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STIMSMITH

Out-of-Order Execution

Concept

Out-of-order execution is a processor execution strategy in which instructions are dispatched and completed in an order different from program order. The available evidence describes it as a defining feature of high-performance pipelined RISC and RISC-V cores (e.g., the BOOM core used in the SNAP hardware-fuzzing platform), links it to enlarged processor state spaces that complicate test generation and verification, and notes that the RVVI-TRACE interface explicitly supports out-of-order cores. Two cited implementation directions include a low-power FPGA-based OoO floating-point engine for the Lagarto II core and the energy-efficient CG-OoO coarse-grain out-of-order architecture, which uses block-level scheduling and Skipahead issue to reduce energy use.

First seen 5/26/2026
Last seen 8/19/2026
Evidence 33 chunks
Wiki v4

WIKI

Overview

Out-of-order execution is described in the provided evidence as a processor execution strategy used in pipelined, high-performance cores. One cited case study states that the Verified Architecture MicroProcessor (VAMP) is "a pipelined reduced instruction set (RISC) processor based on the out-of-order execution principle." [C1] A second source groups it among "complex performance enhancements" of CPU cores alongside pipelines, multiple instruction issue, and branch prediction. [C2] A third source lists "in-order or out-of-order execution strategies" among the features that make microprocessor verification challenging. [C3]

Use in pipelined RISC processors

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NEIGHBORHOOD

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RELATIONSHIPS

23 connections
BOOM ← implements 100% 3e
BOOM is an out-of-order superscalar RISC-V processor.
An integrated concurrency and core-ISA architectural envelope definition, and test oracle, for IBM POWER multiprocessors ← uses 100% 2e
The paper models out-of-order execution as a key feature of weakly consistent multiprocessors.
microarchitectural state space exploration ← mentions 85% 2e
Microarchitectural state space exploration is needed to find bugs related to out-of-order execution.
ISA-Formal ← evaluates 100% 2e
ISA-Formal is applicable to out-of-order processors and is used to verify them.
Microprocessor Verification ← mentions 95% 1e
Out-of-order execution is listed as one of the challenges in microprocessor verification.
Transient Execution Attack ← uses 100% 1e
Transient execution attacks exploit CPU out-of-order execution.
openC910 RTL mentions → 85% 1e
The openC910 features out-of-order execution which increases performance but complicates simulation.
XiangShan ← implements 100% 1e
XiangShan is an out-of-order superscalar CPU.
data-flow non-determinism ← mentions 85% 1e
Data-flow non-determinism is related to out-of-order execution and load reordering.
Toooba ← implements 100% 1e
Toooba is an out-of-order superscalar CPU.
NaxRiscv ← implements 100% 1e
NaxRiscv is an out-of-order superscalar CPU.
SNAP ← uses 90% 1e
SNAP is designed and implemented based on the out-of-order BOOM core.
RISC-V BOOM ← implements 100% 1e
RISC-V BOOM is an out-of-order processor core with modern performance optimizations.
weakly consistent multiprocessor part of → 95% 1e
Out-of-order execution is a key feature of weakly consistent multiprocessors.
RVVI-Trace ← uses 95% 1e
RVVI-TRACE includes capabilities for Out-of-Order execution cores.
reorder buffer ← part of 95% 1e
The reorder buffer is a core component implementing out-of-order execution with in-order commit.
Register Renaming ← part of 90% 1e
Register renaming is used in out-of-order processors to handle data hazards.
Micro-op (micro-operation) ← part of 85% 1e
Micro-ops are fundamental to out-of-order execution where load instructions may be split into multiple micro-ops.
VAMP (Verified Architecture Microprocessor) ← implements 100% 1e
VAMP is based on the out-of-order execution principle.
BOOM Core ← implements 1e
BOOM Core implements out-of-order execution as a complex superscalar processor.
Spectre mentions → 90% 1e
Out-of-order execution exposes potential security vulnerabilities such as Spectre.
Meltdown mentions → 90% 1e
Out-of-order execution exposes potential security vulnerabilities such as Meltdown.
RISC-V ← mentions 88% 1e
RISC-V implementations using out-of-order execution increase complexity and security risks.

LINKED ENTITIES

1 links
RVVI-Trace USES Extracted graph relationship

CITATIONS

14 sources
14 citations — click to expand
[1] VAMP is a pipelined RISC processor based on the out-of-order execution principle. Verisoft VAMP case study (previous article evidence)
[2] Out-of-order execution is listed among complex CPU core performance enhancements that can break functional correctness. TVS asureISG / EETimes constrained-random verification article (previous article evidence)
[3] In-order or out-of-order execution strategies are among features that make microprocessor verification challenging. EETimes constrained-random verification article (previous article evidence)
[4] VAMPasm contains 56 instructions grouped into several categories. Verisoft VAMP case study (previous article evidence)
[5] Enabling out-of-order execution increases reachable states, transitions, and test-program counts by roughly an order of magnitude over in-order configurations. "Generating Test Programs to Cover Pipeline Interactions" Table 1 (previous article evidence)
[6] Test programs are generated by traversing a global FSM composed of operation- and hardware-level extended FSMs; ISA in MADL, micro-architecture in XML. "Generating Test Programs to Cover Pipeline Interactions" (previous article evidence)
[7] VAMP testing verifies conformance of the gate level against the assembly-level formal model. Verisoft VAMP case study (previous article evidence)
[8] Out-of-order and speculative execution are common high-performance RISC-V techniques that increase complexity and can expose security vulnerabilities (Spectre, Meltdown). RISC-V microarchitecture verification source (previous article evidence)
[9] Verification challenges in microarchitecture and pipeline motivate formal verification beyond simulation-based methods. RISC-V microarchitecture verification source (previous article evidence)
[10] RVVI-TRACE (Version 1.7) explicitly supports simple single-hart in-order cores to multi-hart, multi-issue, Out-of-Order cores with asynchronous interrupts and debug modes; it is defined in SystemVerilog and propagates net changes. riscv-verification/RVVI (RVVI GitHub README)
[11] SNAP is built on the out-of-order BOOM core, an open-source RISC-V processor with modern performance optimizations, and adds Trace Decision Logic that tags traced instructions with uses_buq and uses_lbq. Hardware Support to Improve Fuzzing Performance and Precision (SNAP)
[12] SNAP's Bitmap Update Queue exploits out-of-order semantics: loads may proceed speculatively ahead of stores; stores are executed in program order; speculative loads stall only when an older store to the same bitmap location is pending. Hardware Support to Improve Fuzzing Performance and Precision (SNAP)
[13] An FPGA-based out-of-order execution engine of floating-point arithmetic operations for the Lagarto II core includes a low-power issue queue, register bank, bypass network, and IEEE-754-conformant add/sub, mul, div/reciprocal, and FMAC units supporting double-precision and denormals; a second FMAC-pair-based design is reported to improve area, performance, and energy efficiency. Design and implementation of an out-of-order execution engine of floating-point arithmetic operations (arXiv 2111.01948)
[14] CG-OoO achieves close to In-Order energy while maintaining OoO performance via block-level code processing, distributed tables, compiler optimizations, and Skipahead issue; it closes 64% of the average energy gap between In-Order and Out-of-Order baselines at OoO performance and is 1.9× more efficient than OoO on the energy-delay product inverse metric. CG-OoO: Energy-Efficient Coarse-Grain Out-of-Order Execution (arXiv 1606.01607)