tests: [diag] dump JIT'd vmi type_info (_ZTI1D) bytes to pin the corrupt field

Author: jcelerier

.github/workflows/jitmin.yaml

@@ -109,6 +109,12 @@ jobs:
             SKIP_INIT=1 ./build/jitmin.exe "$ORC" "$n.ll" entry "$LIBCXXA"
             echo "  >>> $n skipinit exit=$?"
             echo "::endgroup::"
+            # Dump the JIT'd vmi type_info bytes for the crashing MI cast case.
+            if [ "$n" = "c10_mi_cast_only" ]; then
+              echo "::group::DUMP $n _ZTI1D"
+              SKIP_INIT=1 DUMP_TI=_ZTI1D ./build/jitmin.exe "$ORC" "$n.ll" entry "$LIBCXXA" 2>&1 | grep -aE "\[ti\]"
+              echo "::endgroup::"
+            fi
             # Native-TLS variant: does JITLink handle COFF .tls without emutls?
             case "$n" in c03*|c04*)
               "$CXX" -target x86_64-w64-windows-gnu -stdlib=libc++ -fno-emulated-tls \

tests/jit-min/main.cpp

@@ -197,6 +197,33 @@ int main(int argc, char** argv)
   if(!sym)
     fail("lookup", sym.takeError());
 
+  // DIAG: dump a JIT'd type_info object's raw bytes to find which field is
+  // mis-materialized (vtable ptr / name / flags / base array). In-process JIT, so
+  // the executor address is directly readable here.
+  if(const char* tiName = std::getenv("DUMP_TI"))
+  {
+    if(auto a = (*jit)->lookup(tiName))
+    {
+      const uint64_t* ti = (const uint64_t*)a->getValue();
+      errs() << "[ti] " << tiName << " @" << (void*)ti << "\n";
+      for(int i = 0; i < 7; i++)
+        errs() << "[ti]   +" << (i * 8) << " = " << (void*)ti[i] << "\n";
+      errs().flush();
+      // The vtable ptr (ti[0]) should point ~16 bytes into a cxxabi vtable.
+      // Read a couple of slots around it (may itself fault if ti[0] is garbage,
+      // but the bytes above are already flushed).
+      const uint64_t* vt = (const uint64_t*)ti[0];
+      errs() << "[ti]   *vtable[-2..1] = " << (void*)vt[-2] << " " << (void*)vt[-1]
+             << " " << (void*)vt[0] << " " << (void*)vt[1] << "\n";
+      errs().flush();
+    }
+    else
+    {
+      consumeError(a.takeError());
+      errs() << "[ti] " << tiName << " NOTFOUND\n";
+    }
+  }
+
   if(std::getenv("SKIP_INIT"))
   {
     step("SKIP_INIT set -- not calling initialize()");