[Lldb-commits] [lldb] [lldb] Skip the prologue before reading `this` when stepping into std::function (PR #209629)

[Augusto Noronha via lldb-commits]

https://github.com/augusto2112 updated https://github.com/llvm/llvm-project/pull/209629

>From 1260647daf3d759795afe89a62c0cb2fdc0d053e Mon Sep 17 00:00:00 2001
From: Augusto Noronha <anoronha at apple.com>
Date: Wed, 15 Jul 2026 12:50:06 -0700
Subject: [PATCH] [lldb] Skip the prologue before reading `this` when stepping
 into std::function

Commit 40152b8 broke CPPLanguageRuntime::GetStepThroughTrampolinePlan by
changing operator() from a multi-line function to a single line function.

This used to work by accident, because the fallback step-in plan first advanced
past the prologue and the trampoline detection was re-invoked there with a valid
`this`. Once operator() became a one-line inline definition that intermediate
step no longer happened.

Run past the prologue when stopped at the function entry and let the
step-through machinery re-invoke the plan, so the thread plan has a
valid `this` to read from.
---
 .../CPlusPlus/CPPLanguageRuntime.cpp            | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/lldb/source/Plugins/LanguageRuntime/CPlusPlus/CPPLanguageRuntime.cpp b/lldb/source/Plugins/LanguageRuntime/CPlusPlus/CPPLanguageRuntime.cpp
index c517ec8611932..74203c1887b6a 100644
--- a/lldb/source/Plugins/LanguageRuntime/CPlusPlus/CPPLanguageRuntime.cpp
+++ b/lldb/source/Plugins/LanguageRuntime/CPlusPlus/CPPLanguageRuntime.cpp
@@ -470,6 +470,23 @@ CPPLanguageRuntime::GetStepThroughTrampolinePlan(Thread &thread,
   StackFrameSP frame = thread.GetStackFrameAtIndex(0);
 
   if (frame) {
+    Address func_start_address =
+        sc.function ? sc.function->GetAddress() : symbol->GetAddress();
+    lldb::addr_t func_start =
+        func_start_address.GetLoadAddress(target_sp.get());
+
+    if (func_start == LLDB_INVALID_ADDRESS)
+      return ret_plan_sp;
+
+    // Advance past the prologue if we stopped there.
+    uint32_t prologue_size = sc.function ? sc.function->GetPrologueByteSize()
+                                         : symbol->GetPrologueByteSize();
+    if (curr_pc < func_start + prologue_size) {
+      func_start_address.Slide(prologue_size);
+      return std::make_shared<ThreadPlanRunToAddress>(
+          thread, func_start_address, stop_others);
+    }
+
     ValueObjectSP value_sp = frame->FindVariable(g_this);
 
     CPPLanguageRuntime::LibCppStdFunctionCallableInfo callable_info =