Adds support for transforming integer division and remainder by constants
into sequences that do not involve division instructions.
* div/rem by constant powers of two are turned into right shifts, plus some
fixups for the signed cases.
* div/rem by constant non-powers of two are turned into double length
multiplies by a magic constant, plus some fixups involving shifts,
addition and subtraction, that depends on the constant, the word size and
the signedness involved.
* The following cases are transformed: div and rem, signed or unsigned, 32
or 64 bit. The only un-transformed cases are: unsigned div and rem by
zero, signed div and rem by zero or -1.
* This is all incorporated within a new transformation pass, "preopt", in
lib/cretonne/src/preopt.rs.
* In preopt.rs, fn do_preopt() is the main driver. It is designed to be
extensible to transformations of other kinds of instructions. Currently
it merely uses a helper to identify div/rem transformation candidates and
another helper to perform the transformation.
* In preopt.rs, fn get_div_info() pattern matches to find candidates, both
cases where the second arg is an immediate, and cases where the second
arg is an identifier bound to an immediate at its definition point.
* In preopt.rs, fn do_divrem_transformation() does the heavy lifting of the
transformation proper. It in turn uses magic{S,U}{32,64} to calculate the
magic numbers required for the transformations.
* There are many test cases for the transformation proper:
filetests/preopt/div_by_const_non_power_of_2.cton
filetests/preopt/div_by_const_power_of_2.cton
filetests/preopt/rem_by_const_non_power_of_2.cton
filetests/preopt/rem_by_const_power_of_2.cton
filetests/preopt/div_by_const_indirect.cton
preopt.rs also contains a set of tests for magic number generation.
* The main (non-power-of-2) transformation requires instructions that return
the high word of a double-length multiply. For this, instructions umulhi
and smulhi have been added to the core instruction set. These will map
directly to single instructions on most non-intel targets.
* intel does not have an instruction exactly like that. For intel,
instructions x86_umulx and x86_smulx have been added. These map to real
instructions and return both result words. The intel legaliser will
rewrite {s,u}mulhi into x86_{s,u}mulx uses that throw away the lower half
word. Tests:
filetests/isa/intel/legalize-mulhi.cton (new file)
filetests/isa/intel/binary64.cton (added x86_{s,u}mulx encoding tests)
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