[all-commits] [llvm/llvm-project] 0c4f53: Reland: Drop the ZeroBehavior parameter from count...

[kazutakahirata via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 0c4f53fcff794d8ad4b1b43443affc37c2061f55
      https://github.com/llvm/llvm-project/commit/0c4f53fcff794d8ad4b1b43443affc37c2061f55
  Author: Kazu Hirata <kazu at google.com>
  Date:   2023-01-20 (Fri, 20 Jan 2023)

  Changed paths:
    M llvm/include/llvm/ADT/bit.h
    M llvm/include/llvm/Support/MathExtras.h
    M llvm/unittests/ADT/BitTest.cpp

  Log Message:
  -----------
  Reland: Drop the ZeroBehavior parameter from countLeadingZeros and the like (NFC)

This patch drops the ZeroBehavior parameter from bit counting
functions like countLeadingZeros.  ZeroBehavior specifies the behavior
when the input to count{Leading,Trailing}Zeros is zero and when the
input to count{Leading,Trailing}Ones is all ones.

ZeroBehavior was first introduced on May 24, 2013 in commit
eb91eac9fb866ab1243366d2e238b9961895612d.  While that patch did not
state the intention, I would guess ZeroBehavior was for performance
reasons.  The x86 machines around that time required a conditional
branch to implement countLeadingZero<uint32_t> that returns the 32 on
zero:

        test    edi, edi
        je      .LBB0_2
        bsr     eax, edi
        xor     eax, 31
.LBB1_2:
        mov     eax, 32

That is, we can remove the conditional branch if we don't care about
the behavior on zero.

IIUC, Intel's Haswell architecture, launched on June 4, 2013,
introduced several bit manipulation instructions, including lzcnt and
tzcnt, which eliminated the need for the conditional branch.

I think it's time to retire ZeroBehavior as its utility is very
limited.  If you care about compilation speed, you should build LLVM
with an appropriate -march= to take advantage of lzcnt and tzcnt.
Even if not, modern host compilers should be able to optimize away
quite a few conditional branches because the input is often known to
be nonzero from dominating conditional branches.

In this iteration, I've moved the forward declarations of
_BitScanForward outside the llvm space to fix builds on Windows.

Differential Revision: https://reviews.llvm.org/D141798