[CodeGen] Add skipFunction() check to MachineFunctionSplitter (#166260)

MachineFunctionSplitter was missing a skipFunction() check, causing it
to incorrectly split functions that should be skipped (e.g., functions
with optnone attribute).

This patch adds an early skipFunction() check in runOnMachineFunction()
to ensure these functions are never split, regardless of profile data
availability or other splitting conditions.

Author: grigorypas

llvm/lib/CodeGen/MachineFunctionSplitter.cpp

@@ -129,6 +129,9 @@ static bool isColdBlock(const MachineBasicBlock &MBB,
 }
 
 bool MachineFunctionSplitter::runOnMachineFunction(MachineFunction &MF) {
+  if (skipFunction(MF.getFunction()))
+    return false;
+
   // Do not split functions when -basic-block-sections=all is specified.
   if (MF.getTarget().getBBSectionsType() == llvm::BasicBlockSection::All)
     return false;

llvm/test/CodeGen/Generic/machine-function-splitter-optnone.ll

@@ -0,0 +1,50 @@
+; REQUIRES: x86-registered-target
+
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -split-machine-functions -O0 -mfs-psi-cutoff=0 -mfs-count-threshold=10000 | FileCheck %s
+
+;; Check that functions with optnone attribute are not split.
+; CHECK-LABEL: foo_optnone:
+; CHECK-NOT:   .section .text.split.foo_optnone
+; CHECK-NOT:   foo_optnone.cold:
+; CHECK:       .LBB0_2:
+; CHECK:       .size   foo_optnone
+
+define void @foo_optnone(i1 zeroext %0) nounwind optnone noinline !prof !14 !section_prefix !15 {
+entry:
+  br i1 %0, label %hot, label %cold, !prof !17
+
+hot:
+  %1 = call i32 @bar()
+  br label %exit
+
+cold:
+  %2 = call i32 @baz()
+  br label %exit
+
+exit:
+  %3 = tail call i32 @qux()
+  ret void
+}
+
+declare i32 @bar()
+declare i32 @baz()
+declare i32 @qux()
+
+!llvm.module.flags = !{!0}
+!0 = !{i32 1, !"ProfileSummary", !1}
+!1 = !{!2, !3, !4, !5, !6, !7, !8, !9}
+!2 = !{!"ProfileFormat", !"InstrProf"}
+!3 = !{!"TotalCount", i64 10000}
+!4 = !{!"MaxCount", i64 10}
+!5 = !{!"MaxInternalCount", i64 1}
+!6 = !{!"MaxFunctionCount", i64 1000}
+!7 = !{!"NumCounts", i64 3}
+!8 = !{!"NumFunctions", i64 5}
+!9 = !{!"DetailedSummary", !10}
+!10 = !{!11, !12, !13}
+!11 = !{i32 10000, i64 100, i32 1}
+!12 = !{i32 999900, i64 100, i32 1}
+!13 = !{i32 999999, i64 1, i32 2}
+!14 = !{!"function_entry_count", i64 7000}
+!15 = !{!"function_section_prefix", !"hot"}
+!17 = !{!"branch_weights", i32 7000, i32 0}