IA64 Complier Optimizations Alex Bobrek Jonathan Bradbury

Outline EPIC Style ISA Predication Register Model Speculative Control Flow Data Speculation

Explicitly Parallel Instruction Computing (EPIC) Style ISA IA-64 is just one implementation of EPIC Main idea behind EPIC: –Make Hardware Simpler –Make Compiler Smarter Similar to VLIW –Three Instructions per “bundle” –Includes template that describes dependencies

Predication Conditional execution of an instruction based on predicate register Reduces branching –Increases ILP Allows instructions to be moved across branches

Predication (Cont.) if(a<5) then b=c+d; else b=c-d; cmp.lt p1= ra,#5 (p1) add rc,rd,rb (p1) sub rc,rd,rb cmp ra,#5 bgeL1 add rc,rd,rb jmpL2 L1: subrc,rd,rb L2: Traditional: Predicated:

Register Model IA-64 has 128 integer registers –R0-R31 are always program visible –R32-R127 are stacked Each procedure can have it’s own variable sized stack frame Register Stack Engine (RSE) handles spills in hardware Compiler doesn’t have to worry about managing fills/spills –Done dynamically in hardware –Allows for shorter critical path length through code OS has to flush stack on context switch

Speculative Control Flow Allows for loads to be moved outside of a basic block even if address is not known to be safe Speculative Load (ld.s) instruction Speculation Check (chk.s) instruction TraditionalIA-64

Speculative Control Flow (cont.) Every register has a NaT bit set if there is an exception NaT bits are propagated through all instructions using the speculated value The chk.s instruction will branch to recovery code if speculation fails Deferral Models to allow for tradeoffs between OS and hardware exception handling

Data Speculation Allows for loads to be moved ahead of stores if compiler is unsure if addresses are the same Advanced load (ld.a) Alias check (chk.a) TraditionalIA-64

Data Speculation (cont.) Implemented using an Advanced Load Address Table (ALAT) –All speculative load addresses stored in table –All stores remove entries with same address –On chk, if address is in the table, speculation is successful The chk.a will branch to recovery code if speculation has not succeeded

Interesting Research Questions How do the new IA-64 instructions impact existing compiler optimizations (partial redundancy elimination, liveness analysis...) ? Does the EPIC approach outperform the runtime optimizations of traditional superscalar processors? What other hardware aspects can be exposed to the compiler?

Additional Papers Used J. Huck, et. al., Introducing the IA-64 Architecture. IEEE Micro, Sept./Oct R. Krishnaiyer, et. al., An Advanced Optimizer for the IA-64 Architecture. IEEE Micro, Nov./Dec M. Schlansker, B. Ramakrishna Rau. EPIC: Explicitly Parallel Instruction Computing. IEEE Computer, Feb