#1052: NCG doesn't realise shift instructions trash shifted input?
-------------------------------+--------------------------------------------
Reporter: igloo | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.8
Component: Compiler (NCG) | Version: 6.6
Severity: normal | Keywords:
Difficulty: Unknown | Testcase: arith011
Architecture: x86_64 (amd64) | Os: Linux
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It looks like the NCG on amd64/Linux doesn't realise that shifting
instructions trash the shifted input (spotted due to arith011). With this
input file:
{{{
module Main where
import Data.Bits
import GHC.Exts
main = print ((2 :: Int) `qrotate` 1)
{-# NOINLINE qrotate #-}
(I# x#) `qrotate` (I# i#) =
(I# (word2Int# (a# `or#` b#)), W# a#, W# b#)
where
x'# = int2Word# x#
i'# = word2Int# (int2Word# i# `and#` int2Word# (wsib -# 1#))
a# = x'# `uncheckedShiftL#` i'#
b# = x'# `uncheckedShiftRL#` (wsib -# i'#)
wsib = 64#
}}}
compiling with `-O -fglasgow-exts -v9` if I merge the Cmm and Asm output I
get:
{{{
R2 == rsi
R3 == rdi
_sTh = R3; movq %rdi,%rax rax=_sTh
_sTj = _sTh & 63; andq $63,%rax rax=_sTj
_sTl = _sTj; rax=_sTl
_sTp = R2; movq %rsi,%rcx rcx=_sTp
movq %rcx,64(%rsp)
_sTs = 64 - _sTl; movl $64,%ecx
subq %rax,%rcx
movq 64(%rsp),%rdx
_sTu = _sTp >> _sTs; shrq %cl,%rdx
movq %rdx,%rcx \
movq %rcx,72(%rsp) / why?
movq %rax,%rcx
_sTx = _sTp << _sTl; shlq %cl,%rdx but rdx contains _sTp >> _sTs!
I64[Hp + (-40)] = base_GHCziWord_Wzh_con_info;
I64[Hp + (-32)] = _sTu;
I64[Hp + (-24)] = base_GHCziWord_Wzh_con_info;
I64[Hp + (-16)] = _sTx;
_sTz = _sTx | _sTu;
_sTB = _sTz;
I64[Hp + (-8)] = base_GHCziBase_Izh_con_info;
I64[Hp + 0] = _sTB;
R1 = Hp + (-8);
R2 = Hp + (-24);
R3 = Hp + (-40);
jump (I64[Sp + 0]);
}}}
Thanks
Ian
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1052>
GHC <http://www.haskell.org/ghc/>
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