Re: [Qemu-devel] [PATCH 1/2] tcg-x86_64: Special-case all 32-bit AND operands.

[Richard Henderson <rth@twiddle.net>]

On 01/14/2010 07:57 AM, Aurelien Jarno wrote:
> On Tue, Jan 05, 2010 at 04:03:00PM -0800, Richard Henderson wrote:
>> This avoids an unnecessary REX.W prefix when dealing with AND
>> operands that fit into a 32-bit quantity.  The most common change
>> actually seen is movz[wb]q ->  movz[wb]l.
>>
>> Similarly, avoid REXW in ext{8,16}u_i64 tcg opcodes.
>>
>> Signed-off-by: Richard Henderson<rth@twiddle.net>
>> ---
>>   tcg/x86_64/tcg-target.c |   26 ++++++++------------------
>>   1 files changed, 8 insertions(+), 18 deletions(-)
>>
>> diff --git a/tcg/x86_64/tcg-target.c b/tcg/x86_64/tcg-target.c
>> index 2339091..f584c94 100644
>> --- a/tcg/x86_64/tcg-target.c
>> +++ b/tcg/x86_64/tcg-target.c
>> @@ -426,24 +426,18 @@ static inline void tgen_arithi64(TCGContext *s, int c, int r0, int64_t val)
>>       } else if ((c == ARITH_ADD&&  val == -1) || (c == ARITH_SUB&&  val == 1)) {
>>           /* dec */
>>           tcg_out_modrm(s, 0xff | P_REXW, 1, r0);
>> -    } else if (val == (int8_t)val) {
>> -        tcg_out_modrm(s, 0x83 | P_REXW, c, r0);
>> -        tcg_out8(s, val);
>> -    } else if (c == ARITH_AND&&  val == 0xffu) {
>> -        /* movzbl */
>> -        tcg_out_modrm(s, 0xb6 | P_EXT | P_REXW, r0, r0);
>> -    } else if (c == ARITH_AND&&  val == 0xffffu) {
>> -        /* movzwl */
>> -        tcg_out_modrm(s, 0xb7 | P_EXT | P_REXW, r0, r0);
>>       } else if (c == ARITH_AND&&  val == 0xffffffffu) {
>>           /* 32-bit mov zero extends */
>>           tcg_out_modrm(s, 0x8b, r0, r0);
>> +    } else if (c == ARITH_AND&&  (uint64_t)val<= 0xffffffffu) {
>> +        /* AND with no high bits set can use a 32-bit operation.  */
>> +        tgen_arithi32(s, c, r0, val);
>
> Do we really want to call tgen_arithi32() here, that will redo part of
> the above tests again? It might be better to simply remove the REX.W
> prefix above instead.

Pardon?  Do you mean the inc/dec tests?  Otherwise I don't see what 
"above tests again" you're talking about.

I am looking to handle more than 0xff, 0xffff with the new test in 
gen_arithi64 -- 0x1234 is an appropriate mask to shorten to 32-bit AND 
as well.  I have no idea if that answers your question.


r~