[flang][driver] Bring --gcc-triple to flang

[kwk]

When there are multiple gcc versions installed, we want flang to be able
to find the right one based on the triple. Here's flang selecting an
unwanted gcc candidate installation, namely
/usr/lib/gcc/x86_64-linux-gnu/15:

~/src/llvm-project/main/build-RelWithDebInfo > ./bin/flang -v
flang version 22.0.0custombuild
Target: x86_64-redhat-linux-gnu
Thread model: posix
InstalledDir: /home/fedora/src/llvm-project/main/build-RelWithDebInfo/bin
System configuration file directory: /etc/clang/
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/15
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/15
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/15
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64

When passing --gcc-triple=x86_64-redhat-linux we get the desired gcc
candidate installation:

~/src/llvm-project/main/build-RelWithDebInfo > ./bin/flang --gcc-triple=x86_64-redhat-linux -v
flang version 22.0.0custombuild
Target: x86_64-redhat-linux-gnu
Thread model: posix
InstalledDir: /home/fedora/src/llvm-project/main/build-RelWithDebInfo/bin
System configuration file directory: /etc/clang/
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/15
Selected GCC installation: /usr/lib/gcc/x86_64-redhat-linux/15
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64

• Test: LIT_FILTER="Flang :: Driver/gcc-triple.f90" ninja check-flang
  • Copied flang/test/Driver/Inputs/fedora_39_tree from clang/test/Driver/Inputs/fedora_39_tree.
  • Testing what default triple is selected when two are possible.
  • Testing that we can select an existing triple.
  • Testing that triple is not selected if it doesn't exist.

[llvmbot]

@llvm/pr-subscribers-flang-driver

@llvm/pr-subscribers-clang

Author: Konrad Kleine (kwk)

Changes

When there are multiple gcc versions installed, we want flang to be able to find the right one based on the triple. Here's flang selecting an unwanted gcc candidate installation, namely
/usr/lib/gcc/x86_64-linux-gnu/15:

~/src/llvm-project/main/build-RelWithDebInfo > ./bin/flang -v
flang version 22.0.0custombuild
Target: x86_64-redhat-linux-gnu
Thread model: posix
InstalledDir: /home/fedora/src/llvm-project/main/build-RelWithDebInfo/bin
System configuration file directory: /etc/clang/
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/15
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/15
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/15
Candidate multilib: .;@<!-- -->m64
Candidate multilib: 32;@<!-- -->m32
Selected multilib: .;@<!-- -->m64

When passing --gcc-triple=x86_64-redhat-linux we get the desired gcc candidate installation, namely /usr/lib/gcc/x86_64-redhat-linux/15:

~/src/llvm-project/main/build-RelWithDebInfo > ./bin/flang --gcc-triple=x86_64-redhat-linux -v
flang version 22.0.0custombuild
Target: x86_64-redhat-linux-gnu
Thread model: posix
InstalledDir: /home/fedora/src/llvm-project/main/build-RelWithDebInfo/bin
System configuration file directory: /etc/clang/
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/15
Selected GCC installation: /usr/lib/gcc/x86_64-redhat-linux/15
Candidate multilib: .;@<!-- -->m64
Candidate multilib: 32;@<!-- -->m32
Selected multilib: .;@<!-- -->m64

---

Full diff: https://github.com/llvm/llvm-project/pull/165886.diff

1 Files Affected:

• (modified) clang/include/clang/Driver/Options.td (+1)

diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index cb5cb888c6da7..8b728e9acbb8b 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -741,6 +741,7 @@ def gcc_toolchain : Joined<["--"], "gcc-toolchain=">, Flags<[NoXarchOption]>,
     "Specify a directory where Flang can find 'lib{,32,64}/gcc{,-cross}/$triple/$version'. "
     "Flang will use the GCC installation with the largest version">;
 def gcc_triple_EQ : Joined<["--"], "gcc-triple=">,
+  Visibility<[ClangOption, FlangOption]>,
   HelpText<"Search for the GCC installation with the specified triple.">;
 def CC : Flag<["-"], "CC">, Visibility<[ClangOption, CC1Option]>,
   Group<Preprocessor_Group>,

[tuliom]

I believe it's important to add a test.
Similar to clang/test/Driver/gcc-triple.cpp.

