r225244 - Sema: analyze I,J,K,M,N,O constraints

[Joerg Sonnenberger]

On Tue, Jan 06, 2015 at 04:26:34AM -0000, Saleem Abdulrasool wrote:
> Author: compnerd
> Date: Mon Jan  5 22:26:34 2015
> New Revision: 225244
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=225244&view=rev
> Log:
> Sema: analyze I,J,K,M,N,O constraints
> 
> Add additional constraint checking for target specific behaviour for inline
> assembly constraints.  We would previously silently let all arguments through
> for these constraints.  In cases where the constraints were violated, we could
> end up failing to select instructions and triggering assertions or worse,
> silently ignoring instructions.

As discussion on IRC, I think the change is incorrect at least for 'K'.
It makes it impossible to encapsulate the equivalent to certain MMX/SSE
intrinsincs with always_inline functions, creating a regression for
pixman for example. We already have logic in the backend to ensure that
doesn't happen. In short: it is fine to verify that the constraint *can
be* be fulfilled, but it is not acceptable to require immediates in
cases where the inliner or GVN would be able to compute constant
arguments later.

Joerg