Just for the heck of it I decided to try compilation at -O1, mostly to see what would happpen. What happened was disaster. I used the in-development printf.c file as a test, and I got this:

printf.c: In function ‘printf’:
printf.c:60: internal compiler error: in df_ref_record, at df-scan.c:2799
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EMW>> df_ref_record : GET_CODE = 41

If -Os is used, there is no problem. Odd.

The code 41 line was added to help debug this mess. What I've fallen into is that a CONCAT RTX is being used where a register RTX is expected. the assertion fails, and I get the error above. This is during step 172r.ira. I think a CONCAT is a construct where a value is spread over several registers, but these are only supposed to show up during initial RTX generation. This seems a little late for concats, based on what I've read. Need more research.