[Hexagon] Emit lookup tables in text section based on a flag

[sgundapa]

Summary

The flag "-hexagon-emit-lut-text" (defaulted to false) is added to decide
on where to keep the switch generated lookup table.

[kparzysz]

lib/Target/Hexagon/HexagonTargetObjectFile.cpp

142–145

This could be

if (const Function *Fn = getLutUsedFunction(GO))
  return selectSectionForLookupTable(G0, TM, Fn);

419

Maybe replace it with "if (!I) continue" to reduce indentation.

421

Are there legitimate cases where instruction does not have a parent? If not, you could remove this test (maybe add an assertion instead).

[sgundapa]

lib/Target/Hexagon/HexagonTargetObjectFile.cpp

419

I will work on indentation and push a new one

421

I am not sure about this which is why I am trying to be safe by returning a nullptr. There might be scenarios of dead basic blocks or functions . I probably need to run a big code base to see what might happen.

[sgundapa]

Updated the patch with the indentation edit

lib/Target/Hexagon/HexagonTargetObjectFile.h

 
     unsigned getSmallDataSize() const;
 
+    const Function *getLutUsedFunction(const GlobalObject *GO) const;
+
   private:
     MCSectionELF *SmallDataSection;
     MCSectionELF *SmallBSSSection;

     MCSection *selectSmallSectionForGlobal(const GlobalObject *GO,
                                            SectionKind Kind,
                                            const TargetMachine &TM) const;
+
+    MCSection *selectSectionForLookupTable(const GlobalObject *GO,
+                                           const TargetMachine &TM,
+                                           const Function *Fn) const;
   };
 
 } // namespace llvm

lib/Target/Hexagon/HexagonTargetObjectFile.cpp

   cl::Hidden, cl::init(false),
   cl::desc("Trace global value placement"));
 
+static cl::opt<bool>
+    EmitLutInText("hexagon-emit-lut-text", cl::Hidden, cl::init(false),
+                 cl::desc("Emit hexagon lookup tables in function section"));
+
 // TraceGVPlacement controls messages for all builds. For builds with assertions
 // (debug or release), messages are also controlled by the usual debug flags
 // (e.g. -debug and -debug-only=globallayout)

          << (Kind.isBSS() ? "kind_bss " : "" )
          << (Kind.isBSSLocal() ? "kind_bss_local " : "" ));
 
+  // If the lookup table is used by more than one function, do not place
+  // it in text section.
+  if (EmitLutInText && GO->getName().startswith("switch.table")) {
+    if (const Function *Fn = getLutUsedFunction(GO))
+      return selectSectionForLookupTable(GO, TM, Fn);
+  }
+
   if (isGlobalInSmallSection(GO, TM))
     return selectSmallSectionForGlobal(GO, Kind, TM);
 

   // Otherwise, we work the same as ELF.
   return TargetLoweringObjectFileELF::SelectSectionForGlobal(GO, Kind, TM);
 }
+
+// Return the function that uses the lookup table. If there are more
+// than one live function that uses this look table, bail out and place
+// the lookup table in default section.
+const Function *
+HexagonTargetObjectFile::getLutUsedFunction(const GlobalObject *GO) const {
+  const Function *ReturnFn = nullptr;
+  for (auto U : GO->users()) {
+    // validate each instance of user to be a live function.
+    auto *I = dyn_cast<Instruction>(U);
+    if (!I)
+      continue;
+    auto *Bb = I->getParent();
+    if (!Bb)
+      continue;
+    auto *UserFn = Bb->getParent();
+    if (!ReturnFn)
+      ReturnFn = UserFn;
+    else if (ReturnFn != UserFn)
+      return nullptr;
+  }
+  return ReturnFn;
+}
+
+MCSection *HexagonTargetObjectFile::selectSectionForLookupTable(
+    const GlobalObject *GO, const TargetMachine &TM, const Function *Fn) const {
+
+  SectionKind Kind = SectionKind::getText();
+  // If the function has explicit section, place the lookup table in this
+  // explicit section.
+  if (Fn->hasSection())
+    return getExplicitSectionGlobal(Fn, Kind, TM);
+
+  const auto *FuncObj = dyn_cast<GlobalObject>(Fn);
+  return SelectSectionForGlobal(FuncObj, Kind, TM);
+}