[llvm-dev] RFC: Use closures to delay construction of optimization remarks

[Adam Nemet via llvm-dev]

> On Sep 16, 2017, at 4:49 PM, Sean Silva <chisophugis at gmail.com> wrote:
> 
> Actually maybe something like:
> 
> if (auto &E = ORE.emitMissed(DEBUG_TYPE)) {
>   E.emit(...) << ...;
> }

Well, the point of this interface was exactly to avoid writing a conditional.  If you’re willing to use a conditional you can already write this:

if (ORE.allowExtraAnalysis(DEBUG_TYPE))
  ORE.emit(OptimizationRemark(…) << …;

But again the point was to hide all this.  I find the closure-based interface more concise and easier to identify visually.  One reason is that the block is contained within ORE.emit(…).  I just wish you could omit the return in a lambda as in Python.

Adam

> 
> Credit to Chandler for that (if I'm remembering it right. This is vaguely recollected from a LLVM social conversation a long time ago about reducing overhead of clang diagnostics that are not enabled, which sounds like the same problem)
> 
> On Sep 16, 2017 4:39 PM, "Sean Silva" <chisophugis at gmail.com <mailto:chisophugis at gmail.com>> wrote:
> Another alternative could be:
> 
> ORE.emitMissed(DEBUG_TYPE, ...) << ...
> 
> Then the first line of emitMissed does a check if it is enabled and if not then returns a dummy stream that does nothing for operator<< (and short-circuits all the stream operations)
> 
> On Sep 15, 2017 2:21 PM, "Adam Nemet via llvm-dev" <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> For better readability we typically create remarks and call OptimizationRemarkEmitter::emit unconditionally.  E.g.:
> 
> Transforms/IPO/Inliner.cpp:    ORE.emit(OptimizationRemarkMissed(DEBUG_TYPE, "TooCostly", Call)
> Transforms/IPO/Inliner.cpp-             << NV("Callee", Callee) << " not inlined into "
> Transforms/IPO/Inliner.cpp-             << NV("Caller", Caller) << " because too costly to inline (cost="
> Transforms/IPO/Inliner.cpp-             << NV("Cost", IC.getCost())
> Transforms/IPO/Inliner.cpp-             << ", threshold=" << NV("Threshold", IC.getThreshold()) << ")");
> 
> Then inside ORE we return right away if the remarks for the given pass is not enabled.
> 
> This is nice and concise however there is still some overhead involved in this if remarks are not enabled:
> 
> 1. Constructing the remark object
> 2. Computing and inserting the strings, named-value arguments
> 3. Performing the comparison between the pass name and the passes enabled by the user
> 4. Virtual destructor
> 
> Now that Vivek added support for asking LLVMContext for what remarks are enabled by the user [1] we can improve this.  The idea is to allow ORE::emit to take a closure.  This delays all this work until we know remarks are enabled.  E.g. the above code with closure:
> 
>     ORE.emit([&]() {
>       return OptimizationRemarkMissed(DEBUG_TYPE, "TooCostly", Call)
>              << NV("Callee", Callee) << " not inlined into "
>              << NV("Caller", Caller) << " because too costly to inline (cost="
>              << NV("Cost", IC.getCost())
>              << ", threshold=" << NV("Threshold", IC.getThreshold()) << ")";
>     });
> 
> 
> I have a proof-of-concept implementation at https://reviews.llvm.org/D37921 <https://reviews.llvm.org/D37921>.
>  
> The main change is that since in the lambda the remark is now returned by value, we need to preserve its type in the insertion operator.  This requires making the insertion operator generic.  I am also curious if people see C++ portability problems with the code.
> 
> Feedback welcome.
> 
> Adam
> 
> [1] https://reviews.llvm.org/D33514 <https://reviews.llvm.org/D33514>
> 
> 
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