[Mlir-commits] [mlir] [mlir] Skip the promised-interface lookup for an emptied dialect set (NFC) (PR #219545)

[llvmlistbot at llvm.org]

================
@@ -228,6 +228,9 @@ class Dialect {
   void handleUseOfUndefinedPromisedInterface(TypeID interfaceRequestorID,
                                              TypeID interfaceID,
                                              StringRef interfaceName = "") {
+    // Most dialects promise nothing; skip hashing a key and probing the set.
+    if (unresolvedPromisedInterfaces.empty())
+      return;
     if (unresolvedPromisedInterfaces.count(
----------------
khaki3 wrote:

>Can you clarify what you mean by that?

This is how I implemented the new empty check in `doFind`:

```c++
if (NumBuckets == 0) return nullptr;
if (getNumEntries() == 0) return nullptr;   // new: runs every time
// ... hash the key, probe the buckets
```

This adds a load of `NumEntries` and a subsequent branch to every call to `doFind`. The cost becomes visible when the early-out is combined with this PR: -0.190%, against -0.290% for this PR alone.

| variant | mlir-opt | clang self-host |
|---|---:|---:|
| this PR (guard in `Dialect`) | **-0.290%** | 0.00% (MLIR not linked into clang) |
| `doFind` early-out | -0.115% | **+0.063%** |
| + `LLVM_UNLIKELY` | -0.144% | **-0.042%** |
| both together | -0.190% | -0.042% |


https://github.com/llvm/llvm-project/pull/219545