Intel 64bit Architecture

Assembly Language AI-64 Motaz K. Saad Spring 2007 Motaz K. Saad, Dept. of CS

Background to IA-64 Pentium 4 appears to be last in x86 line Intel & Hewlett-Packard (HP) jointly developed New architecture 64 bit architecture Not extension of x86 Not adaptation of HP 64bit RISC architecture Exploits vast circuitry and high speeds Systematic use of parallelism Departure from superscalar Motaz K. Saad, Dept. of CS

Motivation Instruction level parallelism Implicit in machine instruction Not determined at run time by processor Long or very long instruction words (LIW/VLIW) Branch predication (not the same as branch prediction) Speculative loading Intel & HP call this Explicit Parallel Instruction Computing (EPIC) IA-64 is an instruction set architecture intended for implementation on EPIC Itanium is first Intel product Motaz K. Saad, Dept. of CS

Superscalar v IA-64 Motaz K. Saad, Dept. of CS

Why New Architecture? Not hardware compatible with x86 Now have tens of millions of transistors available on chip Could build bigger cache Diminishing returns Add more execution units Increase superscaling “ Complexity wall” More units makes processor “wider” More logic needed to orchestrate Improved branch prediction required Longer pipelines required Greater penalty for misprediction Larger number of renaming registers required At most six instructions per cycle Motaz K. Saad, Dept. of CS

Explicit Parallelism Instruction parallelism scheduled at compile time Included with machine instruction Processor uses this info to perform parallel execution Requires less complex circuitry Compiler has much more time to determine possible parallel operations Compiler sees whole program Motaz K. Saad, Dept. of CS

General Organization Motaz K. Saad, Dept. of CS

Key Features Large number of registers IA-64 instruction format assumes 256 128 * 64 bit integer, logical & general purpose 128 * 82 bit floating point and graphic 64 * 1 bit predicated execution registers (see later) To support high degree of parallelism Multiple execution units Expected to be 8 or more Depends on number of transistors available Execution of parallel instructions depends on hardware available 8 parallel instructions may be spilt into two lots of four if only four execution units are available Motaz K. Saad, Dept. of CS

IA-64 Execution Units I-Unit Integer arithmetic Shift and add Logical Compare Integer multimedia ops M-Unit Load and store Between register and memory Some integer ALU B-Unit Branch instructions F-Unit Floating point instructions Motaz K. Saad, Dept. of CS

Instruction Format Diagram Motaz K. Saad, Dept. of CS

Instruction Format 128 bit bundle Holds three instructions (syllables) plus template Can fetch one or more bundles at a time Template contains info on which instructions can be executed in parallel Not confined to single bundle e.g. a stream of 8 instructions may be executed in parallel Compiler will have re-ordered instructions to form contiguous bundles Can mix dependent and independent instructions in same bundle Instruction is 41 bit long More registers than usual RISC Predicated execution registers (see later) Motaz K. Saad, Dept. of CS

Assembly Language Format [qp] mnemonic [.comp] dest = srcs // qp - predicate register 1 at execution then execute and commit result to hardware 0 result is discarded mnemonic - name of instruction comp – one or more instruction completers used to qualify mnemonic dest – one or more destination operands srcs – one or more source operands // - comment Instruction groups and stops indicated by ;; Sequence without read after write or write after write Do not need hardware register dependency checks Motaz K. Saad, Dept. of CS

Assembly Examples ld8 r1 = [r5] ;; //first group add r3 = r1, r4 //second group Second instruction depends on value in r1 Changed by first instruction Can not be in same group for parallel execution Motaz K. Saad, Dept. of CS

Predication Motaz K. Saad, Dept. of CS

Speculative Loading Motaz K. Saad, Dept. of CS

Control & Data Speculation Control AKA Speculative loading Load data from memory before needed Data Load moved before store that might alter memory location Subsequent check in value Motaz K. Saad, Dept. of CS

