cxxmodules: support command templates for BMI compilation

Some compilers (Clang) warn when using their BMI-only flag with `-c`. Support a complete template for BMI compilation rather than an additional flag to support such toolchains. Fixes: #27600
2026-06-30 19:57:41 +00:00 · 2026-03-25 22:39:32 -04:00
parent 8ba08a0000
commit 4a4986d28a
12 changed files with 64 additions and 30 deletions
--- a/Help/manual/cmake-cxxmodules.7.rst
+++ b/Help/manual/cmake-cxxmodules.7.rst
@@ -470,13 +470,18 @@ Additionally, toolchains should set the following variables:
 * ``CMAKE_CXX_MODULE_MAP_FLAG``: The arguments used to inform the compiler of
  the :term:`module map` file.  It should use the ``<MODULE_MAP_FILE>``
  placeholder.
+* ``CMAKE_CXX_COMPILE_BMI``: The command template to compile a :term:`BMI`
+  file from a :term:`module interface unit`.  Used when
+  ``CMAKE_CXX_MODULE_BMI_ONLY_FLAG`` is not completely additive to an
+  object compilation template.
 * ``CMAKE_CXX_MODULE_BMI_ONLY_FLAG``: The arguments used to compile only a
  :term:`BMI` file from a :term:`module interface unit`.  This is used when
  consuming modules from external projects to compile :term:`BMI` files for
  use within the current build.

-If a toolchain does not provide the ``CMAKE_CXX_MODULE_BMI_ONLY_FLAG``, it
-will not be able to consume modules provided by ``IMPORTED`` targets.
+If a toolchain does not provide the ``CMAKE_CXX_COMPILE_BMI`` or
+``CMAKE_CXX_MODULE_BMI_ONLY_FLAG`` variables, it will not be able to consume
+modules provided by ``IMPORTED`` targets.

 Configure
 ^^^^^^^^^
--- a/Modules/Compiler/Clang-CXX.cmake
+++ b/Modules/Compiler/Clang-CXX.cmake
@@ -56,6 +56,7 @@ if(CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.0)
    unset(_clang_scan_deps_mv)
    set(CMAKE_CXX_MODULE_MAP_FORMAT "clang")
    set(CMAKE_CXX_MODULE_MAP_FLAG "@<MODULE_MAP_FILE>")
-    set(CMAKE_CXX_MODULE_BMI_ONLY_FLAG "--precompile")
+    set(CMAKE_CXX_COMPILE_BMI
+      "<CMAKE_CXX_COMPILER> <DEFINES> <INCLUDES> <FLAGS> -o <OBJECT> --precompile <SOURCE>")
  endif()
 endif()
--- a/Source/cmGeneratorTarget.cxx
+++ b/Source/cmGeneratorTarget.cxx
@@ -2335,6 +2335,7 @@ cmGeneratorTarget::GetClassifiedFlagsForSource(cmSourceFile const* sf,
  }

  // C++ module flags.
+  bool useBmiTemplate = false;
  if (lang == "CXX"_s) {
    FlagClassification cls = FlagClassification::LocationFlag;
    FlagKind kind = FlagKind::BuildSystem;
@@ -2353,13 +2354,17 @@ cmGeneratorTarget::GetClassifiedFlagsForSource(cmSourceFile const* sf,
      }

      if (!this->Target->IsNormal()) {
-        auto flag = mf->GetSafeDefinition("CMAKE_CXX_MODULE_BMI_ONLY_FLAG");
-        cmRulePlaceholderExpander::RuleVariables compileObjectVars;
-        compileObjectVars.Object = sfVars.ObjectFileDir.c_str();
-        auto rulePlaceholderExpander = lg->CreateRulePlaceholderExpander();
-        rulePlaceholderExpander->ExpandRuleVariables(lg, flag,
-                                                     compileObjectVars);
-        lg->AppendCompileOptions(bmiFlags, flag);
+        if (!mf->GetDefinition("CMAKE_CXX_COMPILE_BMI").IsEmpty()) {
+          useBmiTemplate = true;
+        } else {
+          auto flag = mf->GetSafeDefinition("CMAKE_CXX_MODULE_BMI_ONLY_FLAG");
+          cmRulePlaceholderExpander::RuleVariables compileObjectVars;
+          compileObjectVars.Object = sfVars.ObjectFileDir.c_str();
+          auto rulePlaceholderExpander = lg->CreateRulePlaceholderExpander();
+          rulePlaceholderExpander->ExpandRuleVariables(lg, flag,
+                                                       compileObjectVars);
+          lg->AppendCompileOptions(bmiFlags, flag);
+        }
      }
    }

