Ensure functions are aligned properly on AArch64 (#3908)

Previously (as in an hour ago) #3905 landed a new ability for fuzzing to
arbitrarily insert padding between functions. Running some fuzzers
locally though this instantly hit a lot of problems on AArch64 because
the arbitrary padding isn't aligned to 4 bytes like all other functions
are. To fix this issue appending functions now correctly aligns the
output as appropriate for the platform. The alignment argument for
appending was switched to `None` where `None` means "use the platform
default" and otherwise and explicit alignment can be specified for
inserting other data (like arbitrary padding or Windows unwind tables).

cranelift/codegen/src/machinst/buffer.rs

@@ -1616,7 +1616,7 @@ impl<I: VCodeInst> MachTextSectionBuilder<I> {
 }
 
 impl<I: VCodeInst> TextSectionBuilder for MachTextSectionBuilder<I> {
-    fn append(&mut self, named: bool, func: &[u8], align: u32) -> u64 {
+    fn append(&mut self, named: bool, func: &[u8], align: Option<u32>) -> u64 {
         // Conditionally emit an island if it's necessary to resolve jumps
         // between functions which are too far away.
         let size = func.len() as u32;
@@ -1624,7 +1624,7 @@ impl<I: VCodeInst> TextSectionBuilder for MachTextSectionBuilder<I> {
             self.buf.emit_island_maybe_forced(self.force_veneers, size);
         }
 
-        self.buf.align_to(align);
+        self.buf.align_to(align.unwrap_or(I::LabelUse::ALIGN));
         let pos = self.buf.cur_offset();
         if named {
             self.buf.bind_label(MachLabel::from_block(self.next_func));

cranelift/codegen/src/machinst/mod.rs

@@ -370,7 +370,7 @@ pub trait TextSectionBuilder {
     ///
     /// This function returns the offset at which the data was placed in the
     /// text section.
-    fn append(&mut self, labeled: bool, data: &[u8], align: u32) -> u64;
+    fn append(&mut self, labeled: bool, data: &[u8], align: Option<u32>) -> u64;
 
     /// Attempts to resolve a relocation for this function.
     ///

crates/cranelift/src/compiler.rs

@@ -264,9 +264,11 @@ impl wasmtime_environ::Compiler for Compiler {
             traps.push(range.clone(), &func.traps);
             func_starts.push(range.start);
             if self.linkopts.padding_between_functions > 0 {
-                builder
+                builder.text.append(
-                    .text
+                    false,
-                    .append(false, &vec![0; self.linkopts.padding_between_functions], 1);
+                    &vec![0; self.linkopts.padding_between_functions],
+                    Some(1),
+                );
             }
         }
 

crates/cranelift/src/obj.rs

@@ -184,12 +184,12 @@ impl<'a> ObjectBuilder<'a> {
     /// that the function resides within the text section.
     fn append_func(
         &mut self,
-        wat: bool,
+        labeled: bool,
         name: Vec<u8>,
         func: &'a CompiledFunction,
     ) -> (SymbolId, Range<u64>) {
         let body_len = func.body.len() as u64;
-        let off = self.text.append(wat, &func.body, 1);
+        let off = self.text.append(labeled, &func.body, None);
 
         let symbol_id = self.obj.add_symbol(Symbol {
             name,
@@ -215,7 +215,7 @@ impl<'a> ObjectBuilder<'a> {
                 let unwind_size = info.emit_size();
                 let mut unwind_info = vec![0; unwind_size];
                 info.emit(&mut unwind_info);
-                let unwind_off = self.text.append(false, &unwind_info, 4);
+                let unwind_off = self.text.append(false, &unwind_info, Some(4));
                 self.windows_unwind_info.push(RUNTIME_FUNCTION {
                     begin: u32::try_from(off).unwrap(),
                     end: u32::try_from(off + body_len).unwrap(),