Profiling the DMD Compiler Itself

[max haughton]

On Tuesday, 9 May 2023 at 03:28:15 UTC, Walter Bright wrote:
> If the dmd builder builds dmd with the ENABLE_PROFILE=1, the 
> resulting compiler will profile itself compiling a program. The 
> results of the profile will be emitted to the file trace.log, 
> which is described here:
>
> https://www.digitalmars.com/ctg/trace.html
>
> Yes, dmd inherited the profiler from the Digital Mars C++ 
> profiler.
>
> It works by inserting code into the prolog and epilog of each 
> function, calculating the time it takes the function to 
> execute. It turns out that this takes a disastrously long time 
> to run, so it is only useful for profiling with fairly short 
> programs. Trying to profile dmd itself will likely have to wait 
> until the sun becomes a red giant before it finishes.
>
> But anyhow, profiling compiles of smaller programs can work out 
> nicely, and with the report in trace.log one can see where the 
> problems are.
>
> Happy profiling!

perf (available on basically any Linux system) is a much better 
profiler.

Instrumentation-based profiling is almost always a worse option 
than sampling — even if the overhead was zero the data is much 
less useful.

-profile=gc is very useful, despite -profile being not 
particularly insightful

N.B. Compiling programs with base pointers always being emitted 
helps sampling profilers along quite a lot, DWARF implementations 
are not all made equal.