[MC] Reduce size of MCDataFragment by 8 bytes

[aengelke]

Due to alignment, MCEncodedFragment was 1 byte over the 8 byte boundary, so folding to bools into bitfields in MCFragment gives a nice space reduction of MCDataFragment from 224 to 216 bytes.

[llvmbot]

@llvm/pr-subscribers-mc

Author: None (aengelke)

Changes

Due to alignment, MCFragment was 1 byte over the 8 byte boundary, so folding to bools into bitfields gives a nice space reduction from 224 to 216 bytes.

---

Full diff: https://github.com/llvm/llvm-project/pull/95293.diff

2 Files Affected:

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

diff --git a/llvm/include/llvm/MC/MCFragment.h b/llvm/include/llvm/MC/MCFragment.h
index 45599c940659e..d33160f4ff010 100644
--- a/llvm/include/llvm/MC/MCFragment.h
+++ b/llvm/include/llvm/MC/MCFragment.h
@@ -73,8 +73,8 @@ class MCFragment {
   FragmentType Kind;
 
 protected:
-  bool HasInstructions;
-  bool LinkerRelaxable = false;
+  bool HasInstructions : 1;
+  bool LinkerRelaxable : 1;
 
   MCFragment(FragmentType Kind, bool HasInstructions,
              MCSection *Parent = nullptr);
diff --git a/llvm/lib/MC/MCFragment.cpp b/llvm/lib/MC/MCFragment.cpp
index 6d97e8ce552ba..f245f39a87e9f 100644
--- a/llvm/lib/MC/MCFragment.cpp
+++ b/llvm/lib/MC/MCFragment.cpp
@@ -200,7 +200,7 @@ uint64_t llvm::computeBundlePadding(const MCAssembler &Assembler,
 MCFragment::MCFragment(FragmentType Kind, bool HasInstructions,
                        MCSection *Parent)
     : Parent(Parent), Atom(nullptr), Offset(~UINT64_C(0)), LayoutOrder(0),
-      Kind(Kind), HasInstructions(HasInstructions) {
+      Kind(Kind), HasInstructions(HasInstructions), LinkerRelaxable(false) {
   if (Parent && !isa<MCDummyFragment>(*this))
     Parent->addFragment(*this);
 }

[nikic]

Somewhat confused here. We have an unsigned, followed by uint8_t and two bools. That should fit into 8 bytes?

And why is MCFragment currently 224 bytes? Isn't it just 40?

[aengelke]

MCDataFragment goes from 224 to 216 bytes, MCFragment from 39 to 38. MCEncodedFragment adds two bytes and now goes from 41 to 40 bytes. In the C++ Itanium ABI, there is no padding after base classes.

Edit: I made the comment a bit clearer. Sorry for not providing enough data in the initial message.

[nikic]

Thanks for the explanation!

[MaskRay]

Before the removal of SubsectionNumber (#95077) and IsBeingLaidOut (9d0754a), sizeof(MCDataFragment) was 232 It's now 224. Yes! With some bit fields we can decrease it to 216.