Various fixes and cleanup

While working on other patches that ended up not being applicable, I've
gathered these changes unrelated to the irrelevant patch. So, submitting
them as a seperate change, since the bigger change isn't going in.

Author: khyperia

crates/rustc_codegen_spirv/src/lib.rs

@@ -621,15 +621,20 @@ fn get_env_dump_dir(env_var: &str) -> Option<PathBuf> {
 /// This is the entrypoint for a hot plugged `rustc_codegen_spirv`
 #[no_mangle]
 pub fn __rustc_codegen_backend() -> Box<dyn CodegenBackend> {
-    // Override rustc's panic hook with our own to override the ICE error
-    // message, and direct people to `rust-gpu`.
-    std::panic::set_hook(Box::new(|panic_info| {
-        rustc_driver::report_ice(
-            panic_info,
-            "https://github.com/EmbarkStudios/rust-gpu/issues/new",
-        );
-        eprintln!("note: `rust-gpu` version {}\n", env!("CARGO_PKG_VERSION"));
-    }));
+    // Setting the hook nukes the backtrace, so give developers an escape hatch to see the crash,
+    // even if the message is wrong.
+    // TODO: Figure out a way to print both the backtrace and override the bug report URL.
+    if env::var_os("NO_PANIC_HOOK").is_none() {
+        // Override rustc's panic hook with our own to override the ICE error
+        // message, and direct people to `rust-gpu`.
+        std::panic::set_hook(Box::new(|panic_info| {
+            rustc_driver::report_ice(
+                panic_info,
+                "https://github.com/EmbarkStudios/rust-gpu/issues/new",
+            );
+            eprintln!("note: `rust-gpu` version {}\n", env!("CARGO_PKG_VERSION"));
+        }));
+    }
 
     Box::new(SpirvCodegenBackend)
 }

crates/rustc_codegen_spirv/src/linker/dce.rs

@@ -25,6 +25,20 @@ pub fn collect_roots(module: &Module) -> FxHashSet<Word> {
     rooted
 }
 
+// Exactly the same as Function::all_inst_iter, except return type is `impl DoubleEndedIterator`
+// instead of `impl Iterator`
+fn all_inst_iter(func: &Function) -> impl DoubleEndedIterator<Item = &Instruction> {
+    func.def
+        .iter()
+        .chain(func.parameters.iter())
+        .chain(
+            func.blocks
+                .iter()
+                .flat_map(|b| b.label.iter().chain(b.instructions.iter())),
+        )
+        .chain(func.end.iter())
+}
+
 fn spread_roots(module: &Module, rooted: &mut FxHashSet<Word>) -> bool {
     let mut any = false;
     for inst in module.global_inst_iter() {
@@ -40,18 +54,7 @@ fn spread_roots(module: &Module, rooted: &mut FxHashSet<Word>) -> bool {
             // earlier insts, by reversing the iteration order, we're more likely to root the
             // entire relevant function at once.
             // See https://github.com/EmbarkStudios/rust-gpu/pull/691#discussion_r681477091
-            for inst in func
-                .end
-                .iter()
-                .chain(
-                    func.blocks
-                        .iter()
-                        .rev()
-                        .flat_map(|b| b.instructions.iter().rev().chain(b.label.iter())),
-                )
-                .chain(func.parameters.iter().rev())
-                .chain(func.def.iter())
-            {
+            for inst in all_inst_iter(func).rev() {
                 if !instruction_is_pure(inst) {
                     any |= root(inst, rooted);
                 } else if let Some(id) = inst.result_id {
@@ -111,13 +114,13 @@ fn kill_unrooted(module: &mut Module, rooted: &FxHashSet<Word>) {
     module
         .functions
         .retain(|f| is_rooted(f.def.as_ref().unwrap(), rooted));
-    module.functions.iter_mut().for_each(|fun| {
-        fun.blocks.iter_mut().for_each(|block| {
+    for fun in &mut module.functions {
+        for block in &mut fun.blocks {
             block
                 .instructions
                 .retain(|inst| !instruction_is_pure(inst) || is_rooted(inst, rooted));
-        });
-    });
+        }
+    }
 }
 
 pub fn dce_phi(func: &mut Function) {

tests/ui/image/gather.rs

@@ -20,6 +20,7 @@ pub fn main(
 #[cfg(not(any(
     target_env = "vulkan1.0",
     target_env = "vulkan1.1",
+    target_env = "vulkan1.1spv1.4",
     target_env = "vulkan1.2"
 )))]
 #[spirv(fragment)]