From: Stefan Weil <sw@weilnetz.de>
To: Jay Foad <jay.foad@gmail.com>
Cc: qemu-devel@nongnu.org, Richard Henderson <rth@twiddle.net>
Subject: Re: [Qemu-devel] [PATCH] tci: Add implementation of rotl_i64, rotr_i64
Date: Thu, 05 Sep 2013 22:17:35 +0200 [thread overview]
Message-ID: <5228E6DF.6090301@weilnetz.de> (raw)
In-Reply-To: <CANd1uZm=WB=6fcNT8cwQE3PtvxtxzBZTe7qTAgcWtrCYG+Xu1w@mail.gmail.com>
Am 05.09.2013 14:00, schrieb Jay Foad:
>> diff --git a/tci.c b/tci.c
>> index 18c888e..94b7851 100644
>> --- a/tci.c
>> +++ b/tci.c
>> @@ -952,8 +952,16 @@ uintptr_t tcg_qemu_tb_exec(CPUArchState *env, uint8_t *tb_ptr)
>> break;
>> #if TCG_TARGET_HAS_rot_i64
>> case INDEX_op_rotl_i64:
>> + t0 = *tb_ptr++;
>> + t1 = tci_read_ri64(&tb_ptr);
>> + t2 = tci_read_ri64(&tb_ptr);
>> + tci_write_reg64(t0, (t1 << t2) | (t1 >> (64 - t2)));
>> + break;
>> case INDEX_op_rotr_i64:
>> - TODO();
>> + t0 = *tb_ptr++;
>> + t1 = tci_read_ri64(&tb_ptr);
>> + t2 = tci_read_ri64(&tb_ptr);
>> + tci_write_reg64(t0, (t1 >> t2) | (t1 << (64 - t2)));
> << (64 - t2) is undefined behaviour in C when t2 is 0. How about << (-t2 & 63) ?
>
> Jay.
A short test confirms that the behaviour for (t1 << 64) is indeed
unexpected.
I added assertions for (t2 > 0) and (t2 < 64). They never raised an abort.
Are those cases possible? We already have similar code for 32 bit shifts,
and tcg/optimize.c also includes an implementation which is identical to
my rotl_i64, rotr_i64.
Therefore I think my patch can be applied as it is.
Stefan
next prev parent reply other threads:[~2013-09-05 20:18 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-05 12:00 [Qemu-devel] [PATCH] tci: Add implementation of rotl_i64, rotr_i64 Jay Foad
2013-09-05 20:17 ` Stefan Weil [this message]
2013-09-05 20:33 ` Richard Henderson
-- strict thread matches above, loose matches on Subject: below --
2013-09-04 20:49 Stefan Weil
2013-09-04 20:51 ` Stefan Weil
2013-09-09 14:53 ` Aurelien Jarno
2013-09-09 16:14 ` Stefan Weil
2013-09-09 16:47 ` Aurelien Jarno
2013-09-04 23:00 ` Richard Henderson
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