Don't overwrite existing target-cpu attributes.
I've often found the replacement behavior annoying, and this is
inconsistent with how the fast math command line flags interact with
the function attributes.
Does not yet change target-features, since I think that should behave
as a concatenation.
A random set of attributes are implemented by llc/opt forcing the
string attributes on the IR functions before processing anything. This
would not happen for MIR functions, which have not yet been created at
this point.
Use a callback in the MIR parser, purely to avoid dealing with the
ugliness that the command line flags are in a .inc file, and would
require allowing access to these flags from multiple places (either
from the MIR parser directly, or a new utility pass to implement these
flags). It would probably be better to cleanup the flag handling into
a separate library.
This is in preparation for treating more command line flags with a
corresponding function attribute in a more uniform way. The fast math
flags in particular have a messy system where the command line flag
sets the behavior from a function attribute if present, and otherwise
the command line flag. This means if any other pass tries to inspect
the function attributes directly, it will be inconsistent with the
intended behavior. This is also inconsistent with the current behavior
of -mcpu and -mattr, which overwrites any pre-existing function
attributes. I would like to move this to consistenly have the command
line flags not overwrite any pre-existing attributes, and to always
ensure the command line flags are consistent with the function
attributes.
Matt Arsenault