Add hotpatch flag. Proof of concept only working on x86/x64.

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -370,6 +370,15 @@ pub fn from_fn_attrs<'ll, 'tcx>(
         to_add.push(llvm::CreateAttrString(cx.llcx, "use-sample-profile"));
     }
 
+    // patchable-function is only implemented on x86 on LLVM
+    if cx.sess().opts.unstable_opts.hotpatch && cx.sess().target.is_x86() {
+        to_add.push(llvm::CreateAttrStringValue(
+            cx.llcx,
+            "patchable-function",
+            "prologue-short-redirect",
+        ));
+    }
+
     // FIXME: none of these functions interact with source level attributes.
     to_add.extend(frame_pointer_type_attr(cx));
     to_add.extend(function_return_attr(cx));

compiler/rustc_codegen_llvm/src/back/owned_target_machine.rs

@@ -36,6 +36,7 @@ impl OwnedTargetMachine {
         emit_stack_size_section: bool,
         relax_elf_relocations: bool,
         use_init_array: bool,
+        use_hotpatch: bool,
         split_dwarf_file: &CStr,
         output_obj_file: &CStr,
         debug_info_compression: &CStr,
@@ -68,6 +69,7 @@ impl OwnedTargetMachine {
                 emit_stack_size_section,
                 relax_elf_relocations,
                 use_init_array,
+                use_hotpatch,
                 split_dwarf_file.as_ptr(),
                 output_obj_file.as_ptr(),
                 debug_info_compression.as_ptr(),

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -221,6 +221,10 @@ pub fn target_machine_factory(
     let use_init_array =
         !sess.opts.unstable_opts.use_ctors_section.unwrap_or(sess.target.use_ctors_section);
 
+    // this annotes in the debug file that code is hotpatchable. In addtion without it -functionpadmin will be ignored.
+    // See: https://github.com/llvm/llvm-project/blob/d703b922961e0d02a5effdd4bfbb23ad50a3cc9f/lld/COFF/Writer.cpp#L1298
+    let use_hotpatch = sess.opts.unstable_opts.hotpatch && sess.target.is_x86();
+
     let path_mapping = sess.source_map().path_mapping().clone();
 
     let use_emulated_tls = matches!(sess.tls_model(), TlsModel::Emulated);
@@ -293,6 +297,7 @@ pub fn target_machine_factory(
             emit_stack_size_section,
             relax_elf_relocations,
             use_init_array,
+            use_hotpatch,
             &split_dwarf_file,
             &output_obj_file,
             &debuginfo_compression,

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2189,6 +2189,7 @@ extern "C" {
         EmitStackSizeSection: bool,
         RelaxELFRelocations: bool,
         UseInitArray: bool,
+        UseHotpatch: bool,
         SplitDwarfFile: *const c_char,
         OutputObjFile: *const c_char,
         DebugInfoCompression: *const c_char,

compiler/rustc_interface/src/tests.rs

@@ -787,6 +787,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(fuel, Some(("abc".to_string(), 99)));
     tracked!(function_return, FunctionReturn::ThunkExtern);
     tracked!(function_sections, Some(false));
+    tracked!(hotpatch, true);
     tracked!(human_readable_cgu_names, true);
     tracked!(incremental_ignore_spans, true);
     tracked!(inline_in_all_cgus, Some(true));

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -408,7 +408,7 @@ extern "C" LLVMTargetMachineRef LLVMRustCreateTargetMachine(
     LLVMRustCodeGenOptLevel RustOptLevel, bool UseSoftFloat,
     bool FunctionSections, bool DataSections, bool UniqueSectionNames,
     bool TrapUnreachable, bool Singlethread, bool AsmComments,
-    bool EmitStackSizeSection, bool RelaxELFRelocations, bool UseInitArray,
+    bool EmitStackSizeSection, bool RelaxELFRelocations, bool UseInitArray, bool UseHotpatch,
     const char *SplitDwarfFile, const char *OutputObjFile,
     const char *DebugInfoCompression, bool UseEmulatedTls,
     const char *ArgsCstrBuff, size_t ArgsCstrBuffLen) {
@@ -438,6 +438,7 @@ extern "C" LLVMTargetMachineRef LLVMRustCreateTargetMachine(
   Options.MCOptions.AsmVerbose = AsmComments;
   Options.MCOptions.PreserveAsmComments = AsmComments;
   Options.MCOptions.ABIName = ABIStr;
+  Options.Hotpatch = UseHotpatch;
   if (SplitDwarfFile) {
     Options.MCOptions.SplitDwarfFile = SplitDwarfFile;
   }

compiler/rustc_session/src/options.rs

@@ -1745,6 +1745,10 @@ options! {
         "explicitly enable the `cfg(target_thread_local)` directive"),
     hir_stats: bool = (false, parse_bool, [UNTRACKED],
         "print some statistics about AST and HIR (default: no)"),
+    hotpatch: bool = (false, parse_bool, [TRACKED],
+        "ensures hotpatching is always possible by ensuring that the first instruction of \
+        each function is at least two bytes, and no jump within the function goes to the first instruction. \
+        Should be combined with link-arg passing -functionpadmin to the linker. Currently only supported for x86 (default: false)"),
     human_readable_cgu_names: bool = (false, parse_bool, [TRACKED],
         "generate human-readable, predictable names for codegen units (default: no)"),
     identify_regions: bool = (false, parse_bool, [UNTRACKED],

compiler/rustc_target/src/spec/mod.rs

@@ -1963,6 +1963,9 @@ impl Target {
 
         Ok(dl)
     }
+    pub fn is_x86(&self) -> bool {
+        ["x86", "x86_64"].contains(&&self.arch[..])
+    }
 }
 
 pub trait HasTargetSpec {

tests/codegen/hotpatch.rs

@@ -0,0 +1,12 @@
+//@ revisions: x32 x64
+//@[x32] only-x86
+//@[x64] only-x86_64
+//@ compile-flags: -Z hotpatch
+
+#![crate_type = "lib"]
+
+#[no_mangle]
+pub fn foo() {}
+
+// CHECK: @foo() unnamed_addr #0
+// CHECK: attributes #0 = { {{.*}} "patchable-function"="prologue-short-redirect" {{.*}}}