C application 32-bit->64-bit architecture Porting Guide

64-bit Applications:

64-bit programs use aproximately 30% to 40% more memory then their 32-bit counterparts

Advantages

1. Large file caches on systems with large physical memory
2. Large process data space mapped in a large virtual address space
3. Large file support using standard system library calls
4. Processor has more registers, so it doesn't need to access memory that often.
5. Calculations (64-bit numbers etc.) are more efficient.

Disadvantages

1. less code/data can fit into processor caches
   • more cache misses
     • Processor waits more for memory controller to fetch data from RAM.
     • Processor also needs to read more data because it's more bloated.
2. Less free memory available
   • less space for disk caches
   • slower disk access
3. Once we are out of RAM and the system starts swapping
   • disk access is thousands times slower than RAM, so any potential benefits of 64-bit code are counteracted.

Conclusion

Basically, it's like giving up 30% of RAM in exchange of possible gain of 10% CPU speed.

Data Models:

Datatype	ILP32	LP64
char	8	8
short	16	16
int	32	32
long	32	64
pointer	32	64
long long	64	64
float	32	32
double	64	64
long double	128	128
enum	32	32

Common Issues

• Data truncation
• Pointers
• Data type promotion
• Data alignment and data sharing
• Constants
• Bit shifts and bit masks
• Bit fields
• Enumerated types

Data Truncation

pointers, long, off_t, fpos_t etc are all 64bit in LP64 data model. In LP64, truncation can occur during, initialization, assignments, parameter passing, return statements and casting.

• Assigning longs to ints
• Storing long int in double
• Storing Pointers in ints or ints/hexadecimal literal to a pointer
• Truncating Function Return Values
• Not using appropriate Print Specifiers

Pointers:

• Pointer Arithmetic between longs and ints

Data Type Promotion:

• Expression leading to Sign Extension scenarios
• Arithmetic between signed and unsigned numbers

Data alignment and data sharing (Structures & Unions):

• The compilers align data types on a natural boundary. Structure or a Union will be different on 32-bit and 64-bit systems.
• look out for struct member offsets and thier alignment
• give extra care when union's uses pointers and primitive datatypes

Constants:

The datatype of hexadecimal constants may changes between ILP32 and LP64,

Constants	ILP32	LP64
0x7fffffff	int	int
0x7fffffffL	long	long
0x80000000	unsigned int	unsigned int
0x80000000L	unsigned long	long

• Using Literals and Masks that Assume 32 bits
• Using mixed datatypes
• Hardcoding Size of Data Types
• Hardcoding Constants with malloc(), memory(), string()
• Hardcoding Bit Shift Values

Bit Fields:

• Sign extension in bit fields

Enumerated Types:

• In LP64, enumerated types are signed only if one or more of the enumeration constants defined for that type is negative. If all enumeration constants are non-negative, the type is unsigned. In ILP32, enumerated types are always signed.

Endianess:

• little endian - ll (l - least significant bytes, l - lowest memory address)
• bit endian - ml (m - most significant bytes, l - lowest memory address) (network order)

Endianism is important when

• Bit masks are used
• Indirect pointers address portions of an object

Performance Considerations:

• long division is more time-consuming than 32-bit integer division
• 64-bit programs that use 32-bit signed integers as array indexes require additional instructions to perform sign extension each time

How to use the code

• Checkout the git repro using below command

    git clone https://github.com/sridharsridha/c-32bit-to-64bit-porting-issues.git ~/c-32bit-to-64bit-porting-issues

• To run compile the files one by one using

   gcc -m32 filename.c -o filename32
   gcc -m64 filename.c -o filename64

TODO

• create a makefile to build all the files
• seperate the problems into sperate files of thier own
• add more debugging (data representation, intermediate state of variables etc to problem files)
• Update the problem file with explanation and answers