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

[James Greenhalgh via llvm-commits]

jgreenhalgh added a comment.

In http://reviews.llvm.org/D19061#402576, @rengolin wrote:

> In http://reviews.llvm.org/D19061#401361, @sbaranga wrote:
>
> > It is true that GCC would be more efficient in some cases (one example would be FP constants), but we would still fit into the definition of "no constraint whatsoever" and therefore correct - which is an improvement from the current situation, where we'll simply crash on this constraint.
>
>
> I agree bad codegen trumps ICE crashes, but James mentioned it "might well break use cases". I'm interested in those...
>
> James, do you have some pointers on the expected usage of this constraint in the wild? The more the merrier!


A Debian grep gets a few examples out from x86 mmx headers (and a couple for other architectures) https://codesearch.debian.net/results/%22%3DX%22/page_0

I'm making no promises as to how correct those uses are (http://sources.debian.net/src/php7.0/7.0.5-3/Zend/zend_multiply.h/?hl=56#L56 looks pretty fragile!), but I don't know of any better sources for real world code. I was only guessing this would break somebody's use case (because it is so liberal at the moment), rather than having one in particular in mind.


http://reviews.llvm.org/D19061