[Info-vax] yet another sys$qiow question

[John Reagan]

The early VAX compilers didn't do lots of caching fields of structs.  COBOL wasn't optimized at all.  Given the few registers available, it didn't make much sense to cache a piece of the IOSB over a long sequence of code.  You almost always got a refetch.  VAX BLISS is probably the most aggressive about such things.  I don't think VAX FORTRAN even added VOLATILE until later in its history for example.  Architectures like Alpha and Itanium with more registers started making such "fetch-once/use-many" optimizations valuable.  On x86 with fewer registers, it may not be viewed as profitable.  I haven't dug all that deep into LLVM's heuristics to see.  If we need to adjust some things to help paper over older code, we'll figure it out.