[PATCH] D136716: [JumpThreading] Put a limit on the PHI nodes when duplicating a BB.

[Usman Nadeem via Phabricator via llvm-commits]

This revision was automatically updated to reflect the committed changes.
Closed by commit rG32755786e020: [JumpThreading] Put a limit on the PHI nodes when duplicating a BB. (authored by mnadeem).

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D136716/new/

https://reviews.llvm.org/D136716

Files:
  llvm/lib/Transforms/Scalar/JumpThreading.cpp


Index: llvm/lib/Transforms/Scalar/JumpThreading.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -100,6 +100,11 @@
            "condition to use to thread over a weaker condition"),
   cl::init(3), cl::Hidden);
 
+static cl::opt<unsigned> PhiDuplicateThreshold(
+    "jump-threading-phi-threshold",
+    cl::desc("Max PHIs in BB to duplicate for jump threading"), cl::init(76),
+    cl::Hidden);
+
 static cl::opt<bool> PrintLVIAfterJumpThreading(
     "print-lvi-after-jump-threading",
     cl::desc("Print the LazyValueInfo cache after JumpThreading"), cl::init(false),
@@ -518,8 +523,23 @@
                                              Instruction *StopAt,
                                              unsigned Threshold) {
   assert(StopAt->getParent() == BB && "Not an instruction from proper BB?");
+
+  // Do not duplicate the BB if it has a lot of PHI nodes.
+  // If a threadable chain is too long then the number of PHI nodes can add up,
+  // leading to a substantial increase in compile time when rewriting the SSA.
+  unsigned PhiCount = 0;
+  Instruction *FirstNonPHI = nullptr;
+  for (Instruction &I : *BB) {
+    if (!isa<PHINode>(&I)) {
+      FirstNonPHI = &I;
+      break;
+    }
+    if (++PhiCount > PhiDuplicateThreshold)
+      return ~0U;
+  }
+
   /// Ignore PHI nodes, these will be flattened when duplication happens.
-  BasicBlock::const_iterator I(BB->getFirstNonPHI());
+  BasicBlock::const_iterator I(FirstNonPHI);
 
   // FIXME: THREADING will delete values that are just used to compute the
   // branch, so they shouldn't count against the duplication cost.


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D136716.472167.patch
Type: text/x-patch
Size: 1719 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20221031/f0dab55a/attachment.bin>