[llvm] [PredicateInfo] Don't use depth first walk (NFCI) (PR #145016)

[Nikita Popov via llvm-commits]

https://github.com/nikic created https://github.com/llvm/llvm-project/pull/145016

The order in which we collect the predicates does not matter, as they will be sorted anyway. As such, avoid the expensive depth first walk over the dominator tree and instead use plain iteration over the function.

(To be a bit more precise, the predicates and uses for a specific value are sorted, so this change has no impact on that. It can change the order in which values are processed in the first place, but that order is not semantically relevant.)

>From d216fd1bfecae33b8ebebb14cc61e3994742b9f1 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Fri, 20 Jun 2025 10:58:58 +0200
Subject: [PATCH] [PredicateInfo] Don't use depth first walk (NFCI)

The order in which we collect the predicates does not matter, as
they will be sorted anyway. As such, avoid the expensive depth
first walk over the dominator tree and instead use plain iteration
over the function.
---
 llvm/lib/Transforms/Utils/PredicateInfo.cpp | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/llvm/lib/Transforms/Utils/PredicateInfo.cpp b/llvm/lib/Transforms/Utils/PredicateInfo.cpp
index 9b239d9161e7f..67defef3f75f9 100644
--- a/llvm/lib/Transforms/Utils/PredicateInfo.cpp
+++ b/llvm/lib/Transforms/Utils/PredicateInfo.cpp
@@ -488,17 +488,19 @@ void PredicateInfoBuilder::buildPredicateInfo() {
   // Collect operands to rename from all conditional branch terminators, as well
   // as assume statements.
   SmallVector<Value *, 8> OpsToRename;
-  for (auto *DTN : depth_first(DT.getRootNode())) {
-    BasicBlock *BranchBB = DTN->getBlock();
-    if (auto *BI = dyn_cast<BranchInst>(BranchBB->getTerminator())) {
+  for (BasicBlock &BB : F) {
+    if (!DT.isReachableFromEntry(&BB))
+      continue;
+
+    if (auto *BI = dyn_cast<BranchInst>(BB.getTerminator())) {
       if (!BI->isConditional())
         continue;
       // Can't insert conditional information if they all go to the same place.
       if (BI->getSuccessor(0) == BI->getSuccessor(1))
         continue;
-      processBranch(BI, BranchBB, OpsToRename);
-    } else if (auto *SI = dyn_cast<SwitchInst>(BranchBB->getTerminator())) {
-      processSwitch(SI, BranchBB, OpsToRename);
+      processBranch(BI, &BB, OpsToRename);
+    } else if (auto *SI = dyn_cast<SwitchInst>(BB.getTerminator())) {
+      processSwitch(SI, &BB, OpsToRename);
     }
   }
   for (auto &Assume : AC.assumptions()) {