

@@ -0,0 +1,2 @@
+error: incorrect value `16384` for codegen option `min-function-alignment` - a number that is a power of 2 between 1 and 8192 was expected
+