[llvm] Remove __SHORT_FILE__ macro definition in CMake (PR #152344)

[Mehdi Amini via llvm-commits]

https://github.com/joker-eph created https://github.com/llvm/llvm-project/pull/152344

This per-file macro definition on the command line breaks caching of modules. See discussion in #150677

Instead we use a constexpr variable to force the constexpr evaluation by the frontend, and also the __FILE_NAME__ macro when available (clang/gcc) to spare compile-time in the frontend.

>From 91d16a09b18e2d2982341feffa3a0a99d9147adb Mon Sep 17 00:00:00 2001
From: Mehdi Amini <joker.eph at gmail.com>
Date: Wed, 6 Aug 2025 10:25:32 -0700
Subject: [PATCH] Remove __SHORT_FILE__ macro definition in CMake

This per-file macro definition on the command line breaks caching of
modules. See discussion in #150677

Instead we use a constexpr variable to force the constexpr evaluation
by the frontend, and also the __FILE_NAME__ macro when available
(clang/gcc) to spare compile-time in the frontend.
---
 llvm/cmake/modules/LLVMProcessSources.cmake | 15 -------------
 llvm/include/llvm/Support/DebugLog.h        | 24 ++++++++++-----------
 llvm/unittests/Support/DebugLogTest.cpp     |  4 ++--
 3 files changed, 14 insertions(+), 29 deletions(-)

diff --git a/llvm/cmake/modules/LLVMProcessSources.cmake b/llvm/cmake/modules/LLVMProcessSources.cmake
index cf358a88f5fb6..0670d60bf2afd 100644
--- a/llvm/cmake/modules/LLVMProcessSources.cmake
+++ b/llvm/cmake/modules/LLVMProcessSources.cmake
@@ -58,21 +58,6 @@ function(llvm_process_sources OUT_VAR)
   set(sources ${ARG_UNPARSED_ARGUMENTS})
   llvm_check_source_file_list(${sources})
 
-  # Don't generate __SHORT_FILE__ on VS builds as it can prevent build parallelisation.
-  if(NOT CMAKE_GENERATOR MATCHES "Visual Studio")
-    foreach(fn ${sources})
-      get_filename_component(suf ${fn} EXT)
-      if("${suf}" STREQUAL ".cpp" OR "${suf}" STREQUAL ".c")
-        get_filename_component(short_name ${fn} NAME)
-        set_property(
-            SOURCE ${fn}
-            APPEND
-            PROPERTY COMPILE_DEFINITIONS __SHORT_FILE__="${short_name}")
-      endif()
-    endforeach()
-  endif()
-
-
   # This adds .td and .h files to the Visual Studio solution:
   add_td_sources(sources)
   find_all_header_files(hdrs "${ARG_ADDITIONAL_HEADER_DIRS}")
diff --git a/llvm/include/llvm/Support/DebugLog.h b/llvm/include/llvm/Support/DebugLog.h
index a3312950da94e..e3a0854608565 100644
--- a/llvm/include/llvm/Support/DebugLog.h
+++ b/llvm/include/llvm/Support/DebugLog.h
@@ -56,30 +56,30 @@ namespace llvm {
   DEBUGLOG_WITH_STREAM_AND_TYPE(llvm::dbgs(), LEVEL, DEBUG_TYPE)
 #define LDBG_LOG_LEVEL_1() LDBG_LOG_LEVEL(1)
 
+// We want the filename without the full path. We are using the __FILE__ macro
+// and a constexpr function to strip the path prefix. We can avoid the frontend
+// repeated evaluation of __FILE__ by using the __FILE_NAME__ when defined
+// (gcc and clang do) which contains the file name already.
+#if !defined(__FILE_NAME__)
+#define __FILE_NAME__ ::llvm::impl::getShortFileName(__FILE__)
+#endif
+
 #define DEBUGLOG_WITH_STREAM_TYPE_FILE_AND_LINE(STREAM, LEVEL, TYPE, FILE,     \
                                                 LINE)                          \
   for (bool _c =                                                               \
            (::llvm::DebugFlag && ::llvm::isCurrentDebugType(TYPE, LEVEL));     \
        _c; _c = false)                                                         \
-    for (::llvm::impl::RAIINewLineStream NewLineStream{(STREAM)}; _c;          \
-         _c = false)                                                           \
+    for (constexpr auto file = __FILE_NAME__; _c; _c = false)                  \
+      for (::llvm::impl::RAIINewLineStream NewLineStream{(STREAM)}; _c;        \
+           _c = false)                                                         \
   ::llvm::impl::raw_ldbg_ostream{                                              \
       ::llvm::impl::computePrefix(TYPE, FILE, LINE, LEVEL), NewLineStream}     \
       .asLvalue()
 
 #define DEBUGLOG_WITH_STREAM_TYPE_AND_FILE(STREAM, LEVEL, TYPE, FILE)          \
   DEBUGLOG_WITH_STREAM_TYPE_FILE_AND_LINE(STREAM, LEVEL, TYPE, FILE, __LINE__)
-// When __SHORT_FILE__ is not defined, the File is the full path,
-// otherwise __SHORT_FILE__ is defined in CMake to provide the file name
-// without the path prefix.
-#if defined(__SHORT_FILE__)
 #define DEBUGLOG_WITH_STREAM_AND_TYPE(STREAM, LEVEL, TYPE)                     \
-  DEBUGLOG_WITH_STREAM_TYPE_AND_FILE(STREAM, LEVEL, TYPE, __SHORT_FILE__)
-#else
-#define DEBUGLOG_WITH_STREAM_AND_TYPE(STREAM, LEVEL, TYPE)                     \
-  DEBUGLOG_WITH_STREAM_TYPE_AND_FILE(STREAM, LEVEL, TYPE,                      \
-                                     ::llvm::impl::getShortFileName(__FILE__))
-#endif
+  DEBUGLOG_WITH_STREAM_TYPE_AND_FILE(STREAM, LEVEL, TYPE, __FILE__)
 
 namespace impl {
 
diff --git a/llvm/unittests/Support/DebugLogTest.cpp b/llvm/unittests/Support/DebugLogTest.cpp
index c24d1a5693169..05a49163552a5 100644
--- a/llvm/unittests/Support/DebugLogTest.cpp
+++ b/llvm/unittests/Support/DebugLogTest.cpp
@@ -7,9 +7,9 @@
 //===----------------------------------------------------------------------===//
 
 // This macro is defined in the LLVM build system, but we undefine it here
-// so that we test at least once in-tree the case where __SHORT_FILE__ is not
+// so that we test at least once in-tree the case where __FILE_NAME__ is not
 // defined.
-#undef __SHORT_FILE__
+#undef __FILE_NAME__
 
 #include "llvm/Support/DebugLog.h"
 #include "llvm/ADT/Sequence.h"