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Date: Thu, 05 Sep 2013 22:17:35 +0200
From: Stefan Weil <sw@weilnetz.de>
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Subject: Re: [Qemu-devel] [PATCH] tci: Add implementation of rotl_i64,
	rotr_i64
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To: Jay Foad <jay.foad@gmail.com>
Cc: qemu-devel@nongnu.org, Richard Henderson <rth@twiddle.net>

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