[CodeGen] Do not split functions with attr "implicit-section-name".

[snehasish]

Summary

The #pragma clang section can be used at a coarse granularity to specify
the section used for bss/data/text/rodata for global objects. When split
functions is enabled, the function may be split into two parts violating
user expectations.

Reference:
https://clang.llvm.org/docs/LanguageExtensions.html#specifying-section-names-for-global-objects-pragma-clang-section

[rahmanl]

llvm/lib/CodeGen/MachineFunctionSplitter.cpp

104–105

nit: Can you use hasSection() here to be more consistent with TargetLoweringObjectFile.cpp?

[snehasish]

Use hasSection() in the condition.

[snehasish]

Revert hasSection() since it is not semantically equivalent to getSection().empty().

[snehasish]

llvm/lib/CodeGen/MachineFunctionSplitter.cpp

104–105

Looks like this is not consistent even though the documentation suggests that they should be equivalent, so I'll revert it for now.

llvm/lib/CodeGen/MachineFunctionSplitter.cpp

bool MachineFunctionSplitter::runOnMachineFunction(MachineFunction &MF) {
   // also be considered but we don't see performance improvements yet.
   if (!MF.getFunction().hasProfileData())
     return false;
 
   // TODO: We don't split functions where a section attribute has been set
   // since the split part may not be placed in a contiguous region. It may also
   // be more beneficial to augment the linker to ensure contiguous layout of
   // split functions within the same section as specified by the attribute.
-  if (!MF.getFunction().getSection().empty())
+  if (!MF.getFunction().getSection().empty() ||
+      MF.getFunction().hasFnAttribute("implicit-section-name"))
     return false;
 
   // We don't want to proceed further for cold functions
   // or functions of unknown hotness. Lukewarm functions have no prefix.
   Optional<StringRef> SectionPrefix = MF.getFunction().getSectionPrefix();
   if (SectionPrefix.hasValue() &&
       (SectionPrefix.getValue().equals("unlikely") ||
        SectionPrefix.getValue().equals("unknown"))) {

llvm/test/CodeGen/X86/machine-function-splitter.ll

cold:
   %4 = call i32 @baz()
   br label %exit
 
 exit:
   %5 = tail call i32 @qux()
   ret i32 %5
 }
 
+define void @foo9(i1 zeroext %0) nounwind #0 !prof !14 {
+;; Check that function with section attribute is not split.
+; MFS-DEFAULTS-LABEL: foo9
+; MFS-DEFAULTS-NOT:   foo9.cold:
+  br i1 %0, label %2, label %4, !prof !17
+
+2:                                                ; preds = %1
+  %3 = call i32 @bar()
+  br label %6
+
+4:                                                ; preds = %1
+  %5 = call i32 @baz()
+  br label %6
+
+6:                                                ; preds = %4, %2
+  %7 = tail call i32 @qux()
+  ret void
+}
+
+
 declare i32 @bar()
 declare i32 @baz()
 declare i32 @bam()
 declare i32 @qux()
 declare void @_Z1fv()
 declare i32 @__gxx_personality_v0(...)
 
 @_ZTIi = external constant i8*
 
+attributes #0 = { "implicit-section-name"="nosplit" }
+
 !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}