#9340: Implement new `clz` inline primop
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Reporter: hvr | Owner: hvr
Type: feature | Status: closed
request | Milestone: 7.10.1
Priority: normal | Version: 7.8.2
Component: Compiler | Keywords: primop clz ctz
(CodeGen) | Architecture: Unknown/Multiple
Resolution: fixed | Difficulty: Unknown
Operating System: | Blocked By:
Unknown/Multiple | Related Tickets: #9532
Type of failure: |
None/Unknown |
Test Case: |
Blocking: |
Differential Revisions: Phab:D144 |
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Comment (by Herbert Valerio Riedel ):
In [changeset:"612f3d120c65a461a4ad7f212d67bdae005f4975/ghc"]:
{{{
#!CommitTicketReference repository="ghc"
revision="612f3d120c65a461a4ad7f212d67bdae005f4975"
Implement optimized NCG `MO_Ctz W64` op for i386 (#9340)
Summary:
This is an optimization to the CTZ primops introduced for #9340
Previously we called out to `hs_ctz64`, but we can actually generate
better hand-tuned code while avoiding the FFI ccall.
With this patch, the code
{-# LANGUAGE MagicHash #-}
module TestClz0 where
import GHC.Prim
ctz64 :: Word64# -> Word#
ctz64 x = ctz64# x
results in the following assembler generated by NCG on i386:
TestClz.ctz64_info:
movl (%ebp),%eax
movl 4(%ebp),%ecx
movl %ecx,%edx
orl %eax,%edx
movl $64,%edx
je _nAO
bsf %ecx,%ecx
addl $32,%ecx
bsf %eax,%eax
cmovne %eax,%ecx
movl %ecx,%edx
_nAO:
movl %edx,%esi
addl $8,%ebp
jmp *(%ebp)
For comparision, here's what LLVM 3.4 currently generates:
000000fc :
fc: 0f bc 45 04 bsf 0x4(%ebp),%eax
100: b9 20 00 00 00 mov $0x20,%ecx
105: 0f 45 c8 cmovne %eax,%ecx
108: 83 c1 20 add $0x20,%ecx
10b: 8b 45 00 mov 0x0(%ebp),%eax
10e: 8b 55 08 mov 0x8(%ebp),%edx
111: 0f bc f0 bsf %eax,%esi
114: 85 c0 test %eax,%eax
116: 0f 44 f1 cmove %ecx,%esi
119: 83 c5 08 add $0x8,%ebp
11c: ff e2 jmp *%edx
Reviewed By: austin
Auditors: simonmar
Differential Revision: https://phabricator.haskell.org/D163
}}}
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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9340#comment:11
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