[PATCH] D20337: [MC] Support symbolic expressions in assembly directives

[James Y Knight via llvm-commits]

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In http://reviews.llvm.org/D20337#433834, @phosek wrote:

> That would currently trigger an error `error: expected absolute expression` since `MCAsmStreamer` doesn't currently have any layout information. Is this something you would like to support as well?


Yes, I think supporting this is necessary, since "clang -S foo.c" requires exactly that (on platforms that use the integrated assembler).

I think to make that work, everything in AsmParser that currently uses parseAbsoluteExpression/evaluateAsAbsolute will need to be switched to parsing an arbitrary expression, and passing it on to MCStreamer as-is without evaluating. MCObjectStreamer can then assert that you've only given it an absolute expression, and evaluate it. MCAsmStreamer would just emit the expression as-is for the final assembler to deal with.

For many directives, that wouldn't be too bad. ".zero", for example. You can just add an MCStreamer::EmitFill taking two expressions, instead of two integers, and change AsmParser to call that version.

For others, that actually do something with the value other than error-check it and pass it through, it'll be somewhat more complex.


Repository:
  rL LLVM

http://reviews.llvm.org/D20337