[lldb-dev] When should ArchSpecs match?

[Jason Molenda via lldb-dev]

I think the confusing thing is when "unspecified" means "there is no OS" or "there is no vendor" versus "vendor/OS is unspecified".

Imagine debugging a firmware environment where we have a cpu arch, and we may have a vendor, but we specifically do not have an OS.  Say armv7-apple-none (I make up "none", I don't think that's valid).  If lldb is looking for a binary and it finds one with armv7-apple-ios, it should reject that binary, they are incompatible.

As opposed to a triple of "armv7-*-*" saying "I know this is an armv7 system target, but I don't know anything about the vendor or the OS" in which case an armv7-apple-ios binary is compatible.

My naive reading of "arm64-*-*" means vendor & OS are unspecified and should match anything.

My naive reading of "arm64" is that it is the same as "arm64-*-*".

I don't know what a triple string looks like where we specify "none" for a field.  Is it armv7-apple-- ?  I know Triple has Unknown enums, but "Unknown" is ambiguous between "I don't know it yet" versus "It not any Vendor/OS".

Some of the confusion is the textual representation of the triples, some of it is the llvm Triple class not having a way to express (afaik) "do not match this field against anything" aka "none".



> On Dec 6, 2018, at 3:19 PM, Adrian Prantl via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> 
> I was puzzled by the behavior of ArchSpec::IsExactMatch() and IsCompatibleMatch() yesterday, so I created a couple of unit tests to document the current behavior. Most of the tests make perfect sense, but a few edge cases really don't behave like I would have expected them to.
> 
>>  {
>>    ArchSpec A("arm64-*-*");
>>    ArchSpec B("arm64-apple-ios");
>>    ASSERT_FALSE(A.IsExactMatch(B));
>>    // FIXME: This looks unintuitive and we should investigate whether
>>    // this is the desired behavior.
>>    ASSERT_FALSE(A.IsCompatibleMatch(B));
>>  }
>>  {
>>    ArchSpec A("x86_64-*-*");
>>    ArchSpec B("x86_64-apple-ios-simulator");
>>    ASSERT_FALSE(A.IsExactMatch(B));
>>    // FIXME: See above, though the extra environment complicates things.
>>    ASSERT_FALSE(A.IsCompatibleMatch(B));
>>  }
>>  {
>>    ArchSpec A("x86_64");
>>    ArchSpec B("x86_64-apple-macosx10.14");
>>    // FIXME: The exact match also looks unintuitive.
>>    ASSERT_TRUE(A.IsExactMatch(B));
>>    ASSERT_TRUE(A.IsCompatibleMatch(B));
>>  }
>> 
> 
> Particularly, I believe that:
> - ArchSpec("x86_64-*-*") and ArchSpec("x86_64") should behave the same.
> - ArchSpec("x86_64").IsExactMatch("x86_64-apple-macosx10.14") should be false.
> - ArchSpec("x86_64-*-*").IsCompatibleMath("x86_64-apple-macosx") should be true.
> 
> Does anyone disagree with any of these statements?
> 
> I fully understand that changing any of these behaviors will undoubtedly break one or the other edge case, but I think it would be important to build on a foundation that actually makes sense if we want to be able to reason about the architecture matching logic at all.
> 
> let me know what you think!
> -- adrian
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