[llvm] [InstCombine] Improve `foldDeadPhiWeb` compile time (PR #158057)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Mingjie Xu (Enna1)

<details>
<summary>Changes</summary>

The foldDeadPhiWeb function identifies and removes small dead PHI webs, it bails out if the web size exceeds threshold (16).

In the current implementation, when there is a phi node has large number of users that most of them are phi nodes, we still push them on the `Stack` even if the number of phi nodes user exceeds the threshold.

This patch checks the early stop condition when we push an unvisited phi node on the `Stack`.
With this change, the wall duration of total instcombine pass decreased from 523,649.276 ms to 30,239.399 ms in an our internal case.

---
Full diff: https://github.com/llvm/llvm-project/pull/158057.diff


1 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp (+7-6) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp b/llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
index ed9a0be6981fa..c3643a7cb4c08 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
@@ -62,14 +62,15 @@ bool InstCombinerImpl::foldDeadPhiWeb(PHINode &PN) {
   Stack.push_back(&PN);
   while (!Stack.empty()) {
     PHINode *Phi = Stack.pop_back_val();
-    if (!Visited.insert(Phi).second)
-      continue;
-    // Early stop if the set of PHIs is large
-    if (Visited.size() == 16)
-      return false;
     for (User *Use : Phi->users()) {
-      if (PHINode *PhiUse = dyn_cast<PHINode>(Use))
+      if (PHINode *PhiUse = dyn_cast<PHINode>(Use)) {
+        if (!Visited.insert(Phi).second)
+          continue;
+        // Early stop if the set of PHIs is large
+        if (Visited.size() >= 16)
+          return false;
         Stack.push_back(PhiUse);
+      }
       else
         return false;
     }

``````````

</details>


https://github.com/llvm/llvm-project/pull/158057