[clang] [Driver] Change default PCH extension name from .gch to .pch (PR #66165)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-driver
            
<details>
<summary>Changes</summary>
For `c-header` and `c++-header` inputs, Clang driver selects the
`-emit-pch` frontend action and defaults the output suffix name to
`.gch` (except clang-cl, which uses `.pch`) to follow GCC.
```
clang -c a.h               # a.h.gch
clang -xc++-header -c a.h  # a.h.gch
```

In every other place we prefer `.pch` to `.gch`. E.g. `-include a.h`
probes `a.h.pch` before `a.h.gch`, and we have `-include-pch` instead of
`-include-gch`. (Clang and GCC use different extension names for C++ modules as 
well.)

This change isn't disruptive because:

* Most newer build systems specify explicit output filenames and are unaffected
* Implicit output name users almost assuredly use something like
  `-include a.h`, which probes both `a.h.pch` and `a.h.gch`[^1]

[^1]: In the future, we shall stop probing `a.h.gch` for `-include a.h` as the
compatibility intended behavior actually gets in the way
(https://discourse.llvm.org/t/how-to-have-clang-ignore-gch-directories/51835).
This patch is a prerequisite.

--
Full diff: https://github.com/llvm/llvm-project/pull/66165.diff

3 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+3) 
- (modified) clang/include/clang/Driver/Types.def (+1-1) 
- (modified) clang/test/Driver/pch-codegen.cpp (+3-1) 


<pre>
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 3cdad2f7b9f0e5a..d558705f16ca3be 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -41,6 +41,9 @@ code bases.
 C/C++ Language Potentially Breaking Changes
 -------------------------------------------
 
+- The default extension name for PCH generation (``-c -xc-header`` and ``-c
+  -xc++-header``) is now ``.pch`` instead of ``.gch``.
+
 C++ Specific Potentially Breaking Changes
 -----------------------------------------
 
diff --git a/clang/include/clang/Driver/Types.def 
b/clang/include/clang/Driver/Types.def
index 519d51f8ad1e5f0..b889883125c4c16 100644
--- a/clang/include/clang/Driver/Types.def
+++ b/clang/include/clang/Driver/Types.def
@@ -98,7 +98,7 @@ TYPE(&quot;plist&quot;,                    Plist,        
INVALID,         &quot;plist&quot;,  phases
 TYPE(&quot;rewritten-objc&quot;,           RewrittenObjC,INVALID,         
&quot;cpp&quot;,    phases::Compile, phases::Backend, phases::Assemble, 
phases::Link)
 TYPE(&quot;rewritten-legacy-objc&quot;,    RewrittenLegacyObjC,INVALID,   
&quot;cpp&quot;,    phases::Compile, phases::Backend, phases::Assemble, 
phases::Link)
 TYPE(&quot;remap&quot;,                    Remap,        INVALID,         
&quot;remap&quot;,  phases::Compile, phases::Backend, phases::Assemble, 
phases::Link)
-TYPE(&quot;precompiled-header&quot;,       PCH,          INVALID,         
&quot;gch&quot;,    phases::Compile, phases::Backend, phases::Assemble, 
phases::Link)
+TYPE(&quot;precompiled-header&quot;,       PCH,          INVALID,         
&quot;pch&quot;,    phases::Compile, phases::Backend, phases::Assemble, 
phases::Link)
 TYPE(&quot;object&quot;,                   Object,       INVALID,         
&quot;o&quot;,      phases::Link)
 TYPE(&quot;treelang&quot;,                 Treelang,     INVALID,         
nullptr,  phases::Compile, phases::Backend, phases::Assemble, phases::Link)
 TYPE(&quot;image&quot;,                    Image,        INVALID,         
&quot;out&quot;,    phases::Compile, phases::Backend, phases::Assemble, 
phases::Link)
diff --git a/clang/test/Driver/pch-codegen.cpp 
b/clang/test/Driver/pch-codegen.cpp
index f16f6b51bf409ff..ac4ffb978ca59b1 100644
--- a/clang/test/Driver/pch-codegen.cpp
+++ b/clang/test/Driver/pch-codegen.cpp
@@ -2,10 +2,12 @@
 // RUN: mkdir -p %t
 
 // Create PCH without codegen.
-// RUN: %clang -x c++-header %S/../Modules/Inputs/codegen-flags/foo.h -o 
%t/foo-cg.pch -### 2&gt;&amp;1 | FileCheck %s -check-prefix=CHECK-PCH-CREATE
+// RUN: %clang -x c++-header %S/../Modules/Inputs/codegen-flags/foo.h -### 
2&gt;&amp;1 | FileCheck %s -check-prefix=CHECK-PCH-CREATE
 // CHECK-PCH-CREATE: -emit-pch
 // CHECK-PCH-CREATE-NOT: -fmodules-codegen
 // CHECK-PCH-CREATE-NOT: -fmodules-debuginfo
+/// Also test that the default extension name is .pch instead of .gch
+// CHECK-PCH-CREATE: &quot;-o&quot; &quot;{{[^&quot;]+.pch}}&quot;
 
 // Create PCH with -fpch-codegen.
 // RUN: %clang -x c++-header -fpch-codegen 
%S/../Modules/Inputs/codegen-flags/foo.h -o %t/foo-cg.pch -### 2&gt;&amp;1 | 
FileCheck %s -check-prefix=CHECK-PCH-CODEGEN-CREATE
</pre>
</details>


https://github.com/llvm/llvm-project/pull/66165
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