Rollup merge of #142854 - folkertdev:centralize-min-function-alignment, r=workingjubilee

centralize `-Zmin-function-alignment` logic

tracking issue: #82232
discussed in: #142824 (comment)

Apply the `-Zmin-function-alignment` value to the alignment field of the function attributes when those are created, so that individual backends don't need to consider it.

The one exception right now is cranelift, because it can't yet set the alignment for individual functions, but it can (and does) set the global minimum function alignment.

cc ``@RalfJung`` I think this is an improvement regardless, is there anything else that should be done for miri?

Author: matthiaskrgr

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -491,11 +491,7 @@ 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;
-    // 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)
-    {
+    if let Some(align) = codegen_fn_attrs.alignment {
         llvm::set_alignment(llfn, align);
     }
     if let Some(backchain) = backchain_attr(cx) {

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -145,6 +145,12 @@ 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,
+        );
+
         let Some(Ident { name, .. }) = attr.ident() else {
             continue;
         };

compiler/rustc_codegen_ssa/src/mir/naked_asm.rs

@@ -131,12 +131,8 @@ 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;
-    // 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 align_bytes =
-        Ord::max(min_function_alignment, attrs.alignment).map(|a| a.bytes()).unwrap_or(4);
+    // If no alignment is specified, an alignment of 4 bytes is used.
+    let align_bytes = attrs.alignment.map(|a| a.bytes()).unwrap_or(4);
 
     // In particular, `.arm` can also be written `.code 32` and `.thumb` as `.code 16`.
     let (arch_prefix, arch_suffix) = if is_arm {

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -877,12 +877,7 @@ 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;
-                    // 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;
-
-                    Ord::max(global_align, fn_align).unwrap_or(Align::ONE)
+                    self.tcx.codegen_fn_attrs(instance.def_id()).alignment.unwrap_or(Align::ONE)
                 }
                 // Machine-specific extra functions currently do not support alignment restrictions.
                 FnVal::Other(_) => Align::ONE,