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Avoid recomputing COMPILER_/LINKER_ variables when set explicitly

Authored by arichardson on Aug 7 2020, 11:34 AM.



I noticed that when we build libraries for a different ABI (in CheriBSD) we
were calling ${XCC}/${LD} --version for every directory. It turns out that
this was caused by explicitly setting (X_)COMPILER variables
for that build stage and this stops the _can_export logic from working.
To fix this, we change the check to only set _can_export=no if the variable
is set and it is set to a different value than the cached value.
This noticeably speeds up the tree walk while building compat libraries.

Obtained from: CheriBSD

Test Plan

Adding debug output shows fewer cc --version calls and we have been using this
patch for a long time in CheriBSD.

Diff Detail

rS FreeBSD src repository
Automatic diff as part of commit; lint not applicable.
Automatic diff as part of commit; unit tests not applicable.