[PATCH] D58675: [clang] Adds `-ftime-trace` option to clang that produces Chrome `chrome://tracing` compatible JSON profiling output dumps

[Roman Lebedev via Phabricator via cfe-commits]

lebedev.ri added inline comments.


================
Comment at: cfe/trunk/lib/CodeGen/BackendUtil.cpp:1426-1431
   EmitAssemblyHelper AsmHelper(Diags, HeaderOpts, CGOpts, TOpts, LOpts, M);
 
   if (CGOpts.ExperimentalNewPassManager)
     AsmHelper.EmitAssemblyWithNewPassManager(Action, std::move(OS));
   else
     AsmHelper.EmitAssembly(Action, std::move(OS));
----------------
anton-afanasyev wrote:
> lebedev.ri wrote:
> > anton-afanasyev wrote:
> > > anton-afanasyev wrote:
> > > > lebedev.ri wrote:
> > > > > This isn't covered by any timer; if you look in `BackendUtil.cpp`,
> > > > > `EmitAssemblyHelper` actually has `CodeGenerationTime("codegen", "Code Generation Time")` timer.
> > > > Thanks, I'm to add it.
> > > Hmm, I've figured out this isn't needed: such new timer mostly coincides with "Backend" timer (above). Legacy `Timer CodeGenerationTime(...)` is bad example of doing right timing.
> > "Mostly coincides" may not be the best way to approach fine-grained timings, i think? :)
> > 
> > I have noticed this because when i looked at the produced time flame graph,
> > there's large section in the end that is covered only by `"Backend"` timer,
> > but nothing else. Now, i'm not going to say whether or not that extra section
> > should or should not be within `"Backend"` timer, but it certainly should *also*
> > be within `"codegen"` timer. Or is there no codegen time spent there?
> > {F10062322}
> > {F10062316}
> "Mostly coincides" here means "identical" I believe, the difference is auxiliary stuff.
> Please look at `clang::EmitBackendOutput()`, `"Backend"` timer is outer for `"codegen"` one.
Then we are talking about different things using the same name.
There are two distinct codegen steps:
1. clang AST -> LLVM IR codegen
(after that, all the opt passes run)
2. LLVM IR -> final assembly. This happens after all the opt middle-end passes.

Those are *different* codegen's.


Repository:
  rL LLVM

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D58675/new/

https://reviews.llvm.org/D58675