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Chris Tate
33be2a80ee
An if/else-if chain that compares one number local with integer literals
and stores a number literal to the same typed-array element in every arm,
including a final else, such as
if (x === 97) a[i] = 0;
else if (x === 116) a[i] = 1;
else if (x === 99) a[i] = 2;
else a[i] = 3;
is now emitted as one store of an entry of a constant table indexed by
the local's value (constant-choices.ts). On input whose cases do not
repeat in a pattern, such as the bases of a DNA sequence, the branches
mispredict; LLVM keeps them, because cases this far apart are too sparse
for its own switch tables.
The chain needs at least three distinct values within 256 of each other.
The table covers the cases' span, or the local's proven integer range
when that range has at most 256 values, in which case no test is
emitted. Otherwise a value that is not an exact integer in the table's
span (NaN, a fraction, or a value outside it) stores the final else's
literal, chosen with a select. The first arm wins for a repeated
value, and -0 selects the arm for 0, as strict equality does. The store
is the arms' own (the same receiver and index), converted to the
element's type as each arm's would be. Debug builds keep the arms.
k-nucleotide's transform of the input sequence uses this form: its
median wall time drops from 2.71 s to 2.21 s.