Fix ingest_raw_data performance issue in Nested JSON reader due to RVO

[karthikeyann]

Description

Issue is that json::experimental::ingest_raw_data took double the time of json::ingest_raw_data for same data.

After replacing tertiary operator with if else, runtime for 500 MB file is same as json::ingest_raw_data
I suspect, RVO (copy elision) is skipped while using tertiary operator.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[ttnghia]

Interesting, I also didn't know that ternary expression could prevent copy elision
Ref: https://stackoverflow.com/questions/22078029/why-does-the-ternary-operator-prevent-return-value-optimization

If possible, can you make a simple example in goldbolt (class printing to stdout when constructed/copied) to demonstrate that, please?

[karthikeyann]

https://godbolt.org/z/1vzPqqzs4
Here is a sample code.
tertiary operator calls vector(vector const&) for buffer return
but if() return buffer; calls vector(vector&&).

Reason for this could be - for tertiary operator, one is lvalue (buffer), and the other is rvalue (return value of another function).
So, for lvalue, it makes call to copy constructor.
For Rvalue, it called move constructor.

[codecov]

Codecov Report

Base: 87.47% // Head: 88.12% // Increases project coverage by +0.64% 🎉

Coverage data is based on head (ea207bf) compared to base (f817d96).
Patch has no changes to coverable lines.

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-22.12   #12070      +/-   ##
================================================
+ Coverage         87.47%   88.12%   +0.64%     
================================================
  Files               133      135       +2     
  Lines             21826    22011     +185     
================================================
+ Hits              19093    19397     +304     
+ Misses             2733     2614     -119     

Impacted Files	Coverage Δ
python/cudf/cudf/io/text.py	91.66% <0.00%> (-8.34%)	⬇️
python/cudf/cudf/core/_base_index.py	81.28% <0.00%> (-4.27%)	⬇️
python/cudf/cudf/io/json.py	92.06% <0.00%> (-2.68%)	⬇️
python/cudf/cudf/utils/utils.py	89.91% <0.00%> (-0.69%)	⬇️
python/dask_cudf/dask_cudf/core.py	73.72% <0.00%> (-0.41%)	⬇️
python/cudf/cudf/io/parquet.py	90.45% <0.00%> (-0.39%)	⬇️
python/dask_cudf/dask_cudf/backends.py	84.90% <0.00%> (-0.37%)	⬇️
python/cudf/cudf/core/column/datetime.py	89.62% <0.00%> (-0.09%)	⬇️
python/cudf/cudf/io/orc.py	92.94% <0.00%> (-0.09%)	⬇️
python/cudf/cudf/core/dataframe.py	93.67% <0.00%> (-0.06%)	⬇️
... and 36 more

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[karthikeyann]

rerun tests

[mythrocks]

That's a brilliant catch.

[karthikeyann]

@gpucibot merge