Why class size increases when int64_t changes to int32_t

Question

In my first example I have two bitfields using int64_t. When I compile and get the size of the class I get 8.

class Test
{
    int64_t first : 40;
         


        

Answer 1

To add to what others have already said:

If you want to examine it, you can use a compiler option or external program to output the struct layout.

Consider this file:

// test.cpp
#include 

class Test_1 {
    int64_t first  : 40;
    int64_t second : 24;
};

class Test_2 {
    int64_t first  : 40;
    int32_t second : 24;
};

// Dummy instances to force Clang to output layout.
Test_1 t1;
Test_2 t2;

If we use a layout output flag, such as Visual Studio's /d1reportSingleClassLayoutX (where X is all or part of the class or struct name) or Clang++'s -Xclang -fdump-record-layouts (where -Xclang tells the compiler to interpret -fdump-record-layouts as a Clang frontend command instead of a GCC frontend command), we can dump the memory layouts of Test_1 and Test_2 to standard output. [Unfortunately, I'm not sure how to do this directly with GCC.]

If we do so, the compiler will output the following layouts:

• Visual Studio:

cl /c /d1reportSingleClassLayoutTest test.cpp

// Output:
tst.cpp
class Test_1    size(8):
    +---
 0. | first (bitstart=0,nbits=40)
 0. | second (bitstart=40,nbits=24)
    +---



class Test_2    size(16):
    +---
 0. | first (bitstart=0,nbits=40)
 8. | second (bitstart=0,nbits=24)
    |  (size=4)
    +---

• Clang:

clang++ -c -std=c++11 -Xclang -fdump-record-layouts test.cpp

// Output:
*** Dumping AST Record Layout
   0 | class Test_1
   0 |   int64_t first
   5 |   int64_t second
     | [sizeof=8, dsize=8, align=8
     |  nvsize=8, nvalign=8]

*** Dumping IRgen Record Layout
Record: CXXRecordDecl 0x344dfa8  line:3:7 referenced class Test_1 definition
|-CXXRecordDecl 0x344e0c0  col:7 implicit class Test_1
|-FieldDecl 0x344e1a0  col:10 first 'int64_t':'long'
| `-IntegerLiteral 0x344e170  'int' 40
|-FieldDecl 0x344e218  col:10 second 'int64_t':'long'
| `-IntegerLiteral 0x344e1e8  'int' 24
|-CXXConstructorDecl 0x3490d88  col:7 implicit used Test_1 'void (void) noexcept' inline
| `-CompoundStmt 0x34912b0 
|-CXXConstructorDecl 0x3490ee8  col:7 implicit constexpr Test_1 'void (const class Test_1 &)' inline noexcept-unevaluated 0x3490ee8
| `-ParmVarDecl 0x3491030  col:7 'const class Test_1 &'
`-CXXConstructorDecl 0x34910c8  col:7 implicit constexpr Test_1 'void (class Test_1 &&)' inline noexcept-unevaluated 0x34910c8
  `-ParmVarDecl 0x3491210  col:7 'class Test_1 &&'

Layout: 
    
]>

*** Dumping AST Record Layout
   0 | class Test_2
   0 |   int64_t first
   5 |   int32_t second
     | [sizeof=8, dsize=8, align=8
     |  nvsize=8, nvalign=8]

*** Dumping IRgen Record Layout
Record: CXXRecordDecl 0x344e260  line:8:7 referenced class Test_2 definition
|-CXXRecordDecl 0x344e370  col:7 implicit class Test_2
|-FieldDecl 0x3490bd0  col:10 first 'int64_t':'long'
| `-IntegerLiteral 0x344e400  'int' 40
|-FieldDecl 0x3490c70  col:10 second 'int32_t':'int'
| `-IntegerLiteral 0x3490c40  'int' 24
|-CXXConstructorDecl 0x3491438  col:7 implicit used Test_2 'void (void) noexcept' inline
| `-CompoundStmt 0x34918f8 
|-CXXConstructorDecl 0x3491568  col:7 implicit constexpr Test_2 'void (const class Test_2 &)' inline noexcept-unevaluated 0x3491568
| `-ParmVarDecl 0x34916b0  col:7 'const class Test_2 &'
`-CXXConstructorDecl 0x3491748  col:7 implicit constexpr Test_2 'void (class Test_2 &&)' inline noexcept-unevaluated 0x3491748
  `-ParmVarDecl 0x3491890  col:7 'class Test_2 &&'

Layout: 
    
]>

Note that the version of Clang I used to generate this output (the one used by Rextester) appears to default to optimising both bitfields into a single variable, and I'm unsure how to disable this behaviour.