[clang] c4dc21e - [Clang][Modules] Emit C++20 BMIs to working directory with -fmodule-output (#174560)

[via cfe-commits]

Author: CarvedCoder
Date: 2026-01-08T10:55:00Z
New Revision: c4dc21ecc1e92bd5b04c8dcec8d3d3696889423a

URL: https://github.com/llvm/llvm-project/commit/c4dc21ecc1e92bd5b04c8dcec8d3d3696889423a
DIFF: https://github.com/llvm/llvm-project/commit/c4dc21ecc1e92bd5b04c8dcec8d3d3696889423a.diff

LOG: [Clang][Modules] Emit C++20 BMIs to working directory with -fmodule-output (#174560)

Close #173689.

When -fmodule-output is specified without an explicit path, Clang
currently emits the generated .pcm next to the source file. This
contradicts the documentation and breaks out-of-tree builds.

This patch updates the fallback behavior to emit the BMI in the working
directory, instead of the source file directory.

Driver tests are updated accordingly. Explicit -fmodule-output=PATH
behavior is unchanged.

Added: 
    

Modified: 
    clang/lib/Driver/ToolChains/Clang.cpp
    clang/test/Driver/module-fgen-reduced-bmi.cppm
    clang/test/Driver/module-output.cppm

Removed: 
    


################################################################################
diff  --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 699fc31f23946..a0c98bcea85e6 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -3783,8 +3783,10 @@ clang::driver::tools::getCXX20NamedModuleOutputPath(const ArgList &Args,
   if (Arg *FinalOutput = Args.getLastArg(options::OPT_o);
       FinalOutput && Args.hasArg(options::OPT_c))
     OutputPath = FinalOutput->getValue();
-  else
-    OutputPath = BaseInput;
+  else {
+    llvm::sys::fs::current_path(OutputPath);
+    llvm::sys::path::append(OutputPath, llvm::sys::path::filename(BaseInput));
+  }
 
   const char *Extension = types::getTypeTempSuffix(types::TY_ModuleFile);
   llvm::sys::path::replace_extension(OutputPath, Extension);

diff  --git a/clang/test/Driver/module-fgen-reduced-bmi.cppm b/clang/test/Driver/module-fgen-reduced-bmi.cppm
index 4b893ffbfaae8..0199bf8309bfe 100644
--- a/clang/test/Driver/module-fgen-reduced-bmi.cppm
+++ b/clang/test/Driver/module-fgen-reduced-bmi.cppm
@@ -48,14 +48,10 @@
 // RUN: %clang -std=c++20 Hello.cppm --precompile -fmodules-reduced-bmi \
 // RUN:     -o Hello.full.pcm -### 2>&1 | FileCheck Hello.cppm \
 // RUN:     --check-prefix=CHECK-EMIT-MODULE-INTERFACE
-
-// RUN: %clang -std=c++20 Hello.cppm --precompile -fmodules-reduced-bmi \
-// RUN:     -### 2>&1 | FileCheck Hello.cppm \
+//
+// RUN: %clang -std=c++20 %t/Hello.cppm --precompile -fmodules-reduced-bmi \
+// RUN:     -o %t/Hello.pcm -### 2>&1 | FileCheck %t/Hello.cppm \
 // RUN:     --check-prefix=CHECK-OVERRIDE-WARN
-
-// RUN: %clang -std=c++20 Hello.cppm --precompile -fmodules-reduced-bmi \
-// RUN:     -o Hello.pcm -### 2>&1 | FileCheck Hello.cppm \
-// RUN:    --check-prefix=CHECK-OVERRIDE-WARN
 //
 // RUN: %clang -std=c++20 Hello.cc -fmodules-reduced-bmi -Wall -Werror \
 // RUN:     -c -o Hello.o -### 2>&1 | FileCheck Hello.cc
@@ -74,16 +70,14 @@ export module Hello;
 // CHECK-NOT: -emit-module-interface
 // CHECK: "-fmodules-reduced-bmi"
 
-// CHECK-UNSPECIFIED: -fmodule-output=Hello.pcm
-
-// CHECK-NO-O: -fmodule-output=Hello.pcm
-// CHECK-ANOTHER-NAME: -fmodule-output=AnotherName.pcm
+// CHECK-UNSPECIFIED: -fmodule-output={{.*}}Hello.pcm
+// CHECK-NO-O: -fmodule-output={{.*}}Hello.pcm
+// CHECK-ANOTHER-NAME: -fmodule-output={{.*}}AnotherName.pcm
 
 // With `-emit-module-interface` specified, we should still see the `-emit-module-interface`
 // flag.
 // CHECK-EMIT-MODULE-INTERFACE: -emit-module-interface
-
-// CHECK-OVERRIDE-WARN: warning: the implicit output of reduced BMI may be overrided by the output file specified by '--precompile'. {{.*}}-Wreduced-bmi-output-overrided
+// CHECK-OVERRIDE-WARN: implicit output of reduced BMI{{.*}}-Wreduced-bmi-output-overrided
 
 // NO_WARN-NOT: warning
 

diff  --git a/clang/test/Driver/module-output.cppm b/clang/test/Driver/module-output.cppm
index 197f1d85b0f9c..84f49737b1a6a 100644
--- a/clang/test/Driver/module-output.cppm
+++ b/clang/test/Driver/module-output.cppm
@@ -42,7 +42,7 @@ export module Hello;
 
 // CHECK: "-emit-module-interface" {{.*}}"-main-file-name" "Hello.cppm" {{.*}}"-o" "{{.*}}/output/Hello.pcm" "-x" "c++" "{{.*}}/Hello.cppm"
 // CHECK: "-emit-obj" {{.*}}"-main-file-name" "Hello.cppm" {{.*}}"-o" "{{.*}}/output/Hello.o" "-x" "pcm" "{{.*}}/output/Hello.pcm"
-
+// CHECK-CWD: -fmodule-output={{.*}}/build/Hello.pcm
 // MULTIPLE-ARCH: option '-fmodule-output' cannot be used with multiple arch options
 
 // CHECK-SPECIFIED: "-emit-module-interface" {{.*}}"-main-file-name" "Hello.cppm" {{.*}}"-o" "{{.*}}/pcm/Hello.pcm" "-x" "c++" "{{.*}}/Hello.cppm"