[clang] [clang][Driver][SYCL] Fix SYCLInstallationDetector for compiler installs outside sysroot (PR #211351)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Nick Sarnie (sarnex)

<details>
<summary>Changes</summary>

Currently if the compiler install is not under SYSROOT, we don't try to compute the relative path to `libLLVMSYCL.so` based on the compiler binary installation directory, resulting us never setting `SYCLRTLibPath` so when we try to geenerate a linker call we get a raw `libLLVMSYCL.so` which fails unless the user happens to be running the compiler from a directory with `libLLVMSYCL.so` inside it.

It seems we never use the result from SYCLInstallationDetector on Windows, so I can't add a test, however change the code to be consistent in case we start to use it in the future.

---
Full diff: https://github.com/llvm/llvm-project/pull/211351.diff


2 Files Affected:

- (modified) clang/lib/Driver/ToolChains/SYCL.cpp (+2-5) 
- (added) clang/test/Driver/sycl-rtlib-sysroot-mismatch.cpp (+11) 


``````````diff
diff --git a/clang/lib/Driver/ToolChains/SYCL.cpp b/clang/lib/Driver/ToolChains/SYCL.cpp
index d41b5c06da53f..7208a8cac9436 100644
--- a/clang/lib/Driver/ToolChains/SYCL.cpp
+++ b/clang/lib/Driver/ToolChains/SYCL.cpp
@@ -21,7 +21,6 @@ SYCLInstallationDetector::SYCLInstallationDetector(
     : D(D) {
   // When -fsycl is active, locate the SYCL runtime library and record its
   // directory in SYCLRTLibPath for use by the linker.
-  StringRef SysRoot = D.SysRoot;
   SmallString<128> DriverDir(D.Dir);
 
   if (HostTriple.isWindowsMSVCEnvironment() ||
@@ -30,8 +29,7 @@ SYCLInstallationDetector::SYCLInstallationDetector(
     // NOTE: Only checks for LLVMSYCL.lib existence (release variant).
     // Debug vs release library selection happens at link time based on CRT
     // flags.
-    if (DriverDir.starts_with(SysRoot) &&
-        Args.hasFlag(options::OPT_fsycl, options::OPT_fno_sycl, false)) {
+    if (Args.hasFlag(options::OPT_fsycl, options::OPT_fno_sycl, false)) {
       SmallString<128> LibDir(DriverDir);
       llvm::sys::path::append(LibDir, "..", "lib");
 
@@ -52,8 +50,7 @@ SYCLInstallationDetector::SYCLInstallationDetector(
     SmallString<128> FlatLibPath(DriverDir);
     llvm::sys::path::append(FlatLibPath, "..", "lib", "libLLVMSYCL.so");
 
-    if (DriverDir.starts_with(SysRoot) &&
-        Args.hasFlag(options::OPT_fsycl, options::OPT_fno_sycl, false)) {
+    if (Args.hasFlag(options::OPT_fsycl, options::OPT_fno_sycl, false)) {
       // LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON: library is in lib/<triple>/
       if (D.getVFS().exists(LibPath))
         llvm::sys::path::append(DriverDir, "..", "lib", HostTriple.str());
diff --git a/clang/test/Driver/sycl-rtlib-sysroot-mismatch.cpp b/clang/test/Driver/sycl-rtlib-sysroot-mismatch.cpp
new file mode 100644
index 0000000000000..a0edbaa425510
--- /dev/null
+++ b/clang/test/Driver/sycl-rtlib-sysroot-mismatch.cpp
@@ -0,0 +1,11 @@
+// REQUIRES: system-linux && symlinks
+
+// Verify we still generate a path to libLLVMSYCL.so even with sysroot set.
+
+// RUN: rm -rf %t && mkdir -p %t/bin %t/lib
+// RUN: touch %t/lib/libLLVMSYCL.so
+// RUN: ln -s %clang %t/bin/clang
+// RUN: %t/bin/clang -### -no-canonical-prefixes --target=x86_64-unknown-linux-gnu -fsycl --sysroot=/nonexistent-prefix %s 2>&1 \
+// RUN:   | FileCheck %s
+
+// CHECK: "{{.*}}/bin/../lib/libLLVMSYCL.so"

``````````

</details>


https://github.com/llvm/llvm-project/pull/211351