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IA64 Complier Optimizations

This presentation explores IA-64, an implementation of the EPIC (Explicitly Parallel Instruction Computing) style ISA. It covers key optimizations such as predication, a unique register model, and speculative control flow that simplify hardware and enhance compiler efficiency. Through concepts like conditional execution based on predicate registers and advanced load address tables for data speculation, the architecture aims to boost instruction-level parallelism (ILP). It raises critical inquiries about its impact on existing compiler optimizations and performance compared to traditional superscalar processors.

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IA64 Complier Optimizations

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  1. IA64 Complier Optimizations Alex Bobrek Jonathan Bradbury

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

  3. 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

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

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

  6. 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

  7. 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 Traditional IA-64

  8. 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

  9. 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) Traditional IA-64

  10. 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

  11. 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?

  12. Additional Papers Used • J. Huck, et. al., Introducing the IA-64 Architecture. IEEE Micro, Sept./Oct. 2000 • R. Krishnaiyer, et. al., An Advanced Optimizer for the IA-64 Architecture. IEEE Micro, Nov./Dec. 2000. • M. Schlansker, B. Ramakrishna Rau. EPIC: Explicitly Parallel Instruction Computing. IEEE Computer, Feb. 2000.

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