slightly silly assembly for testing whether a Word# is 0##

{-# LANGUAGE MagicHash #-}

module Zero where

import GHC.Exts

f :: Word# -> Word#
f 0## = 1##
f x## = x##

in HEAD produces

0000000000000018 <ZZero_f_info>:
  18:	49 83 fe 01          	cmp    $0x1,%r14
  1c:	72 06                	jb     24 <ZZero_f_info+0xc>
  1e:	4c 89 f3             	mov    %r14,%rbx
  21:	ff 65 00             	jmpq   *0x0(%rbp)
  24:	bb 01 00 00 00       	mov    $0x1,%ebx
  29:	ff 65 00             	jmpq   *0x0(%rbp)

Well, cmp $0x1,%r14/jb isn't wrong. But it's a byte longer than test %r14,%r14/je, so the latter should be preferred.

GHC 7.10 produced the test/je version, so I'm guessing this is a side effect of nomeata's work on #10137 (closed).

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler (CodeGen)
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