[PATCH] D28196: [X86] Tune bypassing of slow division for Intel CPUs

[Nikolai Bozhenov via Phabricator via llvm-commits]

n.bozhenov created this revision.
n.bozhenov added reviewers: spatel, craig.topper, bkramer, jlebar.
n.bozhenov added subscribers: llvm-commits, zansari, DavidKreitzer, aaboud, volkalexey.

64-bit integer division in Intel CPUs is extremely slow, much slower
than 32-bit division. On the other hand, 8-bit and 16-bit divisions
aren't any faster. The only important exception is Atom where DIV8
is fastest. Because of that, the patch

1. Enables bypassing of 64-bit division for Atom, Silvermont and all big cores.
2. Modifies 64-bit bypassing to use 32-bit division instead of 16-bit one. This doesn't make the shorter division slower but increases chances of taking it. Moreover, it's much more likely to prove at compile-time that a value fits 32 bits and doesn't require a run-time check (e.g. zext i32 to i64).


https://reviews.llvm.org/D28196

Files:
  lib/Target/X86/X86.td
  lib/Target/X86/X86ISelLowering.cpp
  lib/Target/X86/X86Subtarget.h
  test/CodeGen/X86/atom-bypass-slow-division-64.ll
  test/CodeGen/X86/slow-div.ll

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