From 136a653ade792ec2e713b0302166ab59a1c91b80 Mon Sep 17 00:00:00 2001
From: Copilot <198982749+Copilot@users.noreply.github.com>
Date: Sun, 12 Jul 2026 20:07:41 -0700
Subject: [PATCH] cmake: prevent install_tactic.deps from triggering spurious
 rebuilds on reconfigure (#10102)
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`z3_add_install_tactic_rule` unconditionally called `file(WRITE)` during
CMake configure, updating `install_tactic.deps`'s mtime on every
reconfiguration — even when the content was identical. Since
`install_tactic.cpp` lists `install_tactic.deps` as a dependency, any
CMake reconfigure (e.g. touching `src/api/ml/CMakeLists.txt`) caused a
full rebuild of the tactic installation target.

## Change

- **`cmake/z3_add_component.cmake` — `z3_add_install_tactic_rule`**:
Replace unconditional `file(WRITE)` with a read-and-compare guard; the
deps file is only rewritten when its content actually changes.

```cmake
# Before
file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/install_tactic.deps" ${_tactic_header_files})

# After
set(_install_tactic_deps_file "${CMAKE_CURRENT_BINARY_DIR}/install_tactic.deps")
if (EXISTS "${_install_tactic_deps_file}")
  file(READ "${_install_tactic_deps_file}" _install_tactic_deps_old)
else()
  set(_install_tactic_deps_old "")
endif()
if (NOT _install_tactic_deps_old STREQUAL "${_tactic_header_files}")
  file(WRITE "${_install_tactic_deps_file}" "${_tactic_header_files}")
endif()
```

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
---
 cmake/z3_add_component.cmake | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/cmake/z3_add_component.cmake b/cmake/z3_add_component.cmake
index 962b02f684..e3dc2dd80c 100644
--- a/cmake/z3_add_component.cmake
+++ b/cmake/z3_add_component.cmake
@@ -273,7 +273,19 @@ macro(z3_add_install_tactic_rule)
   unset(_component_tactic_header_files)
 
   string(REPLACE ";" "\n" _tactic_header_files "${_tactic_header_files}")
-  file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/install_tactic.deps" ${_tactic_header_files})
+  # Only write the deps file if content has changed to avoid unnecessary rebuilds
+  # (file(WRITE) always updates the timestamp even if content is unchanged)
+  set(_install_tactic_deps_file "${CMAKE_CURRENT_BINARY_DIR}/install_tactic.deps")
+  if (EXISTS "${_install_tactic_deps_file}")
+    file(READ "${_install_tactic_deps_file}" _install_tactic_deps_old)
+  else()
+    set(_install_tactic_deps_old "")
+  endif()
+  if (NOT _install_tactic_deps_old STREQUAL "${_tactic_header_files}")
+    file(WRITE "${_install_tactic_deps_file}" "${_tactic_header_files}")
+  endif()
+  unset(_install_tactic_deps_old)
+  unset(_install_tactic_deps_file)
   add_custom_command(OUTPUT "install_tactic.cpp"
     COMMAND "${Python3_EXECUTABLE}"
     "${PROJECT_SOURCE_DIR}/scripts/mk_install_tactic_cpp.py"