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More info on this transformation.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@91230 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -801,8 +801,21 @@ void bar(unsigned n) {
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true();
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}
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I think this basically amounts to a dag combine to simplify comparisons against
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multiply hi's into a comparison against the mullo.
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This is equivalent to the following, where 2863311531 is the multiplicative
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inverse of 3, and 1431655766 is ((2^32)-1)/3+1:
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void bar(unsigned n) {
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if (n * 2863311531U < 1431655766U)
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true();
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}
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The same transformation can work with an even modulo with the addition of a
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rotate: rotate the result of the multiply to the right by the number of bits
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which need to be zero for the condition to be true, and shrink the compare RHS
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by the same amount. Unless the target supports rotates, though, that
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transformation probably isn't worthwhile.
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The transformation can also easily be made to work with non-zero equality
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comparisons: just transform, for example, "n % 3 == 1" to "(n-1) % 3 == 0".
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//===---------------------------------------------------------------------===//
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