[COFF] Preserve UniqueID used to create MCSectionCOFF

[HaohaiWen]

This UniqueID can be used later to create associative section.
e.g. A .pseudo_probe associated to the section of the corresponding
function.

[llvmbot]

@llvm/pr-subscribers-backend-x86

@llvm/pr-subscribers-mc

Author: Haohai Wen (HaohaiWen)

Changes

This UniqueID can be used later to create associative section.
e.g. A .pseudo_probe associated to the section of the corresponding
function.

---

Full diff: https://github.com/llvm/llvm-project/pull/123869.diff

2 Files Affected:

• (modified) llvm/include/llvm/MC/MCSectionCOFF.h (+7-2)
• (modified) llvm/lib/MC/MCContext.cpp (+1-1)

diff --git a/llvm/include/llvm/MC/MCSectionCOFF.h b/llvm/include/llvm/MC/MCSectionCOFF.h
index 97540d985dbd1d..9c79c27f2a9152 100644
--- a/llvm/include/llvm/MC/MCSectionCOFF.h
+++ b/llvm/include/llvm/MC/MCSectionCOFF.h
@@ -47,16 +47,19 @@ class MCSectionCOFF final : public MCSection {
   /// section (Characteristics & IMAGE_SCN_LNK_COMDAT) != 0
   mutable int Selection;
 
+  unsigned UniqueID;
+
 private:
   friend class MCContext;
   // The storage of Name is owned by MCContext's COFFUniquingMap.
   MCSectionCOFF(StringRef Name, unsigned Characteristics,
-                MCSymbol *COMDATSymbol, int Selection, MCSymbol *Begin)
+                MCSymbol *COMDATSymbol, int Selection, unsigned UniqueID,
+                MCSymbol *Begin)
       : MCSection(SV_COFF, Name, Characteristics & COFF::IMAGE_SCN_CNT_CODE,
                   Characteristics & COFF::IMAGE_SCN_CNT_UNINITIALIZED_DATA,
                   Begin),
         Characteristics(Characteristics), COMDATSymbol(COMDATSymbol),
-        Selection(Selection) {
+        Selection(Selection), UniqueID(UniqueID) {
     assert((Characteristics & 0x00F00000) == 0 &&
            "alignment must not be set upon section creation");
   }
@@ -72,6 +75,8 @@ class MCSectionCOFF final : public MCSection {
 
   void setSelection(int Selection) const;
 
+  unsigned getUniqueID() const { return UniqueID; }
+
   void printSwitchToSection(const MCAsmInfo &MAI, const Triple &T,
                             raw_ostream &OS,
                             uint32_t Subsection) const override;
diff --git a/llvm/lib/MC/MCContext.cpp b/llvm/lib/MC/MCContext.cpp
index 46222fcaa5b152..56aba5de4445e4 100644
--- a/llvm/lib/MC/MCContext.cpp
+++ b/llvm/lib/MC/MCContext.cpp
@@ -718,7 +718,7 @@ MCSectionCOFF *MCContext::getCOFFSection(StringRef Section,
   StringRef CachedName = Iter->first.SectionName;
   MCSymbol *Begin = getOrCreateSectionSymbol<MCSymbolCOFF>(Section);
   MCSectionCOFF *Result = new (COFFAllocator.Allocate()) MCSectionCOFF(
-      CachedName, Characteristics, COMDATSymbol, Selection, Begin);
+      CachedName, Characteristics, COMDATSymbol, Selection, UniqueID, Begin);
   Iter->second = Result;
   auto *F = allocInitialFragment(*Result);
   Begin->setFragment(F);

[MaskRay]

Best to add .section parser and printer support so that this feature is testable. The new test can be added to llvm/test/MC/COFF/section-unique.s

@mstorsjo might be interested in ensuring gas compatibility.

[HaohaiWen]

Best to add .section parser and printer support so that this feature is testable. The new test can be added to llvm/test/MC/COFF/section-unique.s

@mstorsjo might be interested in ensuring gas compatibility.

Is there any specification about COFF .section directive?
Current format is
.section section_name,"characteristic"[(,selection,comdat_name) | (\n.linkonce selection)]
If we can define .section directive format, I'd like to define it as
.section section_name,"characteristic"[(,selection,comdat_name)][,unique,unique_id]
.section section_name,"characteristic"[,unique,unique_id][(\n.linkonce selection)]
Any suggestion about that?

[HaohaiWen]

Let's revive this PR. Any comments?

[williamweixiao]

LGTM.