[llvm-dev] Runtime-configurable LLVM_DEFAULT_TARGET_TRIPLE by env var

[NAKAMURA Takumi via llvm-dev]

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
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