The aim of this patch is to minimize the compilation time overhead of running Function Specialization. It is about 40% slower to run as a standalone pass (IPSCCP + FuncSpec vs IPSCCP with FuncSpec) according to my measurements. I compiled the llvm testsuite with NewPM-O3 + LTO and measured single threaded [user + system] time of IPSCCP and FuncSpec by passing the '-time-passes' option to lld. Then I compared the two configurations in terms of Instruction Count of the total compilation (not of the individual passes) as in https://llvm-compile-time-tracker.com. Geomean for non-LTO builds is -0.25% and LTO is -0.5% approximately.
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