compiletest powerpc64 arch is useless

[jcowgill]

The ARCH_TABLE in compiletest contains a useless entry for powerpc64.
https://github.com/rust-lang/rust/blob/79b25c666fec/src/tools/compiletest/src/util.rs#L34

The only user of that table is get_arch which scans the table sequentially to see if a target contains an architecture in the table. Since powerpc is a substring of powerpc64, the entry for powerpc64 will never match anything and is therefore useless.

This also means that all ignore-powerpc64 headers in the testsuite currently do nothing. I also found a few ignore-powerpc64le headers which probably do nothing as well.

[debris]

What about arm and arm64? Aren't they also affected?

rust/src/tools/compiletest/src/util.rs

Lines 37 to 38 in 79b25c6

	("arm", "arm"),
	("arm64", "aarch64"),

[jcowgill]

Yes I think arm64 is affected as well. I'm not sure why we need both arm64 and aarch64.

[hanna-kruppe]

It seems... ungreat for compiletest to ignore tests tagged ignore-powerpc on powerpc64 given that powerpc is apparently used as the name for the 32 bit architecture. Likewise for arm. #48732 seems to indicate all current tests that ignore "powerpc32" also ignore powerpc64, but if we ever have a test that should run on 64 bit PPC and not on 32 bit PPC, we'd be hosed. Edit: I guess {enable,ignore}-32bit might help? Not sure, and even if it works, would be inconsistent with x86/x86_64.

I think the right fix here is to make compiletest stricter (i.e., only match entire --separated triple components, not substrings).