[CodeGen] [SelectionDAG] More efficient code for X % C == 0 (UREM case) (try 2)

Authored by lebedev.ri on Jun 27 2019, 9:45 AM.


[CodeGen] [SelectionDAG] More efficient code for X % C == 0 (UREM case) (try 2)

I'm submitting a new revision since i don't understand how to reclaim/reopen/take over the existing one, D50222.
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Original patch D50222 by @hermord (Dmytro Shynkevych)

This implements an optimization described in Hacker's Delight 10-17: when C is constant,
the result of X % C == 0 can be computed more cheaply without actually calculating the remainder.
The motivation is discussed here: https://bugs.llvm.org/show_bug.cgi?id=35479.

Original patch author: @hermord (Dmytro Shynkevych)!


  • In principle, it's possible to also handle the X % C1 == C2 case, as discussed on bugzilla. This seems to require an extra branch on overflow, so I refrained from implementing this for now.
  • An explicit check for when the REM can be reduced to just its LHS is included: the X % C == 0 optimization breaks test1 in test/CodeGen/X86/jump_sign.ll otherwise. I hadn't managed to find a better way to not generate worse output in this case.
  • The test/CodeGen/X86/jump_sign.ll regresses, and is being fixed by a followup patch D63390.

Reviewers: RKSimon, craig.topper, spatel, hermord, xbolva00

Reviewed By: RKSimon, xbolva00

Subscribers: xbolva00, javed.absar, llvm-commits, hermord

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63391

llvm-svn: 364563