[llvm] faa931b - MCFragment: Store the number of variable-size tail fixups as uint8_t

[Fangrui Song via llvm-commits]

Author: Fangrui Song
Date: 2025-08-02T15:17:00-07:00
New Revision: faa931b717c02d57f0814caa9133219040e6a85b

URL: https://github.com/llvm/llvm-project/commit/faa931b717c02d57f0814caa9133219040e6a85b
DIFF: https://github.com/llvm/llvm-project/commit/faa931b717c02d57f0814caa9133219040e6a85b.diff

LOG: MCFragment: Store the number of variable-size tail fixups as uint8_t

Decrease sizeof(MCFragment) by 8 on 64-bit machines.

Added: 
    

Modified: 
    llvm/include/llvm/MC/MCSection.h
    llvm/lib/MC/MCCodeView.cpp
    llvm/lib/MC/MCSection.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/MC/MCSection.h b/llvm/include/llvm/MC/MCSection.h
index cec2387912aea..4022ea7763e50 100644
--- a/llvm/include/llvm/MC/MCSection.h
+++ b/llvm/include/llvm/MC/MCSection.h
@@ -80,11 +80,13 @@ class MCFragment {
 
   FragmentType Kind;
 
-protected:
+  //== Used by certain fragment types for better packing.
+
+  // The number of fixups for the optional variable-size tail must be small.
+  uint8_t VarFixupSize = 0;
+
   bool LinkerRelaxable : 1;
 
-  /// Used by certain fragment types for better packing.
-  ///
   /// FT_Data, FT_Relaxable
   bool HasInstructions : 1;
   /// FT_Relaxable, x86-specific
@@ -103,7 +105,6 @@ class MCFragment {
   uint32_t VarContentStart = 0;
   uint32_t VarContentEnd = 0;
   uint32_t VarFixupStart = 0;
-  uint32_t VarFixupEnd = 0;
 
   const MCSubtargetInfo *STI = nullptr;
 
@@ -643,11 +644,10 @@ inline ArrayRef<MCFixup> MCFragment::getFixups() const {
 
 inline MutableArrayRef<MCFixup> MCFragment::getVarFixups() {
   return MutableArrayRef(getParent()->FixupStorage)
-      .slice(VarFixupStart, VarFixupEnd - VarFixupStart);
+      .slice(VarFixupStart, VarFixupSize);
 }
 inline ArrayRef<MCFixup> MCFragment::getVarFixups() const {
-  return ArrayRef(getParent()->FixupStorage)
-      .slice(VarFixupStart, VarFixupEnd - VarFixupStart);
+  return ArrayRef(getParent()->FixupStorage).slice(VarFixupStart, VarFixupSize);
 }
 
 //== FT_Relaxable functions

diff  --git a/llvm/lib/MC/MCCodeView.cpp b/llvm/lib/MC/MCCodeView.cpp
index 7d528a55432e6..3a5f01c2af607 100644
--- a/llvm/lib/MC/MCCodeView.cpp
+++ b/llvm/lib/MC/MCCodeView.cpp
@@ -695,5 +695,7 @@ void CodeViewContext::encodeDefRange(const MCAssembler &Asm,
   }
 
   Frag.setVarContents(Contents);
+  assert(Fixups.size() < 256 && "Store fixups outside of MCFragment's VarFixup "
+                                "storage if the number ever exceeds 256");
   Frag.setVarFixups(Fixups);
 }

diff  --git a/llvm/lib/MC/MCSection.cpp b/llvm/lib/MC/MCSection.cpp
index 4f282672afbbf..27ca1314074fe 100644
--- a/llvm/lib/MC/MCSection.cpp
+++ b/llvm/lib/MC/MCSection.cpp
@@ -83,12 +83,14 @@ void MCFragment::appendFixups(ArrayRef<MCFixup> Fixups) {
 }
 
 void MCFragment::setVarFixups(ArrayRef<MCFixup> Fixups) {
+  assert(Fixups.size() < 256 &&
+         "variable-size tail cannot have more than 256 fixups");
   auto &S = getParent()->FixupStorage;
-  if (VarFixupStart + Fixups.size() > VarFixupEnd) {
+  if (Fixups.size() > VarFixupSize) {
     VarFixupStart = S.size();
     S.resize_for_overwrite(S.size() + Fixups.size());
   }
-  VarFixupEnd = VarFixupStart + Fixups.size();
+  VarFixupSize = Fixups.size();
   // Source fixup offsets are relative to the variable part's start. Add the
   // fixed part size to make them relative to the fixed part's start.
   std::transform(Fixups.begin(), Fixups.end(), S.begin() + VarFixupStart,