NCG doesn't realise shift instructions trash shifted input?

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

Trac metadata

Trac field	Value
Version	6.6
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (NCG)
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