[llvm-bugs] [Bug 42968] New: Missed inlining causes significat slowdown (C-ray case study)

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https://bugs.llvm.org/show_bug.cgi?id=42968

            Bug ID: 42968
           Summary: Missed inlining causes significat slowdown (C-ray case
                    study)
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Interprocedural Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: llvm-bugs at lists.llvm.org

This is case study for C-ray benchmark, but it is not so unrealistic for other
real word code. In C-ray's hot loop there is a call to a quite big function
'ray_sphere'. Clang does not inline it, GCC does. GCC's code is much faster
(see below).

So in this case, Inliner should be enhanced with some sort of heuristic so it
will realise that inlining of 'ray_sphere' is profitable since most of the
calculations on ray are loop invariant and will be hoisted.

Tested on Intel Haswell.

'make' - GCC 9
./c-ray-mt -t 8 -s 800x400 -r 1 -i sphfract -o output.ppm
c-ray-mt v1.1
Rendering took: 0 seconds (518 milliseconds)

'make' - Clang 9
./c-ray-mt -t 8 -s 800x400 -r 1 -i sphfract -o output.ppm
c-ray-mt v1.1
Rendering took: 0 seconds (609 milliseconds)


Added __attribute((always_inline)) to ray_sphere:
GCC 9:
./c-ray-mt -t 8 -s 800x400 -r 1 -i sphfract -o output.ppm
c-ray-mt v1.1
Rendering took: 0 seconds (514 milliseconds)


 ./c-ray-mt -t 8 -s 800x400 -r 1 -i sphfract -o output.ppm
c-ray-mt v1.1
Rendering took: 0 seconds (413 milliseconds)


As we can see, a significant win:
609 -> 413 milliseconds


There is also some haswell codegen issue, since with -march=haswell:
Clang 9
 ./c-ray-mt -t 8 -s 800x400 -r 1 -i sphfract -o output.ppm
c-ray-mt v1.1
Rendering took: 0 seconds (403 milliseconds)


GCC 9
 ./c-ray-mt -t 8 -s 800x400 -r 1 -i sphfract -o output.ppm
c-ray-mt v1.1
Rendering took: 0 seconds (371 milliseconds)

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