wip patch: Create SymbolBody for all symbols

[Rafael Espíndola via llvm-commits]

On 9 March 2016 at 23:51, Rui Ueyama <ruiu at google.com> wrote:
> I think this is towards a good direction, but I want to understand what is
> going on more as 9% slowdown is somewhat significant.
>
> On Wed, Mar 9, 2016 at 7:11 PM, Rafael Espíndola
> <rafael.espindola at gmail.com> wrote:
>>
>> OK, looking at perf, what I got was that the "slow" instructions were
>>
>>
>> mov    (%rcx,%rax,8),%rax
>> mov    0x28(%rax),%rax
>> mov    (%rax),%r13
>>
>> Which correspond to:
>>  * given a symbol index, get the SymbolBody*
>>  * get the backref
>>  * get the repl
>>
>> I think these are only slow because we then proceed to use the data
>> and pref blames the instruction that is computing the dependency of an
>> expensive computations.
>
>
> Unless added to the symbol table, SymbolBodies does not have backrefs. Their
> backrefs are nullptrs. So I think for local symbols, that code would not be
> executed. Is this actually a bottleneck for local symbols?

But it still returns itself as a repl. So a hot code path went from
checking a value with null to checking a field of the returned pointer
(Body.isLocal()).

Cheers,
Rafael