From d433448d6fb87df87e1c8ff555bce6566a1162a7 Mon Sep 17 00:00:00 2001
From: Folkert <folkert@folkertdev.nl>
Date: Sun, 28 Aug 2022 13:17:50 +0200
Subject: [PATCH 1/2] don't regenerate the dynhost file if it is unchanged

---
 crates/linker/src/lib.rs | 58 +++++++++++++++++++++++++++++++++++++---
 1 file changed, 55 insertions(+), 3 deletions(-)

diff --git a/crates/linker/src/lib.rs b/crates/linker/src/lib.rs
index c8a8853a1d..035c2b8ec4 100644
--- a/crates/linker/src/lib.rs
+++ b/crates/linker/src/lib.rs
@@ -136,10 +136,62 @@ fn generate_dynamic_lib(
         }
     }
 
-    let bytes = crate::generate_dylib::generate(target, &custom_names)
-        .unwrap_or_else(|e| internal_error!("{}", e));
+    if !dynhost_is_up_to_date(target, dummy_lib_path, &custom_names) {
+        let bytes = crate::generate_dylib::generate(target, &custom_names)
+            .unwrap_or_else(|e| internal_error!("{}", e));
 
-    std::fs::write(dummy_lib_path, &bytes).unwrap_or_else(|e| internal_error!("{}", e))
+        std::fs::write(dummy_lib_path, &bytes).unwrap_or_else(|e| internal_error!("{}", e))
+    }
+}
+
+fn object_matches_target<'a>(target: &Triple, object: &object::File<'a, &'a [u8]>) -> bool {
+    use target_lexicon::{Architecture as TLA, OperatingSystem as TLO};
+
+    match target.architecture {
+        TLA::X86_64 => {
+            if object.architecture() != object::Architecture::X86_64 {
+                return false;
+            }
+
+            let actual = object.format();
+
+            let expected = match target.operating_system {
+                TLO::Linux => object::BinaryFormat::Elf,
+                other => internal_error!("unexpected target {:?}", other),
+            };
+
+            actual == expected
+        }
+        TLA::Aarch64(_) => object.architecture() == object::Architecture::Aarch64,
+        _ => false,
+    }
+}
+
+fn dynhost_is_up_to_date(target: &Triple, dummy_lib_path: &Path, custom_names: &[String]) -> bool {
+    if !std::path::Path::exists(dummy_lib_path) {
+        return false;
+    }
+
+    let exec_file = fs::File::open(dummy_lib_path).unwrap_or_else(|e| internal_error!("{}", e));
+    let exec_mmap = unsafe { Mmap::map(&exec_file).unwrap_or_else(|e| internal_error!("{}", e)) };
+    let exec_data = &*exec_mmap;
+
+    let object = object::File::parse(exec_data).unwrap();
+
+    // the user may have been cross-compiling.
+    // The dynhost on disk must match our current target
+    if !object_matches_target(target, &object) {
+        return false;
+    }
+
+    // we made this dynhost file. For the file to be the same as what we'd generate,
+    // we need all symbols to be there and in the correct order
+    let dynamic_symbols: Vec<_> = object.exports().unwrap();
+
+    let it1 = dynamic_symbols.iter().map(|e| e.name());
+    let it2 = custom_names.iter().map(|s| s.as_bytes());
+
+    it1.eq(it2)
 }
 
 fn is_roc_symbol(sym: &object::Symbol) -> bool {

From 3e78395237fbed815e5eee491b3879b0e7c1ed6c Mon Sep 17 00:00:00 2001
From: Folkert <folkert@folkertdev.nl>
Date: Tue, 30 Aug 2022 12:32:19 +0200
Subject: [PATCH 2/2] changes after review

---
 crates/linker/src/lib.rs | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/crates/linker/src/lib.rs b/crates/linker/src/lib.rs
index 035c2b8ec4..bd9356150c 100644
--- a/crates/linker/src/lib.rs
+++ b/crates/linker/src/lib.rs
@@ -136,7 +136,7 @@ fn generate_dynamic_lib(
         }
     }
 
-    if !dynhost_is_up_to_date(target, dummy_lib_path, &custom_names) {
+    if !dummy_lib_is_up_to_date(target, dummy_lib_path, &custom_names) {
         let bytes = crate::generate_dylib::generate(target, &custom_names)
             .unwrap_or_else(|e| internal_error!("{}", e));
 
@@ -153,21 +153,27 @@ fn object_matches_target<'a>(target: &Triple, object: &object::File<'a, &'a [u8]
                 return false;
             }
 
-            let actual = object.format();
-
-            let expected = match target.operating_system {
+            let target_format = match target.operating_system {
                 TLO::Linux => object::BinaryFormat::Elf,
-                other => internal_error!("unexpected target {:?}", other),
+                TLO::Windows => object::BinaryFormat::Pe,
+                _ => todo!("surgical linker does not support target {:?}", target),
             };
 
-            actual == expected
+            object.format() == target_format
         }
         TLA::Aarch64(_) => object.architecture() == object::Architecture::Aarch64,
-        _ => false,
+        _ => todo!("surgical linker does not support target {:?}", target),
     }
 }
 
-fn dynhost_is_up_to_date(target: &Triple, dummy_lib_path: &Path, custom_names: &[String]) -> bool {
+/// Checks whether the dummy `.dll/.so` is up to date, in other words that it exports exactly the
+/// symbols that it is supposed to export, and is built for the right target. If this is the case,
+/// we can skip rebuildingthe dummy lib.
+fn dummy_lib_is_up_to_date(
+    target: &Triple,
+    dummy_lib_path: &Path,
+    custom_names: &[String],
+) -> bool {
     if !std::path::Path::exists(dummy_lib_path) {
         return false;
     }