[kwk]

I believe it's important to add a test. Similar to clang/test/Driver/gcc-triple.cpp.

@tuliom, thank you for pointing this out. I've added tests now.

[tarunprabhu]

Thanks for working on this. Barring some minor changes, this LGTM.

Please change the title to [flang][driver] Bring --gcc-triple to flang. This should also be the first line of the commit message.

Out of curiosity, did the absence of --gcc-triple cause any problems? Did you file an issue?

[kwk]

Thanks for working on this. Barring some minor changes, this LGTM.

@tarunprabhu Thank you for reviewing this!

Please change the title to [flang][driver] Bring --gcc-triple to flang. This should also be the first line of the commit message.

Done.

Out of curiosity, did the absence of --gcc-triple cause any problems?

Yes, it caused a problem. In case of multiple gcc installations candidate, we couldn't select the right one by using a triple but instead had to specify the --gcc-install-dir which includes the gcc version information. In case the major gcc version changes we would have to rebuild llvm which isn't what we wanted.

Did you file an issue?

No I didn't. Is that required in this case? Want me to create one?

[tarunprabhu]

Out of curiosity, did the absence of --gcc-triple cause any problems?

Yes, it caused a problem. In case of multiple gcc installations candidate, we couldn't select the right one by using a triple but instead had to specify the --gcc-install-dir which includes the gcc version information. In case the major gcc version changes we would have to rebuild llvm which isn't what we wanted.

Got it.

No I didn't. Is that required in this case? Want me to create one?

Not required at all. I was just wondering if I missed something that you might have filed.

[DavidSpickett]

This test was failing on non-x86 systems (or rather, builds where that wasn't the default triple):
https://lab.llvm.org/buildbot/#/builders/29/builds/17827
https://lab.llvm.org/buildbot/#/builders/201/builds/6934

So I have added an x86 target to the first command to fix it. The test now passes, but please check this change does not defeat the point of the test.

If you really need it to check what happens without any target argument, we can probably find a set of REQUIRES to only run it in the right places.

[tarunprabhu]

Apologies for missing this. I tried running clang with -### and got this:

$ ./bin/clang -### -v --sysroot=/path/to/llvm-project/clang/test/Driver/Inputs/fedora_39_tree
clang version 22.0.0git
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /path/to/bin
Build config: +assertions
Found candidate GCC installation: /path/to/llvm-project/clang/test/Driver/Inputs/fedora_39_tree/usr/lib/gcc/x86_64-linux-gnu/13
Found candidate GCC installation: /path/to/llvm-project/clang/test/Driver/Inputs/fedora_39_tree/usr/lib/gcc/x86_64-redhat-linux/13
Selected GCC installation: /path/to/llvm-project/clang/test/Driver/Inputs/fedora_39_tree/usr/lib/gcc/x86_64-linux-gnu/13
Candidate multilib: .;@m64
Selected multilib: .;@m64

This might be a better approach since it doesn't require either a REQUIRES directive, or an explicit target triple. The absence of the latter makes it clear that we are checking for the default behavior. This should also just work with flang, but I can check later today.

[kwk]

This test was failing on non-x86 systems (or rather, builds where that wasn't the default triple): https://lab.llvm.org/buildbot/#/builders/29/builds/17827 https://lab.llvm.org/buildbot/#/builders/201/builds/6934

Thank you for bringing this up. I was out shortly after the commit was auto-merged. That wasn't ideal and I hope you accept my apologies.

So I have added an x86 target to the first command to fix it. The test now passes, but please check this change does not defeat the point of the test.

If you really need it to check what happens without any target argument, we can probably find a set of REQUIRES to only run it in the right places.

Honeslty, I thought clang would take the sysroot (Inputs/fedora_39_tree) and only make the containing targets available. I hope no more change or revert is needed.