[XRay] Make the xray_instr_map section specification more correct

[dberris]

Summary

We also add a test to show what currently happens when we create a
section per function and emit an xray_instr_map. This illustrates the
relationship (or lack thereof) between the per-function section and the
xray_instr_map section.

We also change the code generation slightly so that we don't always
create group sections, but rather only do so if a function where the
table is associated with is in a group.

Also in this change:

• Remove the "merge" flag on the xray_instr_map section.
• Test that we're generating the right table for comdat and non-comdat functions.

[dberris]

• Only use groups if the original function is defined in a group too

[majnemer]

LGTM

lib/Target/X86/X86MCInstLower.cpp

1110

you have a SwitchSection a few lines down.

lib/Target/X86/X86MCInstLower.cpp

 }
 
 void X86AsmPrinter::EmitXRayTable() {
   if (Sleds.empty())
     return;
   if (Subtarget->isTargetELF()) {
     auto *Section = OutContext.getELFSection(
         "xray_instr_map", ELF::SHT_PROGBITS,
-        ELF::SHF_ALLOC | ELF::SHF_GROUP | ELF::SHF_MERGE, 0,
+        ELF::SHF_ALLOC | ELF::SHF_GROUP | ELF::SHF_MERGE, 32,
         CurrentFnSym->getName());
     auto PrevSection = OutStreamer->getCurrentSectionOnly();
     OutStreamer->SwitchSection(Section);
     for (const auto &Sled : Sleds) {
       OutStreamer->EmitSymbolValue(Sled.Sled, 8);
       OutStreamer->EmitSymbolValue(CurrentFnSym, 8);
       auto Kind = static_cast<uint8_t>(Sled.Kind);
       OutStreamer->EmitBytes(
           StringRef(reinterpret_cast<const char *>(&Kind), 1));
       OutStreamer->EmitBytes(
           StringRef(reinterpret_cast<const char *>(&Sled.AlwaysInstrument), 1));
       OutStreamer->EmitZeros(14);
     }
     OutStreamer->SwitchSection(PrevSection);
   }
   Sleds.clear();
 }

test/CodeGen/X86/xray-section-group.ll

+; RUN: llc -filetype=asm -o - -mtriple=x86_64-unknown-linux-gnu -function-sections < %s | FileCheck %s
+
+define i32 @foo() nounwind noinline uwtable "function-instrument"="xray-always" {
+; CHECK: .section .text.foo,"ax",@progbits
+  ret i32 0
+; CHECK: .section xray_instr_map,"aGM",@progbits,32,foo,comdat
+}