[NFC][MC] `MCInst`: `Operands` small size optimization: store 10, not 8, inline `MCOperand`

[lebedev.ri]

Summary

This improves the torture test of

./bin/llvm-exegesis -mcpu=znver3 -mode=inverse_throughput --opcode-index=-1 --benchmarks-file=/dev/null --dump-object-to-disk=0 --measurements-print-progress --skip-measurements

from ~2m16s to ~2min07s, and has the following effect on memory:

heaptrack stats:
        allocations:            100828624 -> 77362343 (-23.2%)
        leaked allocations:     1128
        temporary allocations:  24911300  ->  1576308 (-93.7%) !!!

peak heap memory consumption:
        78.2MB after 02.121s    ->   76.4MB after 01.985s (-2.3%)
peak RSS (including heaptrack overhead):
        193.4MB                 ->   192.6MB              (-0.4%)

The reasoning is that having more Operands than the SSO is costly,
because we go to global allocator, but having larger SSO is fine,
even if it's not always needed, because MCInst is hopefully pool-allocated.

I'm not sure who is the code owner of this component.

[craig.topper]

More than 8 operands seem high for x86. What am I missing?

[craig.topper]

In D139882#3991000, @craig.topper wrote:

More than 8 operands seem high for x86. What am I missing?

Ok I checked. I guess the highest for x86 is 10?

[lebedev.ri]

@craig.topper good point!
10 is still a win, and even decreases total memory footprint.

[craig.topper]

LGTM

[lebedev.ri]

In D139882#3992461, @craig.topper wrote:

LGTM

Thank you for the review!

llvm/include/llvm/MC/MCInst.h

 class MCInst {
   unsigned Opcode = 0;
   // These flags could be used to pass some info from one target subcomponent
   // to another, for example, from disassembler to asm printer. The values of
   // the flags have any sense on target level only (e.g. prefixes on x86).
   unsigned Flags = 0;
 
   SMLoc Loc;
-  SmallVector<MCOperand, 8> Operands;
+  SmallVector<MCOperand, 10> Operands;
 
 public:
   MCInst() = default;
 
   void setOpcode(unsigned Op) { Opcode = Op; }
   unsigned getOpcode() const { return Opcode; }
 
   void setFlags(unsigned F) { Flags = F; }