[rocjitsu] Replace decoder exceptions with results

emulation/rocjitsu/CONTRIBUTING.md

@@ -49,8 +49,11 @@ Before writing new infrastructure, check these existing libraries:
 - **Intrusive list** — `util/intrusive_list.h` for linked data structures
   that need O(1) insert/erase without heap allocation per node.
 - **Arena allocator** — `util/arena_alloc.h` for fixed-size block pools.
-- **Exceptions** — `util/except.h` for `InvalidInst`, `UnimplementedInst`,
-  `ConfigError`.
+- **Expected failures** — Return `Result` or `FailureOr<T>` and accept a
+  `DiagnosticEmitter` when the caller needs a reason. Its sink is non-owning
+  and must not outlive its callable.
+- **Exceptions** — `util/except.h` for unrecoverable initialization,
+  configuration, and execution failures.
 - **Register sets** — `isa/register_set.h` for ISA-independent register
   file modeling (SGPR/VGPR/AccVGPR bitsets).
 - **Spill manager** — `code/patch/spill_manager.h` for scratch layout
@@ -88,16 +91,13 @@ Before writing new infrastructure, check these existing libraries:
   `stderr` — it runs inside another runtime's `OnLoad`, its messages must
   survive a log-groups-OFF build, and its signal-backtrace path must stay
   async-signal-safe (`rj_hsa_dbt_hooks.cpp` uses raw `::write`).
-- **Exceptions.** Exceptions are used only for unrecoverable errors
-  during initialization and configuration (`ConfigError`) or when
-  encountering invalid/unimplemented instructions during code object
-  parsing (`InvalidInst`, `UnimplementedInst`). All exception types
-  live in `util/except.h`. Do not throw exceptions in simulation hot
-  paths (event handlers, instruction execution, cache lookups). Do not
-  add `try`/`catch` blocks unless you are at a boundary that must
-  translate an error (e.g., the C API layer). There is no general
-  exception safety guarantee — assume code is not exception-safe
-  unless explicitly documented.
+- **Errors.** Expected failures such as a rejected instruction encoding return
+  `Result` or `FailureOr<T>` and optionally emit a diagnostic. Exceptions are
+  reserved for unrecoverable initialization, configuration, or execution
+  failures. Do not throw exceptions in simulation hot paths (event handlers,
+  instruction execution, cache lookups). Do not add `try`/`catch` blocks unless
+  you are at a boundary that must translate an error (e.g., the C API layer).
+  There is no general exception safety guarantee unless explicitly documented.
 
 ## ISA codegen workflow
 

emulation/rocjitsu/docs/style.md

@@ -148,10 +148,11 @@ Foo foo = static_cast<Foo>(bar); // OK but could use auto instead to avoid repet
 
 ## Error Handling
 
-- Exceptions are only for unrecoverable errors: initialization/configuration
-  failures (`ConfigError`) and invalid or unimplemented instructions during
-  code-object parsing (`InvalidInst`, `UnimplementedInst`). All exception types
-  live in `util/except.h`.
+- Expected failures such as a rejected instruction encoding return `Result` or
+  `FailureOr<T>`. Accept a `DiagnosticEmitter` when the caller needs the reason;
+  its sink is non-owning and must not outlive its callable.
+- Exceptions are only for unrecoverable initialization, configuration, or
+  execution failures. Exception types live in `util/except.h`.
 - Do not throw in simulation hot paths (event handlers, instruction execution,
   cache lookups).
 - Do not add `try`/`catch` except at a boundary that must translate an error

emulation/rocjitsu/fuzz/decode/README.md

@@ -77,7 +77,7 @@ ninja -C build-decode-afl rj_decode_fuzz
   build-decode-afl/fuzz/decode/rj_decode_fuzz --afl --target gfx1250
 ```
 
-`InvalidInst` is an ordinary rejection. Other exceptions, signals, sanitizer
+Decoder failure is an ordinary rejection. Exceptions from unrelated failures, signals, sanitizer
 findings, and decoder invariant failures remain crashes for AFL++ to retain.
 
 ## Offline LLVM comparison

emulation/rocjitsu/fuzz/decode/decode_fuzz_core.cpp

@@ -8,7 +8,6 @@
 #include "rocjitsu/isa/instruction.h"
 #include "rocjitsu/isa/operand.h"
 #include "rocjitsu/isa/target_registry.h"
-#include "util/except.h"
 
 #include <array>
 #include <cstdlib>
@@ -91,17 +90,14 @@ DecodeRecord decode_window(Decoder &decoder, std::span<const uint8_t> bytes) {
       words[word] |= static_cast<uint32_t>(bytes[word * sizeof(uint32_t) + byte]) << (byte * 8);
   }
 
-  std::unique_ptr<Instruction> inst;
-  try {
-    inst.reset(decoder.decode(words.data()));
-  } catch (const util::InvalidInst &error) {
+  util::StringDiagnostic rejection;
+  DecodeResult decoded = decoder.decode(words.data(), rejection.emitter());
+  if (decoded.failed()) {
     DecodeRecord record;
-    record.rejection = error.what();
+    record.rejection = rejection.message();
     return record;
   }
-
-  if (!inst)
-    fail_invariant("a successful decode returned null");
+  std::unique_ptr<Instruction> inst = std::move(decoded).value();
 
   const int size = inst->size();
   if (size != 4 && size != 8 && size != 12 && size != 16)