Category
Pragma equivalent
None.
Purpose
Specifies the cache configuration for a specific execution machine.
If you know the type of execution system for a program, and that system has its instruction or data cache configured differently from the default case, use this option to specify the exact cache characteristics. The compiler uses this information to calculate the benefits of cache-related optimizations.
Syntax
.-:-------------------. .-:------------------------. V | V | >>- -q--cache--=----+-level--=--+-1-+-+-+----+----------------------+-+->< | +-2-+ | +-assoc--=----number---+ | '-3-' | +-auto-----------------+ '-type--=--+-c-+--' +-cost--=--cycles------+ +-d-+ +-line--=--bytes-------+ '-i-' '-size--=--Kbytes------'
Defaults
Automatically determined by the setting of the -qtune option.
Parameters
- assoc
- Specifies the set associativity of the cache.
- number
- Is one of:
- 0
- Direct-mapped cache
- 1
- Fully associative cache
- N>1
- n-way set associative cache
- auto
- Automatically detects the specific cache configuration of the compiling machine. This assumes that the execution environment will be the same as the compilation environment.
- cost
- Specifies the performance penalty resulting from a cache miss.
- cycles
- level
- Specifies the level of cache affected. If a machine has more than one level of cache, use a separate -qcache option.
- level
- Is one of:
- 1
- Basic cache
- 2
- Level-2 cache or, if there is no level-2 cache, the table lookaside buffer (TLB)
- 3
- TLB
- line
- Specifies the line size of the cache.
- bytes
- An integer representing the number of bytes of the cache line.
- size
- Specifies the total size of the cache.
- Kbytes
- An integer representing the number of kilobytes of the total cache.
- type
- Specifies that the settings apply to the specified cache_type.
- cache_type
- Is one of:
- c
- Combined data and instruction cache
- d
- Data cache
- i
- Instruction cache
Usage
The -qtune setting determines the optimal default -qcache settings for most typical compilations. You can use the -qcache to override these default settings. However, if you specify the wrong values for the cache configuration, or run the program on a machine with a different configuration, the program will work correctly but may be slightly slower.
Use the following guidelines when specifying -qcache suboptions:
- Specify information for as many configuration parameters as possible.
- If the target execution system has more than one level of cache, use a separate -qcache option to describe each cache level.
- If you are unsure of the exact size of the cache(s) on the target execution machine, specify an estimated cache size on the small side. It is better to leave some cache memory unused than it is to experience cache misses or page faults from specifying a cache size larger than actually present.
- The data cache has a greater effect on program performance than the instruction cache. If you have limited time available to experiment with different cache configurations, determine the optimal configuration specifications for the data cache first.
- If you specify the wrong values for the cache configuration, or run the program on a machine with a different configuration, program performance may degrade but program output will still be as expected.
- The -O4 and -O5 optimization options automatically select the cache characteristics of the compiling machine. If you specify the -qcache option together with the -O4 or -O5 options, the option specified last takes precedence.
- Unless -qcache=auto is specified, you must specify both the type and level suboptions when you use the -qcache option. Otherwise, a warning message is issued.
Predefined macros
None.
Examples
To tune performance for a system with a combined instruction and data level-1 cache, where cache is 2-way associative, 8 KB in size and has 64-byte cache lines, enter:
xlc -O4 -qcache=type=c:level=1:size=8:line=64:assoc=2 file.c