Software Pipelining L1: ld4 r4=[r5],4 ;; //cycle 0 load postinc 4 add r7=r4,r9 ;; //cycle 2 st4 [r6]=r7,4 //cycle 3 store postinc 4 br.cloop L1 ;; //cycle 3 Adds constant to one vector and stores result in another No opportunity for instruction level parallelism Instruction in iteration x all executed before iteration x+1 begins If no address conflicts between loads and stores can move independent instructions from loop x+1 to loop x Motaz K. Saad, Dept. of CS

Unrolled Loop ld4 r32=[r5],4;; //cycle 0 ld4 r33=[r5],4;; //cycle 1 ld4 r34=[r5],4 //cycle 2 add r36=r32,r9;; //cycle 2 ld4 r35=[r5],4 //cycle 3 add r37=r33,r9 //cycle 3 st4 [r6]=r36,4;; //cycle 3 ld4 r36=[r5],4 //cycle 3 add r38=r34,r9 //cycle 4 st4 [r6]=r37,4;; //cycle 4 add r39=r35,r9 //cycle 5 st4 [r6]=r38,4;; //cycle 5 add r40=r36,r9 //cycle 6 st4 [r6]=r39,4;; //cycle 6 st4 [r6]=r40,4;; //cycle 7 Motaz K. Saad, Dept. of CS

Unrolled Loop Detail Completes 5 iterations in 7 cycles Compared with 20 cycles in original code Assumes two memory ports Load and store can be done in parallel Motaz K. Saad, Dept. of CS

Software Pipeline Example Diagram Motaz K. Saad, Dept. of CS

Support For Software Pipelining Automatic register renaming Fixed size are of predicate and fp register file (p16-P32, fr32-fr127) and programmable size area of gp register file (max r32-r127) capable of rotation Loop using r32 on first iteration automatically uses r33 on second Predication Each instruction in loop predicated on rotating predicate register Determines whether pipeline is in prolog, kernel or epilog Special loop termination instructions Branch instructions that cause registers to rotate and loop counter to decrement Motaz K. Saad, Dept. of CS

IA-64 Register Set Motaz K. Saad, Dept. of CS

IA-64 Registers (1) General Registers 128 gp 64 bit registers r0-r31 static references interpreted literally r32-r127 can be used as rotating registers for software pipeline or register stack References are virtual Hardware may rename dynamically Floating Point Registers 128 fp 82 bit registers Will hold IEEE 745 double extended format fr0-fr31 static, fr32-fr127 can be rotated for pipeline Predicate registers 64 1 bit registers used as predicates pr0 always 1 to allow unpredicated instructions pr1-pr15 static, pr16-pr63 can be rotated Motaz K. Saad, Dept. of CS

IA-64 Registers (2) Branch registers 8 64 bit registers Instruction pointer Bundle address of currently executing instruction Current frame marker State info relating to current general register stack frame Rotation info for fr and pr User mask Set of single bit values Allignment traps, performance monitors, fp register usage monitoring Performance monitoring data registers Support performance monitoring hardware Application registers Special purpose registers Motaz K. Saad, Dept. of CS

Register Stack Avoids unnecessary movement of data at procedure call & return Provides procedure with new frame up to 96 registers on entry r32-r127 Compiler specifies required number Local output Registers renamed so local registers from previous frame hidden Output registers from calling procedure now have numbers starting r32 Physical registers r32-r127 allocated in circular buffer to virtual registers Hardware moves register contents between registers and memory if more registers needed Motaz K. Saad, Dept. of CS

Register Stack Behaviour Motaz K. Saad, Dept. of CS

Register Formats Motaz K. Saad, Dept. of CS

Itanium Organization Superscalar features Six wide, ten stage deep hardware pipeline Dynamic prefetch branch prediction register scoreboard to optimise for compile time nondeterminism EPIC features Hardware support for predicated execution Control and data speculation Software pipelining Motaz K. Saad, Dept. of CS

Itanium Processor Diagram Motaz K. Saad, Dept. of CS

Intel 64bit Architecture

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Intel 64bit Architecture