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To finish off this appendix, here’s an example of the power of
OIL; this is the optimization of an idiom from the
INTERCAL-72 system library, as shown with -H;
this should give a good idea of how OIL programs
work. (All the relevant idioms are in idiotism.oil as of
the time of writing.) Note how the expression is reduced one small step
at a time; the smallness of the steps makes the optimizer more general,
because if the original expression had been slightly different, the
optimizer wouldn’t have come to the same result but could have
optimized it quite a bit of the way, up to the point where the
optimizations were no longer valid; in an older version of
INTERCAL, this idiom was simply hardcoded as a
special case and so slight variations of it weren’t optimized at
all. If you look at the idioms themselves, it’ll also be apparent
how c
(the record of which bits of an expression can be 1
and which bits can’t be) is important information in being able
to apply an optimization more aggressively.
.3 <- ((((((((.3 $ 0x0) ~ (0x7fff $ 0x1)) $ 0x0) ~ (0x7fff $ 0x1)) $ 0x0) ~ (0x3fff $ 0x3)) $ 0x0) ~ (0xfff $ 0xf)) [minglefold] .3 <- ((((((((.3 $ 0x0) ~ 0x2aaaaaab) $ 0x0) ~ (0x7fff $ 0x1)) $ 0x0) ~ (0x3fff $ 0x3)) $ 0x0) ~ (0xfff $ 0xf)) [lshift16] .3 <- ((((((((((.3 >> 0x0) & 0x7fff) << 0x1) | 0x0) $ 0x0) ~ (0x7fff $ 0x1)) $ 0x0) ~ (0x3fff $ 0x3)) $ 0x0) ~ (0xfff $ 0xf)) [noopor] .3 <- (((((((((.3 >> 0x0) & 0x7fff) << 0x1) $ 0x0) ~ (0x7fff $ 0x1)) $ 0x0) ~ (0x3fff $ 0x3)) $ 0x0) ~ (0xfff $ 0xf)) [minglefold] .3 <- (((((((((.3 >> 0x0) & 0x7fff) << 0x1) $ 0x0) ~ 0x2aaaaaab) $ 0x0) ~ (0x3fff $ 0x3)) $ 0x0) ~ (0xfff $ 0xf)) [lshift16] .3 <- (((((((((((.3 >> 0x0) & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) | 0x0) $ 0x0) ~ (0x3fff $ 0x3)) $ 0x0) ~ (0xfff $ 0xf)) [noopor] .3 <- ((((((((((.3 >> 0x0) & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) $ 0x0) ~ (0x3fff $ 0x3)) $ 0x0) ~ (0xfff $ 0xf)) [minglefold] .3 <- ((((((((((.3 >> 0x0) & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) $ 0x0) ~ 0xaaaaaaf) $ 0x0) ~ (0xfff $ 0xf)) [lshift16] .3 <- ((((((((((((.3 >> 0x0) & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) >> 0x1) & 0x1fff) << 0x3) | 0x0) $ 0x0) ~ (0xfff $ 0xf)) [noopor] .3 <- (((((((((((.3 >> 0x0) & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) >> 0x1) & 0x1fff) << 0x3) $ 0x0) ~ (0xfff $ 0xf)) [minglefold] .3 <- (((((((((((.3 >> 0x0) & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) >> 0x1) & 0x1fff) << 0x3) $ 0x0) ~ 0xaaaaff) [lshift16] .3 <- (((((((((((((.3 >> 0x0) & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) >> 0x1) & 0x1fff) << 0x3) >> 0x3) & 0x1ff) << 0x7) | 0x0) [noopor] .3 <- ((((((((((((.3 >> 0x0) & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) >> 0x1) & 0x1fff) << 0x3) >> 0x3) & 0x1ff) << 0x7) [nullshift] .3 <- (((((((((((.3 & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) >> 0x1) & 0x1fff) << 0x3) >> 0x3) & 0x1ff) << 0x7) [combinelrshift] .3 <- ((((((((((.3 & 0x7fff) << 0x1) & 0x7fff) << 0x1) >> 0x1) & 0x1fff) << 0x3) >> 0x3) & 0x1ff) << 0x7) [andintolshift] .3 <- ((((((((((.3 & 0x7fff) & 0x3fff) << 0x1) << 0x1) >> 0x1) & 0x1fff) << 0x3) >> 0x3) & 0x1ff) << 0x7) [combinellshift] .3 <- (((((((((.3 & 0x7fff) & 0x3fff) << 0x2) >> 0x1) & 0x1fff) << 0x3) >> 0x3) & 0x1ff) << 0x7) [combinelrshift] .3 <- ((((((((.3 & 0x7fff) & 0x3fff) << 0x1) & 0x1fff) << 0x3) >> 0x3) & 0x1ff) << 0x7) [andintolshift] .3 <- ((((((((.3 & 0x7fff) & 0x3fff) & 0xfff) << 0x1) << 0x3) >> 0x3) & 0x1ff) << 0x7) [combinellshift] .3 <- (((((((.3 & 0x7fff) & 0x3fff) & 0xfff) << 0x4) >> 0x3) & 0x1ff) << 0x7) [combinelrshift] .3 <- ((((((.3 & 0x7fff) & 0x3fff) & 0xfff) << 0x1) & 0x1ff) << 0x7) [andintolshift] .3 <- ((((((.3 & 0x7fff) & 0x3fff) & 0xfff) & 0xff) << 0x1) << 0x7) [combinellshift] .3 <- (((((.3 & 0x7fff) & 0x3fff) & 0xfff) & 0xff) << 0x8) [combineand] .3 <- ((((.3 & 0x3fff) & 0xfff) & 0xff) << 0x8) [combineand] .3 <- (((.3 & 0xfff) & 0xff) << 0x8) [combineand] .3 <- ((.3 & 0xff) << 0x8)
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