Only split cold blocks with more than a given number of instructions

[dhoekwater]

Summary

On Arm, splitting a cold block may incur a thunk, a 16-byte snippet of code
that extends the range of an unconditional branch. Consequently, splitting
the block may actually inflate cold code size and hurt performance.

While thunk-aware splitting is a complex problem, only splitting cold blocks
larger than a thunk will get some wins without the risk of regression.

[kristof.beyls]

Adding Snehasish as a reviewer as git blame suggests snehasish has designed and written most of the MachineFunctionSplitter.

[kristof.beyls]

It seems to me that this patch would also need:

• Some regression test to test it actually changes code generation (only?) when targeting Arm.
• As is, this patch will not result in changed code generation IIUC?

[snehasish]

In D145212#4174199, @kristof.beyls wrote:

It seems to me that this patch would also need:

• Some regression test to test it actually changes code generation (only?) when targeting Arm.
• As is, this patch will not result in changed code generation IIUC?

Yes, a small test for this would be great. The existing tests live in llvm-project/llvm/test/CodeGen/X86/machine-function-splitter.ll. I don't think this enhancement has much utility on X86, so perhaps the tests should be under CodeGen/AArch64 instead?

[dhoekwater]

Dropping for the time being as it doesn't look like thresholding is necessary for MFS performance.

llvm/lib/CodeGen/MachineFunctionSplitter.cpp

PercentileCutoff("mfs-psi-cutoff",
                      cl::init(999950), cl::Hidden);
 
 static cl::opt<unsigned> ColdCountThreshold(
     "mfs-count-threshold",
     cl::desc(
         "Minimum number of times a block must be executed to be retained."),
     cl::init(1), cl::Hidden);
 
+static cl::opt<unsigned>
+    ColdSizeThreshold("mfs-size-threshold",
+                      cl::desc("Maximum number of instructions a cold block "
+                               "may have and still be retained."),
+                      cl::init(0), cl::Hidden);
+
 static cl::opt<bool> SplitAllEHCode(
     "mfs-split-ehcode",
     cl::desc("Splits all EH code and it's descendants by default."),
     cl::init(false), cl::Hidden);
 
 namespace {
 
 class MachineFunctionSplitter : public MachineFunctionPass {
static void setDescendantEHBlocksCold(MachineFunction &MF) {
   for (auto Block : EHBlocks) {
     Block->setSectionID(MBBSectionID::ColdSectionID);
   }
 }
 
 static bool isColdBlock(const MachineBasicBlock &MBB,
                         const MachineBlockFrequencyInfo *MBFI,
                         ProfileSummaryInfo *PSI) {
+  if (MBB.size() <= ColdSizeThreshold)
+    return false;
+
   std::optional<uint64_t> Count = MBFI->getBlockProfileCount(&MBB);
   if (!Count)
     return true;
 
   if (PercentileCutoff > 0) {
     return PSI->isColdCountNthPercentile(PercentileCutoff, *Count);
   }
   return (*Count < ColdCountThreshold);