[PATCH] D19061: [ARM] Add support for the X asm constraint

[James Greenhalgh via llvm-commits]

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jgreenhalgh added a comment.

Decaying this to "w" or "r" would potentially pessimize code generation, and might well break use cases. The whole point of "X" is to prevent the compiler from having to reload an operand you don't actually care about using in the output assembly (a scratch, or in that blog post, a fasle dependency). As GCC isn't going to put any effort in to forcing the form of the operand, I'd expect most uses that actually try to print out an "X" constrained register to be using it as a shorthand for getting constants or labels out. There are normally more expressive constraints if that is what you need.


http://reviews.llvm.org/D19061