Rollup merge of rust-lang#117744 - quininer:add-z-sync-uw, r=bjorn3

Add -Zuse-sync-unwind

Currently Rust uses async unwind by default, but async unwind will bring non-negligible size overhead. it would be nice to allow users to choose this.

In addition, async unwind currently prevents LLVM from generate compact unwind for MachO, if one wishes to generate compact unwind for MachO, then also needs this flag.

Author: matthiaskrgr

compiler/rustc_codegen_llvm/src/allocator.rs

@@ -134,7 +134,8 @@ fn create_wrapper_function(
             llvm::LLVMRustSetVisibility(llfn, llvm::Visibility::Hidden);
         }
         if tcx.sess.must_emit_unwind_tables() {
-            let uwtable = attributes::uwtable_attr(llcx);
+            let uwtable =
+                attributes::uwtable_attr(llcx, tcx.sess.opts.unstable_opts.use_sync_unwind);
             attributes::apply_to_llfn(llfn, llvm::AttributePlace::Function, &[uwtable]);
         }
 

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -95,11 +95,12 @@ pub fn sanitize_attrs<'ll>(
 
 /// Tell LLVM to emit or not emit the information necessary to unwind the stack for the function.
 #[inline]
-pub fn uwtable_attr(llcx: &llvm::Context) -> &Attribute {
+pub fn uwtable_attr(llcx: &llvm::Context, use_sync_unwind: Option<bool>) -> &Attribute {
     // NOTE: We should determine if we even need async unwind tables, as they
     // take have more overhead and if we can use sync unwind tables we
     // probably should.
-    llvm::CreateUWTableAttr(llcx, true)
+    let async_unwind = !use_sync_unwind.unwrap_or(false);
+    llvm::CreateUWTableAttr(llcx, async_unwind)
 }
 
 pub fn frame_pointer_type_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> Option<&'ll Attribute> {
@@ -333,7 +334,7 @@ pub fn from_fn_attrs<'ll, 'tcx>(
     // You can also find more info on why Windows always requires uwtables here:
     //      https://bugzilla.mozilla.org/show_bug.cgi?id=1302078
     if cx.sess().must_emit_unwind_tables() {
-        to_add.push(uwtable_attr(cx.llcx));
+        to_add.push(uwtable_attr(cx.llcx, cx.sess().opts.unstable_opts.use_sync_unwind));
     }
 
     if cx.sess().opts.unstable_opts.profile_sample_use.is_some() {

compiler/rustc_session/src/options.rs

@@ -1960,6 +1960,8 @@ written to standard error output)"),
         "adds unstable command line options to rustc interface (default: no)"),
     use_ctors_section: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "use legacy .ctors section for initializers rather than .init_array"),
+    use_sync_unwind: Option<bool> = (None, parse_opt_bool, [TRACKED],
+        "Generate sync unwind tables instead of async unwind tables (default: no)"),
     validate_mir: bool = (false, parse_bool, [UNTRACKED],
         "validate MIR after each transformation"),
     #[rustc_lint_opt_deny_field_access("use `Session::verbose_internals` instead of this field")]