[all-commits] [llvm/llvm-project] 696406: [SelectionDAG] Use Magic Algorithm for Splitting U...

[Marius Kamp via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 696406fbd585e497ebc766f0f066fa660b4b57fc
      https://github.com/llvm/llvm-project/commit/696406fbd585e497ebc766f0f066fa660b4b57fc
  Author: Marius Kamp <msk at posteo.org>
  Date:   2026-05-01 (Fri, 01 May 2026)

  Changed paths:
    M llvm/include/llvm/CodeGen/TargetLowering.h
    M llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
    M llvm/test/CodeGen/RISCV/urem-vector-lkk.ll
    M llvm/test/CodeGen/X86/divide-by-constant.ll
    M llvm/test/CodeGen/X86/divmod128.ll
    M llvm/test/CodeGen/X86/i128-udiv.ll

  Log Message:
  -----------
  [SelectionDAG] Use Magic Algorithm for Splitting UDIV/UREM by Constant (#154968)

For integer types twice as large as a legal type, we have previously
generated a library call if another splitting technique was not
applicable.
    
With this change, we use an adaption of the Magic algorithm. This
algorithm is also used for UDIV/UREM by constants on legal types. The
implementation introduced here is a simple port of the already existing
implementation to types twice the size of a legal type. The core idea of
this algorithm is to replace (udiv x c) for a constant c with the bits
higher or equal to the s-th bit of the multiplication of x by (2^s +
o)/c
for some s and o. More details are available in Henry S. Warren, Jr.:
"Hacker's Delight", chapter 10.
    
An efficient handling of UDIV/UREM by constants on types twice as large
as a legal type is mostly relevant for 32-bit platforms. But some
projects may also benefit on 64-bit platforms. For example, the `fmt`
library for C++ uses 128-bit unsigned divisions by 100 and 10000, which
have not been covered by the previously existing optimizations.
    
Closes #137514.



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