[Clang][Driver] Support relative paths for CLANG_CONFIG_FILE_SYSTEM_DIR

[carlocab]

Shipping a system configuration file for Clang is useful, but it limits
the relocatability of the toolchain because it bakes in a reference to
an absolute path on the file system.

Let's fix that by allowing for CLANG_CONFIG_FILE_SYSTEM_DIR to be set
to a relative path, and then interpreting that relative to the location
of the driver if applicable.

This would be useful for the LLVM package we ship at Homebrew. We
currently have to bake in a DEFAULT_SYSROOT in order to ship a
toolchain that works out of the box on macOS. If
CLANG_CONFIG_FILE_SYSTEM_DIR supported relative paths, we could
replace that with a configuration file which would be easier to update
when the compiled-in DEFAULT_SYSROOT becomes stale (e.g. on macOS
version upgrades).

We could, of course, set CLANG_CONFIG_FILE_SYSTEM_DIR to an absolute
path to begin with. However, we do have users who install Homebrew into
a prefix that is different from the one used on our buildbots, so doing
this would result in a broken toolchain for those users.

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[llvmbot]

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-driver

Author: Carlo Cabrera (carlocab)

Changes

Shipping a system configuration file for Clang is useful, but it limits
the relocatability of the toolchain because it bakes in a reference to
an absolute path on the file system.

Let's fix that by allowing for CLANG_CONFIG_FILE_SYSTEM_DIR to be set
to a relative path, and then interpreting that relative to the location
of the driver if applicable.

This would be useful for the LLVM package we ship at Homebrew. We
currently have to bake in a DEFAULT_SYSROOT in order to ship a
toolchain that works out of the box on macOS. If
CLANG_CONFIG_FILE_SYSTEM_DIR supported relative paths, we could
replace that with a configuration file which would be easier to update
when the compiled-in DEFAULT_SYSROOT becomes stale (e.g. on macOS
version upgrades).

We could, of course, set CLANG_CONFIG_FILE_SYSTEM_DIR to an absolute
path to begin with. However, we do have users who install Homebrew into
a prefix that is different from the one used on our buildbots, so doing
this would result in a broken toolchain for those users.

---

Full diff: https://github.com/llvm/llvm-project/pull/110962.diff

1 Files Affected:

• (modified) clang/lib/Driver/Driver.cpp (+10-1)

diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index 10d72be2c3d658..aaedfbc31579b7 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -229,7 +229,16 @@ Driver::Driver(StringRef ClangExecutable, StringRef TargetTriple,
   }
 
 #if defined(CLANG_CONFIG_FILE_SYSTEM_DIR)
-  SystemConfigDir = CLANG_CONFIG_FILE_SYSTEM_DIR;
+  {
+    if (llvm::sys::path::is_absolute(CLANG_CONFIG_FILE_SYSTEM_DIR)) {
+      SystemConfigDir = CLANG_CONFIG_FILE_SYSTEM_DIR;
+    } else {
+      SmallString<128> configFileDir(Dir);
+      llvm::sys::path::append(configFileDir, CLANG_CONFIG_FILE_SYSTEM_DIR);
+      llvm::sys::path::remove_dots(configFileDir, true);
+      SystemConfigDir = static_cast<std::string>(configFileDir);
+    }
+  }
 #endif
 #if defined(CLANG_CONFIG_FILE_USER_DIR)
   {

[MaskRay]

LGTM if you remove the unneeded, outer brace

[carlocab]

Thanks, done! I thought that was preferred given the handling of CLANG_CONFIG_FILE_USER_DIR below it.

[MaskRay]

Note: you are using github's private email address feature, but llvm-project prefers to show the real email address. Actually, there is a bot detecting the issue and making a message, though it has some issues and does not trigger on this PR.

[carlocab]

I authored my commit using a working email address. See the header of the patch:

From e31545f666eb4ca32030956a38dbc4078a64068c Mon Sep 17 00:00:00 2001
From: Carlo Cabrera <[email protected]>
Date: Thu, 3 Oct 2024 14:15:07 +0800
Subject: [PATCH] [Clang][Driver] Support relative paths for
 CLANG_CONFIG_FILE_SYSTEM_DIR

Is that insufficient?

[MaskRay]

This commit will be authored by [email protected]

"Squash and merge" uses the PR author's github's setting. The commit email is completely ignored (unless there is another contributor, which would lead to a Co-authored-by:)

[carlocab]

That's annoying. Thanks GitHub. I've updated my settings now.

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