Runtime-configurable LLVM_DEFAULT_TARGET_TRIPLE by env var

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#1

I have been working for extending test coverage for years.
Nowadays, I have several cross-testing (target != host). See http://bb.pgr.jp/console
Each of them (test-*-linux) is doing;

  • Assume a preceding builder passes with warming ccache.
  • All compilation units will hit ccache whenever the tree is built before lit.
  • Almost all compilation units will hit ccache except for Host.cpp when the tree is built with different LLVM_DEFAULT_TARGET_TRIPLE.

FYI, I am doing a trick to let ccache recognize compilation units identical for build.ninja;

$ sed -i -r ‘s=(-I|_COMPILER\S* )/home/bb/\w[^/]*/llvm-project=\1…/llvm-project=g’ build.ninja

In other words, testing builders have almost same build except for Host.cpp.
I have to build the tree for each testing target.

I propose an option to override LLVM_DEFAULT_TARGET_TRIPLE in run-time.

The change will be small but I wonder which option I could take.
Please let me know your suggestions.

  • Should it be configurable option?

I think it shouldn’t be required for release binaries, and it wouldn’t be required for most developers.
I think it may be off by default.

  • What is the name of environment variable?

I suggest the name itself could be configurable.
If I would introduce the envvar name as “LLVM_TARGET”, clang users would complain why clang didn’t have like “CLANG_TARGET”.

$ cmake -DLLVM_DEFAULT_TARGET_ENVVAR_NAME=LLVM_TARGET
$ LLVM_TARGET=i686-win32 bin/llvm-lit path/to/tests

This feature may be disabled if LLVM_DEFAULT_TARGET_ENVVAR_NAME is blank.
I wonder the name LLVM_DEFAULT_TARGET_ENVVAR_NAME might be weird.
Better suggestions are welcome.

As a future plan, I will implement, in Lit, run tests with the matrix of triples,
like {arm|mips|i686|x86_64}-{darwin|linux|win32}.
Some of tests should be skipped if it isn’t affected to default target triple.
For example, FileCheck

Regards, Takumi

#2

Hypothetically you could hack this in with our existing environment override option, CCC_OVERRIDE_OPTIONS, like so:
ninja all
export CCC_OVERRIDE_OPTIONS=#^–target=$triple

run lit tests

I’d rather not make a new, easier to use environment variable because it will encourage people to use it for production cross-compilation. The existing mechanism is probably good enough for testing purposes.

#3

Hypothetically you could hack this in with our existing environment override option, CCC_OVERRIDE_OPTIONS, like so:
ninja all
export CCC_OVERRIDE_OPTIONS=#^–target=$triple

run lit tests

It’s an option just in clang driver. It doesn’t affect to other clang tools (including libclang), and LLVM.

I’d rather not make a new, easier to use environment variable because it will encourage people to use it for production cross-compilation. The existing mechanism is probably good enough for testing purposes.

Agreed partially. I won’t introduce such an option in release binaries.

#4

I have proposed a patch, thank you.

[CMake] Introduce LLVM_TARGET_TRIPLE_ENV as an option to override LLVM_DEFAULT_TARGET_TRIPLE at runtime.
https://reviews.llvm.org/D33662