[all-commits] [llvm/llvm-project] 83d56f: Drop the ZeroBehavior parameter from countLeadingZ...
kazutakahirata via All-commits
all-commits at lists.llvm.org
Wed Jan 18 19:58:57 PST 2023
Branch: refs/heads/main
Home: https://github.com/llvm/llvm-project
Commit: 83d56fb17a4d78471125df4249c3557bd4ddb5c2
https://github.com/llvm/llvm-project/commit/83d56fb17a4d78471125df4249c3557bd4ddb5c2
Author: Kazu Hirata <kazu at google.com>
Date: 2023-01-18 (Wed, 18 Jan 2023)
Changed paths:
M llvm/include/llvm/Support/MathExtras.h
M llvm/lib/Target/AMDGPU/AMDGPUCallLowering.cpp
M llvm/lib/Target/AMDGPU/SIISelLowering.cpp
M llvm/lib/Target/RISCV/MCTargetDesc/RISCVMatInt.cpp
M llvm/lib/Transforms/IPO/LowerTypeTests.cpp
M llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
M mlir/lib/Bytecode/Reader/BytecodeReader.cpp
Log Message:
-----------
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.
Differential Revision: https://reviews.llvm.org/D141798
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