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Re: [PATCH] tcg,riscv: Fix illegal shift instructions

From: Richard Henderson
Subject: Re: [PATCH] tcg,riscv: Fix illegal shift instructions
Date: Wed, 16 Dec 2020 10:10:16 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 12/16/20 2:12 AM, Zihao Yu wrote:
> * This bug can be reproduced by running the following guest instructions
>   on a RISC-V host.
>   (1) xor %ecx,%ecx
>   (2) sar %cl,%eax
>   (3) cmovne %edi,%eax
>   After optimization, the tcg instructions of (2) are
>   movi_i32 tmp3,$0xffffffffffffffff  pref=all
>   sar_i32 tmp3,eax,tmp3              dead: 2  pref=all
>   mov_i32 cc_dst,eax                 sync: 0  dead: 1 pref=0xffc0300
>   mov_i32 cc_src,tmp3                sync: 0  dead: 0 1  pref=all
>   movi_i32 cc_op,$0x31               sync: 0  dead: 0  pref=all
>   And the target assembly instructions of (2) are
>   0x200808d618:  fffa5b9b          illegal
>   0x200808d61c:  03442423          sw              s4,40(s0)
>   0x200808d620:  03742623          sw              s7,44(s0)
>   0x200808d624:  03100b93          addi            s7,zero,49
>   0x200808d628:  03742a23          sw              s7,52(s0)
> * Note that the `illegal` target instruction above should be
>   `sraiw s7,s4,0x1f` (41fa5b9b).

More precisely, the 'illegal' instruction should be *anything*, except for
'illegal'.  The result of the out-of-range shift is undefined, but TCG requires
that it not trap.  The undefined result should be unused.

In this particular case this is so, because cc_op == 0x31 == CC_OP_CLR, which
does not use either of cc_dst or cc_src.

We have make the same patch for other hosts, e.g. 1fd959466574 for tcg/sparc/.

I have queued the patch to tcg-next.


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