[IRGen] Reorder generic struct metadata so that field offsets follow …

[xedin]

…generic parameters

Right now, field offsets come before generic arguments in instances
of a struct's metadata. It would be more resilient to put the generic
arguments first, since they're always used by compiler-generated code,
and cannot change resiliently, unlike fields.

Resolves: rdar://problem/36560486

[xedin]

@swift-ci please test

[xedin]

@jckarter please take a look, I'm going to try and move from pointer size to uint32_t for field offsets separately.

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - cfa8b82387a87ddf3ad3ffbd515464cc396b5d67

[xedin]

@swift-ci please test

[jckarter]

Looks good! Changing the field offsets to 32-bit as a separate patch makes sense to me.

[xedin]

@swift-ci please test source compatibility

[xedin]

Source compatibility failed with known certificate problems, merging.

[slavapestov]

@xedin Can you also add a test to validation-test/Evolution to test this case? Look at the existing tests to see how they work. The setup should be:

• Library file defines a generic resilient struct with a protocol-constrained parameter
• Before: a generic resilient struct has N fields
• After: a generic resilient struct has N+1 fields
• The test program itself defines an extension that uses type metadata for the generic parameter in some way, perhaps calling a static protocol requirement on the type

This test should fail before your change, because the offset at which the generic arguments are stored will change, causing failures in the after/before and before/after combinations.

Also can you verify that for enums, the generic arguments are always at a fixed offset? Some enum metadata has a field for the payload size, and I don't remember if this comes before or after the generic arguments.

[xedin]

@slavapestov Will do!