[mips] Add assembler support for .set push/pop directive.

[tomatabacu]

Summary

These directives are used to save the current assembler options (in the case of ".set push") and restore the previously saved options (in the case of ".set pop").

Contains work done by Matheus Almeida.

[tomatabacu]

Rebased.

[tomatabacu]

Improved a couple of error messages.

[tomatabacu]

Changed the CurrentFeatures and InitialFeatures data members of MipsAssemblerOptions from unsigned type to uint64_t (because of r215887).

[dsanders]

lib/Target/Mips/AsmParser/MipsAsmParser.cpp

54–56

Nit: variables should start with a capital

84

I've just realized the reason we shouldn't use a static member. A static member prevents multiple instances of llvm being used in the same application. For example, we wouldn't be able to assemble two different files with two different llvm instances in the same process.

I think we need to switch to the other implementation we discussed which is to replace the static InitialFeatures with an extra MipsAssemblerOptions object on the stack.

98

We should use unique_ptr here.

315–319

Please pass the current features as an argument to the constructor so we can't forget to call setCurrentFeatures()

test/MC/Mips/set-push-pop-directives.s

9

We should test the default values too

[tomatabacu]

Addressed comments.
Improved the set-push-pop-directives.s test.

[tomatabacu]

Fixed a minor formatting issue.

[tomatabacu]

Replaced the use of std::stack with SmallVector to make it easier to implement the .set mips0 directive (http://reviews.llvm.org/D4957).

[dsanders]

LGTM with a few nits.

lib/Target/Mips/AsmParser/MipsAsmParser.cpp

46

The argument should be const.

305

Nit: The aim isn't to cache the options. The aim is to treat all levels of the stack uniformly. The only special bits about the bottom value is that the user can't change it (because they can't pop that far), and that we will overwrite the top stack element with the values from the bottom when '.set mips0' is used.

306–307

Nit: Please declare and assign in the same statement

2684–2685

Always keep an element on the stack.

Two elements.

This is how we cache the initial options.

As mentioned above, caching isn't the purpose of the outermost environment.

2702–2703

Nit: Please declare and assign in the same statement.

2705

Nit: This isn't necessary. It's about to go out of scope.

[tomatabacu]

Addressed reviewer's concerns.
Further clarified comments.
Rebased.

[dsanders]

LGTM, Thanks.