(gcc-295.info)Pattern Ordering


When the Order of Patterns Matters
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   Sometimes an insn can match more than one instruction pattern.  Then
the pattern that appears first in the machine description is the one
used.  Therefore, more specific patterns (patterns that will match
fewer things) and faster instructions (those that will produce better
code when they do match) should usually go first in the description.

   In some cases the effect of ordering the patterns can be used to hide
a pattern when it is not valid.  For example, the 68000 has an
instruction for converting a fullword to floating point and another for
converting a byte to floating point.  An instruction converting an
integer to floating point could match either one.  We put the pattern
to convert the fullword first to make sure that one will be used rather
than the other.  (Otherwise a large integer might be generated as a
single-byte immediate quantity, which would not work.)  Instead of
using this pattern ordering it would be possible to make the pattern
for convert-a-byte smart enough to deal properly with any constant
value.


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