cache Batch Compiler Option

cache

Option Type	Default Value	#pragma options	C	C++
-qoption			x	x

Syntax

-qcache=
{
        assoc=number |
        auto |
        cost=cycles | 
        level=level | 
        line=bytes | 
        size=Kbytes | 
        type=cache_type 
}[: ...|

Purpose
The -qcache option 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.

NotesAllowable values for suboption are:

`assoc=number`	Specifies the set associativity of the cache. 0 Direct-mapped cache 1 Fully associate 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=cycles`	Specifies the performance penalty resulting from a cache miss.
`level=level`	Specifies the level of cache affected. 1 Basic cache 2 Level-2 cache or, if there is no level-2 cache, the table lookaside buffer (TLB) 3 TLB If a machine has more than one level of cache, use a separate -qcache option.
`line=bytes`	Specifies the line size of the cache.
`size=Kbytes`	Specifies the total size of the cache.
`type=C\|c\|D\|d\|I\|i`	The settings apply to the specified type of cache. C or c Combined data and instruction cache D or d Data cache I or i Instruction cache

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.

You must specify -O4 or -qipa with the -qcache option.

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.

Example 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

List of Batch Compiler Options and Their Defaults
Options that Specify Compiler Output
Equivalent Batch Compile-Link and Incremental Build Options