[llvm] [JTS] Add a call count threshold (PR #189782)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 updated https://github.com/llvm/llvm-project/pull/189782

>From 59953f4d8e8b3a9ce0ae7807fa5867b247f66e39 Mon Sep 17 00:00:00 2001
From: Aiden Grossman <aidengrossman at google.com>
Date: Wed, 1 Apr 2026 00:58:49 +0000
Subject: [PATCH] [JTS] Add a call count threshold

When compiling flang with an old libstdc++ version, we were running into
issues with explosive compile times (one module went from taking 18s to
optimize to hours with the pass enabled). This seems to have been due to
JTS exposing inlining opportunities for functions with a large number of
calls, leading to explosive function growth. This patch fixes that
behavior by only allowing the conversion of jump tables with functions
that have under a certain number of calls (set to 2 by default).

This pass still likely needs some proper cost modelling, but this fixes
the compile time issues which should let us enable the pass by default.
---
 .../Transforms/Scalar/JumpTableToSwitch.cpp   | 22 +++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/Scalar/JumpTableToSwitch.cpp b/llvm/lib/Transforms/Scalar/JumpTableToSwitch.cpp
index a3e3b9a207ca1..fd451f03a0a6b 100644
--- a/llvm/lib/Transforms/Scalar/JumpTableToSwitch.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpTableToSwitch.cpp
@@ -42,6 +42,14 @@ static cl::opt<unsigned> FunctionSizeThreshold(
              "or equal than this threshold."),
     cl::init(50));
 
+static cl::opt<unsigned>
+    FunctionCallCountThreshold("jump-table-to-switch-call-count-threshold",
+                               cl::Hidden,
+                               cl::desc("Only split jump tables containing "
+                                        "functions with a call count less than "
+                                        "or equal to this threshold."),
+                               cl::init(2));
+
 namespace llvm {
 extern cl::opt<bool> ProfcheckDisableMetadataFixes;
 } // end namespace llvm
@@ -101,8 +109,18 @@ static std::optional<JumpTableTy> parseJumpTable(GetElementPtrInst *GEP,
     Constant *C =
         ConstantFoldLoadFromConst(GV->getInitializer(), PtrTy, Offset, DL);
     auto *Func = dyn_cast_or_null<Function>(C);
-    if (!Func || Func->isDeclaration() ||
-        Func->getInstructionCount() > FunctionSizeThreshold)
+    if (!Func || Func->isDeclaration())
+      return std::nullopt;
+    if (Func->getInstructionCount() > FunctionSizeThreshold)
+      return std::nullopt;
+    unsigned FunctionCallCount = 0;
+    for (const auto &BB : *Func) {
+      for (const auto &I : BB) {
+        if (isa<CallInst>(I))
+          ++FunctionCallCount;
+      }
+    }
+    if (FunctionCallCount > FunctionCallCountThreshold)
       return std::nullopt;
     JumpTable.Funcs.push_back(Func);
   }