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More info on this transformation.

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llvm-svn: 91230
This commit is contained in:
Eli Friedman 2009-12-12 23:23:43 +00:00
parent 38a7d3b32e
commit 392adbdd7d
1 changed files with 15 additions and 2 deletions
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17
lib/Target/README.txt
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@ -801,8 +801,21 @@ void bar(unsigned n) {
true();
}
I think this basically amounts to a dag combine to simplify comparisons against
multiply hi's into a comparison against the mullo.
This is equivalent to the following, where 2863311531 is the multiplicative
inverse of 3, and 1431655766 is ((2^32)-1)/3+1:
void bar(unsigned n) {
if (n * 2863311531U < 1431655766U)
true();
}
The same transformation can work with an even modulo with the addition of a
rotate: rotate the result of the multiply to the right by the number of bits
which need to be zero for the condition to be true, and shrink the compare RHS
by the same amount. Unless the target supports rotates, though, that
transformation probably isn't worthwhile.
The transformation can also easily be made to work with non-zero equality
comparisons: just transform, for example, "n % 3 == 1" to "(n-1) % 3 == 0".
//===---------------------------------------------------------------------===//