Intel ARCHITECTURE IA-32 User Manual

General Optimization Guidelines 2
2-15
Branch Prediction
Branch optimizations have a significant impact on performance. By
understanding the flow of branches and improving the predictability of
branches, you can increase the speed of code significantly.
Optimizations that help branch prediction are:
• Keep code and data on separate pages (a very important item, see
more details in the “Memory Accesses” section).
• Whenever possible, eliminate branches.
• Arrange code to be consistent with the static branch prediction
algorithm.
• Use the pause instruction in spin-wait loops.
• Inline functions and pair up calls and returns.
• Unroll as necessary so that repeatedly-executed loops have sixteen
or fewer iterations, unless this causes an excessive code size
increase.
• Separate branches so that they occur no more frequently than every
three
μops where possible.
Eliminating Branches
Eliminating branches improves performance because it:
• reduces the possibility of mispredictions
• reduces the number of required branch target buffer (BTB) entries;
conditional branches, which are never taken, do not consume BTB
resources
There are four principal ways of eliminating branches:
• arrange code to make basic blocks contiguous
• unroll loops, as discussed in the “Loop Unrolling” section
• use the cmov instruction
• use the setcc instruction

Intel ARCHITECTURE IA-32 Specifications

General

Instruction Set	x86
Instruction Set Type	CISC
Memory Segmentation	Supported
Operating Modes	Real mode, Protected mode, Virtual 8086 mode
Max Physical Address Size	36 bits (with PAE)
Max Virtual Address Size	32 bits
Architecture	IA-32 (Intel Architecture 32-bit)
Addressable Memory	4 GB (with Physical Address Extension up to 64 GB)
Floating Point Registers	8 x 80-bit
MMX Registers	8 x 64-bit
SSE Registers	8 x 128-bit
Registers	General-purpose registers (EAX, EBX, ECX, EDX, ESI, EDI, ESP, EBP), Segment registers (CS, DS, SS, ES, FS, GS), Instruction pointer (EIP), Flags register (EFLAGS)
Floating Point Unit	Yes (x87)