@@ -2382,7 +2387,8 @@ cmGeneratorTarget::GetClassifiedFlagsForSource(cmSourceFile const* sf,
    FlagClassification cls = FlagClassification::ExecutionFlag;
    FlagKind kind = FlagKind::NotAFlag;

-    std::string const cmdVar = cmStrCat("CMAKE_", lang, "_COMPILE_OBJECT");
+    std::string const cmdVar =
+      cmStrCat("CMAKE_", lang, "_COMPILE_", useBmiTemplate ? "BMI" : "OBJECT");
    std::string const& compileCmd = mf->GetRequiredDefinition(cmdVar);
    cmList compileCmds(compileCmd); // FIXME: which command to use?
    std::string& cmd = compileCmds[0];
--- a/Source/cmNinjaTargetGenerator.cxx
+++ b/Source/cmNinjaTargetGenerator.cxx
@@ -265,15 +265,19 @@ std::string cmNinjaTargetGenerator::ComputeFlagsForObject(
    }

    if (!this->GeneratorTarget->Target->IsNormal()) {
-      auto flag = this->GetMakefile()->GetSafeDefinition(
-        "CMAKE_CXX_MODULE_BMI_ONLY_FLAG");
-      cmRulePlaceholderExpander::RuleVariables compileObjectVars;
-      compileObjectVars.Object = objectFileName.c_str();
-      auto rulePlaceholderExpander =
-        this->GetLocalGenerator()->CreateRulePlaceholderExpander();
-      rulePlaceholderExpander->ExpandRuleVariables(this->GetLocalGenerator(),
-                                                   flag, compileObjectVars);
-      this->LocalGenerator->AppendCompileOptions(flags, flag);
+      if (this->GetMakefile()
+            ->GetDefinition("CMAKE_CXX_COMPILE_BMI")
+            .IsEmpty()) {
+        auto flag = this->GetMakefile()->GetSafeDefinition(
+          "CMAKE_CXX_MODULE_BMI_ONLY_FLAG");
+        cmRulePlaceholderExpander::RuleVariables compileObjectVars;
+        compileObjectVars.Object = objectFileName.c_str();
+        auto rulePlaceholderExpander =
+          this->GetLocalGenerator()->CreateRulePlaceholderExpander();
+        rulePlaceholderExpander->ExpandRuleVariables(this->GetLocalGenerator(),
+                                                     flag, compileObjectVars);
+        this->LocalGenerator->AppendCompileOptions(flags, flag);
+      }
    }
  }

@@ -652,6 +656,19 @@ void cmNinjaTargetGenerator::WriteCompileRule(std::string const& lang,
  this->WriteCompileRule(lang, config, WithScanning::No);
 }

+std::string cmNinjaTargetGenerator::GetCompileTemplateVar(
+  std::string const& lang) const
+{
+  std::string cmdVar = cmStrCat("CMAKE_", lang, "_COMPILE_OBJECT");
+  if (!this->GetGeneratorTarget()->IsNormal()) {
+    std::string bmiCmdVar = cmStrCat("CMAKE_", lang, "_COMPILE_BMI");
+    if (!this->GetMakefile()->GetDefinition(bmiCmdVar).IsEmpty()) {
+      cmdVar = std::move(bmiCmdVar);
+    }
+  }
+  return cmdVar;
+}
+
 void cmNinjaTargetGenerator::WriteCompileRule(std::string const& lang,
                                              std::string const& config,
                                              WithScanning withScanning)
@@ -928,7 +945,7 @@ void cmNinjaTargetGenerator::WriteCompileRule(std::string const& lang,
  }

  // Rule for compiling object file.
-  std::string const cmdVar = cmStrCat("CMAKE_", lang, "_COMPILE_OBJECT");
+  std::string const cmdVar = this->GetCompileTemplateVar(lang);
  std::string const& compileCmd = mf->GetRequiredDefinition(cmdVar);
  cmList compileCmds(compileCmd);

