[lld][NFC] Pack InputChunk / OutputSegment more tightly

[gchatelet]

Summary

This is a follow up on D139181.
Packing InputChunks going from 88B to 80B.
Packing OutputSegment going from 112B to 104B.

[sbc100]

lld/wasm/InputChunks.h

109

Maybe put this below the last i32 member (outSecOff) and before the first packed value (sectionKind)?

Do we have any tooling for automating this kind of stuff?

[MaskRay]

lld/wasm/InputChunks.h

109

Note that the placement may affect performance as well as code size (mostly for x86 when the assembly may switch from 1-byte to 4-byte displacement). I usually benchmark two runs. I know little about wasm, but I think the field is probably not accessed that frequently.

My comment here is a reminder that field reordering should not be done blindly.

lld/wasm/InputChunks.h

public:
   void generateRelocationCode(raw_ostream &os) const;
 
   bool isTLS() const { return flags & llvm::wasm::WASM_SEG_FLAG_TLS; }
 
   ObjFile *file;
   OutputSection *outputSec = nullptr;
   uint32_t comdat = UINT32_MAX;
   uint32_t inputSectionOffset = 0;
-  llvm::Align alignment;
   uint32_t flags;
 
   // Only applies to data segments.
   uint32_t outputSegmentOffset = 0;
   const OutputSegment *outputSeg = nullptr;
 
   // After assignAddresses is called, this represents the offset from
   // the beginning of the output section this chunk was assigned to.
   int32_t outSecOff = 0;
 
   uint8_t sectionKind : 3;
 
   // Signals that the section is part of the output.  The garbage collector,
   // and COMDAT handling can set a sections' Live bit.
   // If GC is disabled, all sections start out as live by default.
   unsigned live : 1;
 
   // Signals the chunk was discarded by COMDAT handling.
   unsigned discarded : 1;
 
+
+  llvm::Align alignment;
+
 protected:
   InputChunk(ObjFile *f, Kind k, StringRef name, uint32_t alignment = 0,
              uint32_t flags = 0)
-      : name(name), file(f), alignment(1ULL << alignment), flags(flags),
-        sectionKind(k), live(!config->gcSections), discarded(false) {}
+      : name(name), file(f), flags(flags),
+        sectionKind(k), live(!config->gcSections), discarded(false), alignment(1ULL << alignment) {}
   ArrayRef<uint8_t> data() const { return rawData; }
   uint64_t getTombstone() const;
 
   ArrayRef<WasmRelocation> relocations;
   ArrayRef<uint8_t> rawData;
 };
 
 // Represents a WebAssembly data segment which can be included as part of

lld/wasm/OutputSegment.h

public:
   // to the output binary.  However if the memory is imported, and
   // we can't use memory.fill during startup (due to lack of bulk
   // memory feature) then we include BSS segments verbatim.
   bool requiredInBinary() const { return !isBss || config->emitBssSegments; }
 
   bool isTLS() const { return name == ".tdata"; }
 
   StringRef name;
-  bool isBss = false;
   uint32_t index = 0;
   uint32_t linkingFlags = 0;
   uint32_t initFlags = 0;
   uint32_t sectionOffset = 0;
-  llvm::Align alignment;
   uint64_t startVA = 0;
   std::vector<InputChunk *> inputSegments;
+  bool isBss = false;
+  llvm::Align alignment;
 
   // Sum of the size of the all the input segments
   uint32_t size = 0;
 
   // Segment header
   std::string header;
 };
 
 } // namespace wasm
 } // namespace lld
 
 #endif // LLD_WASM_OUTPUT_SEGMENT_H