[PATCH] Long NOP padding

[Shankar Easwaran]

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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