@@ -2313,7 +2330,7 @@ void cmNinjaTargetGenerator::ExportObjectCompileCommand(
    compileObjectVars.CudaCompileMode = cudaCompileMode.c_str();
  }

-  std::string const cmdVar = cmStrCat("CMAKE_", language, "_COMPILE_OBJECT");
+  std::string const cmdVar = this->GetCompileTemplateVar(language);
  std::string const& compileCmd =
    this->Makefile->GetRequiredDefinition(cmdVar);
  cmList compileCmds(compileCmd);
--- a/Source/cmNinjaTargetGenerator.h
+++ b/Source/cmNinjaTargetGenerator.h
@@ -168,6 +168,7 @@ protected:

  void WriteLanguageRules(std::string const& language,
                          std::string const& config);
+  std::string GetCompileTemplateVar(std::string const& lang) const;
  void WriteCompileRule(std::string const& language,
                        std::string const& config);
  void WriteCompileRule(std::string const& language, std::string const& config,
--- a/Tests/RunCMake/CXXModulesCompile/expect/exp-builddb-imped-all-multi.json
+++ b/Tests/RunCMake/CXXModulesCompile/expect/exp-builddb-imped-all-multi.json
@@ -143,7 +143,7 @@
            "REGEX:PATH:-Ddepflag=\"CMakeFiles/CXXModules__export_build_database@synth_<HEX>.dir<CONFIG_DIR>/.d\"",
            "REGEX:<BMI_ONLY_FLAG>",
            "REGEX:PATH:<OUTPUT_FLAG>CMakeFiles/CXXModules__export_build_database@synth_<HEX>.dir<CONFIG_DIR>/",
-            "-c",
+            "REGEX:-[cv]",
            "PATH:<SOURCE_DIR>/exp-builddb/importable.cxx"
          ],
          "baseline-arguments": [
@@ -229,7 +229,7 @@
            "REGEX:PATH:-Ddepflag=\"CMakeFiles/CXXModules__export_build_database@synth_<HEX>.dir<CONFIG_OTHER_DIR>/.d\"",
            "REGEX:<BMI_ONLY_FLAG>",
            "REGEX:PATH:<OUTPUT_FLAG>CMakeFiles/CXXModules__export_build_database@synth_<HEX>.dir<CONFIG_OTHER_DIR>/",
-            "-c",
+            "REGEX:-[cv]",
            "PATH:<SOURCE_DIR>/exp-builddb/importable.cxx"
          ],
          "baseline-arguments": [
--- a/Tests/RunCMake/CXXModulesCompile/expect/exp-builddb-imped-all.json
+++ b/Tests/RunCMake/CXXModulesCompile/expect/exp-builddb-imped-all.json
@@ -87,7 +87,7 @@
            "REGEX:PATH:-Ddepflag=\"CMakeFiles/CXXModules__export_build_database@synth_<HEX>.dir<CONFIG_DIR>/.d\"",
            "REGEX:<BMI_ONLY_FLAG>",
            "REGEX:PATH:<OUTPUT_FLAG>CMakeFiles/CXXModules__export_build_database@synth_<HEX>.dir<CONFIG_DIR>/",
-            "-c",
+            "REGEX:-[cv]",
            "PATH:<SOURCE_DIR>/exp-builddb/importable.cxx"
          ],
          "baseline-arguments": [
--- a/Tests/RunCMake/CXXModulesCompile/expect/exp-builddb-imped-config.json
+++ b/Tests/RunCMake/CXXModulesCompile/expect/exp-builddb-imped-config.json
@@ -87,7 +87,7 @@
            "REGEX:PATH:-Ddepflag=\"CMakeFiles/CXXModules__export_build_database@synth_<HEX>.dir<CONFIG_DIR>/.d\"",
            "REGEX:<BMI_ONLY_FLAG>",
            "REGEX:PATH:<OUTPUT_FLAG>CMakeFiles/CXXModules__export_build_database@synth_<HEX>.dir<CONFIG_DIR>/",
-            "-c",
