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[InstCombine] "X - (X / C) * C == 0" to "X & C-1 == 0"

Authored by EgorBo on Jun 12 2020, 12:08 AM.

Description

[InstCombine] "X - (X / C) * C == 0" to "X & C-1 == 0"

Summary:
"X % C == 0" is optimized to "X & C-1 == 0" (where C is a power-of-two)
However, "X % Y" can also be represented as "X - (X / Y) * Y" so if I rewrite the initial expression:
"X - (X / C) * C == 0" it's not currently optimized to "X & C-1 == 0", see godbolt: https://godbolt.org/z/KzuXUj

This is my first contribution to LLVM so I hope I didn't mess things up

Reviewers: lebedev.ri, spatel

Reviewed By: lebedev.ri

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79369

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