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[X86ISelLowering] Add additional support for multiplication-to-shift conversion.

Authored by chenli on Dec 11 2015, 3:39 PM.

Description

[X86ISelLowering] Add additional support for multiplication-to-shift conversion.

Summary: This patch adds support of conversion (mul x, 2^N + 1) => (add (shl x, N), x) and (mul x, 2^N - 1) => (sub (shl x, N), x) if the multiplication can not be converted to LEA + SHL or LEA + LEA. LLVM has already supported this on ARM, and it should also be useful on X86. Note the patch currently only applies to cases where the constant operand is positive, and I am planing to add another patch to support negative cases after this.

Reviewers: craig.topper, RKSimon

Subscribers: aemerson, llvm-commits

Differential Revision: http://reviews.llvm.org/D14603

llvm-svn: 255391