Matt Pharr 0c20483853 Make all "if" statements "coherent" ifs. Workaround for issue #74. ...

Using blend to do masked stores is unsafe if all of the lanes are off:
it may read from or write to invalid memory.  For now, this workaround
transforms all 'if' statements into coherent 'if's, ensuring that an
instruction only runs if at least on program instance wants to be running
it.

One nice thing about this change is that a number of implementations of
various builtins can be simplified, since they no longer need to confirm
that at least one program instance is running.

It might be nice to re-enable regular if statements in a future checkin,
but we'd want to make sure they don't have any masked loads or blended
masked stores in their statement lists.  There isn't a performance
impact for any of the examples with this change, so it's unclear if
this is important.

Note that this only impacts 'if' statements with a varying condition.

2011-09-12 16:25:08 -07:00

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