Bump coalescing limit

[MatzeB]

Summary

This bumps the "large-interval-freq-threshold" limit in the register coalescer to 256. The limit was introduced in https://reviews.llvm.org/D59143 without much justify for the particular value "100", so I hope bumping it is ok.

This change is motivated by bad codegen for the popular crc32c algorithm; the code is often based/copied from this implementation: https://github.com/htot/crc32c/blob/master/crc32c/crc32intelc.cc which uses a duffs-device pattern with 128 switch-cases. There are examples in RocksDB (https://github.com/facebook/rocksdb/blob/main/util/crc32c.cc) and Folly (https://github.com/facebook/folly/blob/main/folly/hash/detail/Crc32cDetail.cpp) which are important use cases for us.

[MatzeB]

FWIW I noticed that fixing this dramatically improves IPC and instructions executed, but does not actually affect benchmark runtimes. For my microbenchmark.

Before:

5,581,342,454      cycles                    #    3.288 GHz                      (50.02%)
11,940,550,021      instructions              #    2.14  insn per cycle           (50.05%)

After:

5,595,396,911      cycles                    #    3.264 GHz                      (50.01%)
6,809,294,177      instructions              #    1.22  insn per cycle           (50.06%)

so this isn't as critical as I initially thought, but I also don't think it hurt anything and improves code size.

[MatzeB]

I did not notice any change in compiletimes but then again the projects I have here typically don't even hit the 100 limit. Maybe someone using SpeculativeLoadHardening could confirm or report back in case of trouble?

[qcolombet]

Hi @MatzeB,

Given what you observed and this limit is pretty much never hit in practice, I'm fine with bumping it.

If people come back complaining with compile time, we can lower it again.

Cheers,
-Quentin

llvm/lib/CodeGen/RegisterCoalescer.cpp

cl::desc("If the valnos size of an interval is larger than the threshold, "
              "it is regarded as a large interval. "),
     cl::init(100));
 
 static cl::opt<unsigned> LargeIntervalFreqThreshold(
     "large-interval-freq-threshold", cl::Hidden,
     cl::desc("For a large interval, if it is coalesed with other live "
              "intervals many times more than the threshold, stop its "
              "coalescing to control the compile time. "),
-    cl::init(100));
+    cl::init(256));
 
 namespace {
 
   class JoinVals;
 
   class RegisterCoalescer : public MachineFunctionPass,
                             private LiveRangeEdit::Delegate {
     MachineFunction* MF = nullptr;