[llvm-dev] linear-scan RA (original) (raw)

David A. Greene via llvm-dev llvm-dev at lists.llvm.org
Wed Sep 12 06:39:00 PDT 2018

Bruce Hoult <brucehoult at sifive.com> writes:

Another point is that it's important that the default works well for pathological code, such as that produced by other programs.

An example I frequently see is the output from Verilator, a program which compiles the hardware description language Verilog into C, and then compiles that. It likes to produce extremely large function bodies. I've seen cases where engineers designing a CPU make some trivial one line change to their Verilog and see the compile time go from a couple of seconds to many hours. Telling them to use -O0 fixes the compile time problem, but of course then makes their simulation go much more slowly.

That is very true. There are some ridiculously large machine-generated Fortran subroutines in the HPC world. Many years ago a particular code caused our Fortran compiler to abort because it triggered an assert that there would be no more than 10,000 vectorizable loops in a subroutine.

Our customers are sensitive to compile time, even though we are all about execution speed to a ridiculous extreme. Customers will tolerate quite a bit of extra compile time for things like good vectorization and parallelization but a 20% hit from register allocation for a marginal benefit on most codes is not going to fly.

                         -David

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