+            "REGEX:-[cv]",
            "PATH:<SOURCE_DIR>/exp-builddb/importable.cxx"
          ],
          "baseline-arguments": [
--- a/Tests/RunCMake/CXXModulesCompile/expect/exp-builddb-imped-cxx-config.json
+++ b/Tests/RunCMake/CXXModulesCompile/expect/exp-builddb-imped-cxx-config.json
@@ -87,7 +87,7 @@
            "REGEX:PATH:-Ddepflag=\"CMakeFiles/CXXModules__export_build_database@synth_<HEX>.dir<CONFIG_DIR>/.d\"",
            "REGEX:<BMI_ONLY_FLAG>",
            "REGEX:PATH:<OUTPUT_FLAG>CMakeFiles/CXXModules__export_build_database@synth_<HEX>.dir<CONFIG_DIR>/",
-            "-c",
+            "REGEX:-[cv]",
            "PATH:<SOURCE_DIR>/exp-builddb/importable.cxx"
          ],
          "baseline-arguments": [
--- a/Tests/RunCMake/CXXModulesCompile/expect/exp-builddb-imped-cxx-multi.json
+++ b/Tests/RunCMake/CXXModulesCompile/expect/exp-builddb-imped-cxx-multi.json
@@ -143,7 +143,7 @@
            "REGEX:PATH:-Ddepflag=\"CMakeFiles/CXXModules__export_build_database@synth_<HEX>.dir<CONFIG_DIR>/.d\"",
            "REGEX:<BMI_ONLY_FLAG>",
            "REGEX:PATH:<OUTPUT_FLAG>CMakeFiles/CXXModules__export_build_database@synth_<HEX>.dir<CONFIG_DIR>/",
-            "-c",
+            "REGEX:-[cv]",
            "PATH:<SOURCE_DIR>/exp-builddb/importable.cxx"
          ],
          "baseline-arguments": [
@@ -229,7 +229,7 @@
            "REGEX:PATH:-Ddepflag=\"CMakeFiles/CXXModules__export_build_database@synth_<HEX>.dir<CONFIG_OTHER_DIR>/.d\"",
            "REGEX:<BMI_ONLY_FLAG>",
            "REGEX:PATH:<OUTPUT_FLAG>CMakeFiles/CXXModules__export_build_database@synth_<HEX>.dir<CONFIG_OTHER_DIR>/",
-            "-c",
+            "REGEX:-[cv]",
            "PATH:<SOURCE_DIR>/exp-builddb/importable.cxx"
          ],
          "baseline-arguments": [
--- a/Tests/RunCMake/CXXModulesCompile/expect/exp-builddb-imped-cxx.json
+++ b/Tests/RunCMake/CXXModulesCompile/expect/exp-builddb-imped-cxx.json
@@ -87,7 +87,7 @@
            "REGEX:PATH:-Ddepflag=\"CMakeFiles/CXXModules__export_build_database@synth_<HEX>.dir<CONFIG_DIR>/.d\"",
            "REGEX:<BMI_ONLY_FLAG>",
            "REGEX:PATH:<OUTPUT_FLAG>CMakeFiles/CXXModules__export_build_database@synth_<HEX>.dir<CONFIG_DIR>/",
-            "-c",
+            "REGEX:-[cv]",
            "PATH:<SOURCE_DIR>/exp-builddb/importable.cxx"
          ],
          "baseline-arguments": [
--- a/Tests/RunCMake/CXXModulesCompile/export-build-database-setup.cmake
+++ b/Tests/RunCMake/CXXModulesCompile/export-build-database-setup.cmake
@@ -36,6 +36,10 @@ if (CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "MSVC")
 else ()
  set(output_flag "-o")
 endif ()
+if (CMAKE_CXX_COMPILE_BMI) # Only `clang` does this today.
+  set(CMAKE_CXX_COMPILE_BMI
+    "<CMAKE_CXX_COMPILER> <DEFINES> <INCLUDES> <FLAGS> --precompile ${output_flag}<OBJECT> -v <SOURCE>")
+endif ()
 set(CMAKE_CXX_COMPILE_OBJECT
  "<CMAKE_CXX_COMPILER> <DEFINES> <INCLUDES> <FLAGS> ${output_flag}<OBJECT> -c <SOURCE>")