[PATCH] Long NOP padding

[Shankar Easwaran]

LGTM.

On 3/4/2013 4:29 PM, David Sehr wrote:
> Largely because it was that way before I tweaked it :-)  I have updated to
> use uint8_t below.
>
> David
>
>
> On Mon, Mar 4, 2013 at 2:13 PM, Shankar Easwaran <shankare at codeaurora.org>wrote:
>
>>   Hi David,
>>
>> +  // 15 is the longest single nop instruction.  Emit as many 15-byte nops
>> as
>> +  // needed, then emit a nop of the remaining length.
>> +  do {
>> +    uint64_t ThisNopLength = std::min(Count, (uint64_t) 15);
>> +    const uint64_t Prefixes = ThisNopLength <= 10 ? 0 : ThisNopLength -
>> 10;
>> +    for (uint64_t i = 0; i < Prefixes; i++)
>> +      OW->Write8(0x66);
>>
>> Why are the types 64bit ? Looks like a shorter type would suffice here,
>> short/unsigned char ?
>>
>> Thanks
>>
>> Shankar Easwaran
>>
>>
>> On 3/4/2013 4:06 PM, David Sehr wrote:
>>
>> The current X86 NOP padding uses one long NOP followed by the remainder in
>> one-byte NOPs.  If the processor actually executes those NOPs, as it
>> sometimes does with aligned bundling, this can have a performance impact.
>>   From my micro-benchmarks run on my one machine, a 15-byte NOP followed by
>> twelve one-byte NOPs is about 20% worse than a 15 followed by a 12.  This
>> patch changes NOP emission to emit as many 15-byte (the maximum) as
>> possible followed by at most one shorter NOP.
>>
>> David
>>
>>
>>
>>
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>>
>>
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>>


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