add -Zmin-function-alignment

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -480,7 +480,11 @@ 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]);
     }
-    if let Some(align) = codegen_fn_attrs.alignment {
+    // function alignment can be set globally with the `-Zmin-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)
+    {
         llvm::set_alignment(llfn, align);
     }
     if let Some(backchain) = backchain_attr(cx) {

compiler/rustc_interface/src/tests.rs

@@ -25,6 +25,7 @@ use rustc_span::edition::{DEFAULT_EDITION, Edition};
 use rustc_span::source_map::{RealFileLoader, SourceMapInputs};
 use rustc_span::symbol::sym;
 use rustc_span::{FileName, SourceFileHashAlgorithm};
+use rustc_target::abi::Align;
 use rustc_target::spec::{
     CodeModel, FramePointer, LinkerFlavorCli, MergeFunctions, OnBrokenPipe, PanicStrategy,
     RelocModel, RelroLevel, SanitizerSet, SplitDebuginfo, StackProtector, TlsModel, WasmCAbi,
@@ -803,6 +804,7 @@ 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_keep_place_mention, true);

compiler/rustc_session/src/config.rs

@@ -2884,6 +2884,7 @@ pub(crate) mod dep_tracking {
     use std::num::NonZero;
     use std::path::PathBuf;
 
+    use rustc_abi::Align;
     use rustc_data_structures::fx::FxIndexMap;
     use rustc_data_structures::stable_hasher::Hash64;
     use rustc_errors::LanguageIdentifier;
@@ -3003,6 +3004,7 @@ pub(crate) mod dep_tracking {
         InliningThreshold,
         FunctionReturn,
         WasmCAbi,
+        Align,
     );
 
     impl<T1, T2> DepTrackingHash for (T1, T2)

compiler/rustc_session/src/options.rs

@@ -4,6 +4,7 @@ use std::num::{IntErrorKind, NonZero};
 use std::path::PathBuf;
 use std::str;
 
+use rustc_abi::Align;
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::profiling::TimePassesFormat;
 use rustc_data_structures::stable_hasher::Hash64;
@@ -453,6 +454,7 @@ mod desc {
     pub(crate) const parse_wasm_c_abi: &str = "`legacy` or `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 mod parse {
@@ -1521,6 +1523,21 @@ pub mod parse {
 
         true
     }
+
+    pub(crate) fn parse_align(slot: &mut Option<Align>, v: Option<&str>) -> bool {
+        let mut bytes = 0u64;
+        if !parse_number(&mut bytes, v) {
+            return false;
+        }
+
+        let Ok(align) = Align::from_bytes(bytes) else {
+            return false;
+        };
+
+        *slot = Some(align);
+
+        true
+    }
 }
 
 options! {
@@ -1876,6 +1893,8 @@ 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)"),

tests/codegen/min-function-alignment.rs

@@ -0,0 +1,30 @@
+//@ compile-flags: -C no-prepopulate-passes -Z mir-opt-level=0 -Zmin-function-alignment=16
+
+#![crate_type = "lib"]
+#![feature(fn_align)]
+
+// functions without explicit alignment use the global minimum
+//
+// CHECK: align 16
+#[no_mangle]
+pub fn no_explicit_align() {}
+
+// CHECK: align 16
+#[no_mangle]
+#[repr(align(8))]
+pub fn lower_align() {}
+
+// CHECK: align 32
+#[no_mangle]
+#[repr(align(32))]
+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.
+//
+// CHECK: align 16
+#[no_mangle]
+#[cold]
+pub fn no_explicit_align_cold() {}