aaron.ballman added inline comments.
================
Comment at: lib/AST/DeclBase.cpp:854-859
+ auto I = Attrs.begin(), E = Attrs.end();
+ for (; I != E; ++I) {
+ if (!(*I)->isInherited())
+ break;
+ }
+ Attrs.insert(I, A);
----------------
Meinersbur wrote:
> aaron.ballman wrote:
> > The unfortunate part about this is that inherited attributes are fairly
> > common, so this extra searching may happen more often than we'd like. I
> > wonder how bad it would be to keep track of the location within the list
> > where the inherited attributes start. Then again, the list of attributes
> > should be relatively short in most cases, so this search isn't likely to be
> > too expensive.
> >
> > Having some performance measurements might help decide this, too.
> Timed clang-compilation using perf (`perf stat bin/clang
> llvm/unittests/IR/InstructionsTest.cpp ...`), before this patch (r339574):
> ```
> 7647.415800 task-clock (msec) # 0.983 CPUs utilized
> 289 context-switches # 0.038 K/sec
> 26 cpu-migrations # 0.003 K/sec
> 86,494 page-faults # 0.011 M/sec
> 19,068,741,863 cycles # 2.493 GHz
> 26,581,355,844 instructions # 1.39 insn per cycle
> 6,242,394,037 branches # 816.275 M/sec
> 128,405,656 branch-misses # 2.06% of all branches
>
> 7.779848330 seconds time elapsed
> ```
>
> With this patch:
> ```
> 7679.173467 task-clock (msec) # 0.987 CPUs utilized
> 321 context-switches # 0.042 K/sec
> 23 cpu-migrations # 0.003 K/sec
> 86,071 page-faults # 0.011 M/sec
> 19,150,248,538 cycles # 2.494 GHz
> 26,667,465,697 instructions # 1.39 insn per cycle
> 6,262,381,898 branches # 815.502 M/sec
> 128,527,669 branch-misses # 2.05% of all branches
>
> 7.779477815 seconds time elapsed
> ```
> (Intel(R) Xeon(R) Platinum 8180M CPU @ 2.50GHz)
>
> The vector insert operation is also be `O(n)`. If the number of non-inherited
> is expected to be smaller, we can instead search for the first inherited
> attribute starting at the end of the vector. If we want to avoid the `O(n)`
> entirely, we need one list for inherited and another for non-inherited
> attributes.
Thank you for gathering this data -- those perf numbers look reasonable to me.
We can refactor for performance later if it ever turns up on the hot path.
Repository:
rC Clang
https://reviews.llvm.org/D50214
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