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

[Sean Silva via llvm-dev]

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>
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.

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


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