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I could be wrong here but I thought that this form of subregs were put into gcc so that it was obvious to the compiler which bits were being preserved and which bits were being destroyed. This is a message that may not have made it to the masses who support the ports and have hacked on much of the back end, but subregs seem to me a perfectly reasonable way to represent partial word operations until very late in the code generation process.
What danny and I have done is build the transfer functions so that it conveys this information to the client of dataflow so that those clients can take advantage of this.
1 (set x 40000) 2 (set (strict_low x) 23) 3 (use x)
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