[Driver] Drop $sysroot/usr special case from Gentoo gcc-config detection

[MaskRay]

Summary

If --gcc-toolchain is specified, we should detect GCC installation there, and suppress other directories for detection.

[manojgupta]

Thanks @MaskRay for this clean up. I can't speak for all of Gentoo but please give me a couple of days to at least test this on Chrome OS.

[MaskRay]

Thanks. This looks just looks me untested and likely uneeded logic.

My notes about --gcc-toolchain= and --prefix (-B)

https://lore.kernel.org/linux-kbuild/[e-mail]/

In the presence of --gcc-toolchain= (even if it equals $prefix/usr), the magic detection should be avoided.

[mgorny]

Well, I think this was mostly 'just in case' condition, so I guess it's fine. Let's just wait for @manojgupta to give it some more testing.

[MaskRay]

In D97894#2602638, @mgorny wrote:

Well, I think this was mostly 'just in case' condition, so I guess it's fine. Let's just wait for @manojgupta to give it some more testing.

Thanks. I sent D97902 to improve the documentation. Dropping the special case makes it more consistent: if --gcc-toolchain is not empty, suppress sysroot GCC detection

With the special rule, it is: if --gcc-toolchain is $sysroot/usr, suppress sysroot GCC detection as well.

Clang -B and --gcc-toolchain have some weird behaviors. Hope you can share your opinions on https://lists.llvm.org/pipermail/cfe-dev/2021-March/067827.html

[manojgupta]

In Chrome OS, we currently both "-B" and "--prefix". "-B" points to binutils bin directory and the "--prefix" has the binutils directory + target suffix. Should we drop "-B" and still get the same behavior?

Sample invocation: '/usr/bin/clang++' '--sysroot=/usr/x86_64-cros-linux-gnu '-fcrash-diagnostics-dir=/tmp/clang_crash_diagnostics' '-fcommon' '-fstack-protector-strong' '-fPIE' '-pie' '-D_FORTIFY_SOURCE=2' '-fno-omit-frame-pointer' '--prefix=../../../../../../usr/libexec/gcc/x86_64-cros-linux-gnu/x86_64-cros-linux-gnu-' 'foo.o' '-o' 'main' '-B../../../../../../usr/libexec/gcc/x86_64-cros-linux-gnu' '-target' 'x86_64-cros-linux-gnu'

I am not yet able to test this change in Chrome OS thoroughly yet because of some CQ issues. But manually checking the command lines for a few common cases does not show any difference so far.

[manojgupta]

With the special rule, it is: if --gcc-toolchain is $sysroot/usr, suppress sysroot GCC detection as well.

Clang -B and --gcc-toolchain have some weird behaviors. Hope you can share your opinions on https://lists.llvm.org/pipermail/cfe-dev/2021-March/067827.html

Sure, I have replied on the thread with chrome os usage explanation. I also noticed that we also use "--gcc-toolchain" as well so the whole lot.

[manojgupta]

@MaskRay I have verified that Chrome OS builds are not affected by this change.

[MaskRay]

In D97894#2620077, @manojgupta wrote:

@MaskRay I have verified that Chrome OS builds are not affected by this change.

Thank you for testing!

clang/lib/Driver/ToolChains/Gnu.cpp

if (GCCToolchainDir != "") {
     Prefixes.push_back(D.InstalledDir + "/..");
 
     // Next, look for prefix(es) that correspond to distribution-supplied gcc
     // installations.
     if (D.SysRoot.empty()) {
       // Typically /usr.
       AddDefaultGCCPrefixes(TargetTriple, Prefixes, D.SysRoot);
     }
-  }
 
-  // Try to respect gcc-config on Gentoo. However, do that only
-  // if --gcc-toolchain is not provided or equal to the Gentoo install
-  // in /usr. This avoids accidentally enforcing the system GCC version
-  // when using a custom toolchain.
-  if (GCCToolchainDir == "" || GCCToolchainDir == D.SysRoot + "/usr") {
+    // Try to respect gcc-config on Gentoo if --gcc-toolchain is not provided.
+    // This avoids accidentally enforcing the system GCC version when using a
+    // custom toolchain.
     SmallVector<StringRef, 16> GentooTestTriples;
     // Try to match an exact triple as target triple first.
     // e.g. crossdev -S x86_64-gentoo-linux-gnu will install gcc libs for
     // x86_64-gentoo-linux-gnu. But "clang -target x86_64-gentoo-linux-gnu"
     // may pick the libraries for x86_64-pc-linux-gnu even when exact matching
     // triple x86_64-gentoo-linux-gnu is present.
     GentooTestTriples.push_back(TargetTriple.str());
     // Check rest of triples.