[ELF] Shrink binding and type in Symbol

[smeenai]

Summary

STB_HIPROC and STT_HIPROC are both 15, so we can fit the symbol binding
and type in 4 bits. This gives us an additional byte to use for Symbol
flags (without increasing the type's size), which I'll be making use of
in the next diff.

Reorder type and binding based on a suggestion from @MaskRay, to
optimize st_info computation on little-endian systems (see
https://godbolt.org/z/nMn8Yar43).

[MaskRay]

#define ELF32_ST_INFO(bind, type) (((bind) << 4) + ((type) & 0xf))

It may be worth swapping type and bind to optimize for little-endian systems.

[smeenai]

Reorder type and binding

[smeenai]

In D124042#3460961, @MaskRay wrote:

#define ELF32_ST_INFO(bind, type) (((bind) << 4) + ((type) & 0xf))

It may be worth swapping type and bind to optimize for little-endian systems.

Good idea, thanks.

[smeenai]

Mention optimization is specific to little-endian systems

[MaskRay]

Thanks for the update. Seems that the functions which become smaller are more than functions which become larger.
lld::elf::SymbolTable::insert(llvm::StringRef) and SymbolTableSection::writeTo are examples becoming smaller.

[MaskRay]

BTW: the new lld (/tmp/c/1) may be slightly faster when linking chromium

% hyperfine --warmup 2 --min-runs 10 "numactl -C 20-27 "{/tmp/c/0,/tmp/c/1}" -flavor gnu @response.txt --threads=8"
Benchmark 1: numactl -C 20-27 /tmp/c/0 -flavor gnu @response.txt --threads=8
  Time (mean ± σ):      5.568 s ±  0.022 s    [User: 9.089 s, System: 2.293 s]
  Range (min … max):    5.529 s …  5.605 s    10 runs
 
Benchmark 2: numactl -C 20-27 /tmp/c/1 -flavor gnu @response.txt --threads=8
  Time (mean ± σ):      5.543 s ±  0.026 s    [User: 9.038 s, System: 2.287 s]
  Range (min … max):    5.518 s …  5.605 s    10 runs
 
Summary
  'numactl -C 20-27 /tmp/c/1 -flavor gnu @response.txt --threads=8' ran
    1.00 ± 0.01 times faster than 'numactl -C 20-27 /tmp/c/0 -flavor gnu @response.txt --threads=8'

% hyperfine --warmup 2 --min-runs 10 "numactl -C 20-27 "{/tmp/c/0,/tmp/c/1}" -flavor gnu @response.txt --threads=8"
Benchmark 1: numactl -C 20-27 /tmp/c/0 -flavor gnu @response.txt --threads=8
  Time (mean ± σ):      5.596 s ±  0.055 s    [User: 9.052 s, System: 2.322 s]
  Range (min … max):    5.510 s …  5.699 s    10 runs
 
Benchmark 2: numactl -C 20-27 /tmp/c/1 -flavor gnu @response.txt --threads=8
  Time (mean ± σ):      5.548 s ±  0.041 s    [User: 9.018 s, System: 2.320 s]
  Range (min … max):    5.485 s …  5.628 s    10 runs
 
Summary
  'numactl -C 20-27 /tmp/c/1 -flavor gnu @response.txt --threads=8' ran
    1.01 ± 0.01 times faster than 'numactl -C 20-27 /tmp/c/0 -flavor gnu @response.txt --threads=8'

[smeenai]

Ah, neat :) Thanks for checking.

lld/ELF/Symbols.h

public:
   InputFile *file;
 
 protected:
   const char *nameData;
   // 32-bit size saves space.
   uint32_t nameSize;
 
 public:
+  // The next three fields have the same meaning as the ELF symbol attributes.
+  // type and binding are placed in this order to optimize generating st_info,
+  // which is defined as (binding << 4) + (type & 0xf), on a little-endian
+  // system.
+  uint8_t type : 4; // symbol type
+
   // Symbol binding. This is not overwritten by replace() to track
   // changes during resolution. In particular:
   //  - An undefined weak is still weak when it resolves to a shared library.
   //  - An undefined weak will not extract archive members, but we have to
   //    remember it is weak.
-  uint8_t binding;
+  uint8_t binding : 4;
 
-  // The following fields have the same meaning as the ELF symbol attributes.
-  uint8_t type;    // symbol type
   uint8_t stOther; // st_other field value
 
   uint8_t symbolKind;
 
   // The partition whose dynamic symbol table contains this symbol's definition.
   uint8_t partition = 1;
 
   // Symbol visibility. This is the computed minimum visibility of all
private:
 
   bool shouldReplace(const Defined &other) const;
 
   inline size_t getSymbolSize() const;
 
 protected:
   Symbol(Kind k, InputFile *file, StringRef name, uint8_t binding,
          uint8_t stOther, uint8_t type)
-      : file(file), nameData(name.data()), nameSize(name.size()),
-        binding(binding), type(type), stOther(stOther), symbolKind(k),
+      : file(file), nameData(name.data()), nameSize(name.size()), type(type),
+        binding(binding), stOther(stOther), symbolKind(k),
         visibility(stOther & 3), isPreemptible(false),
         isUsedInRegularObj(false), used(false), exportDynamic(false),
         inDynamicList(false), referenced(false), traced(false),
         hasVersionSuffix(false), isInIplt(false), gotInIgot(false),
         folded(false), needsTocRestore(false), scriptDefined(false),
         needsCopy(false), needsGot(false), needsPlt(false), needsTlsDesc(false),
         needsTlsGd(false), needsTlsGdToIe(false), needsGotDtprel(false),
         needsTlsIe(false), hasDirectReloc(false) {}