Compiler-Assisted Dynamic Register Sharing for GPU Occupancy
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Solution Overview
Problem
Modern processors face inefficiencies in register allocation due to static reservation schemes, leading to underutilization of physical registers and reduced occupancy for GPU programs, which limits concurrent execution and resource utilization.
Innovation Solution
Implementing a compiler-assisted register sharing mechanism that allows threads to dynamically acquire and release extended register sets from a shared pool based on register liveness analysis, enabling efficient allocation and deallocation of registers as needed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If static register reservation is used to ensure sufficient registers for each SIMD group, then register availability is improved, but physical register underutilization occurs because reserved registers are not always needed
Solution Approach 1:
The patent implements dynamic register allocation where the number of registers reserved for each SIMD group changes over time based on actual needs. The compiler inserts acquire/release instructions to dynamically adjust register reservations, transitioning from static to dynamic allocation. This allows the system to maintain sufficient register availability when needed while releasing excess registers back to the pool when not needed, eliminating underutilization.
Solution Approach 2:
The patent changes the parameter of register reservation from fixed (static) to variable (dynamic). By using compiler-directed acquire/release mechanisms, the register allocation parameters are adjusted at runtime based on program execution state, allowing the system to adapt register reservations to actual computational needs rather than maintaining constant conservative reservations.
2Reliability
If high number of registers are allocated to a SIMD group, then register availability for that group is improved, but occupancy decreases limiting concurrent execution
Solution Approach 1:
The patent enables dynamic adjustment of register allocations, allowing SIMD groups to acquire additional registers only when needed for specific computational phases. This dynamic approach prevents permanent high register allocations that would block other threads, thereby maintaining high occupancy while ensuring sufficient registers are available when required by individual SIMD groups.
Solution Approach 2:
The patent implements partial register allocation where SIMD groups receive base registers plus additional registers only when needed. Instead of allocating maximum registers to all groups simultaneously (excessive action), the system provides registers partially and selectively based on actual requirements, enabling more groups to coexist with adequate resources.
3Reliability
If static conservative register allocation is used, then worst-case register needs are met, but registers go unused during most of execution time
Solution Approach 1:
The patent uses compiler analysis to identify phases where additional registers will be needed and inserts acquire instructions in advance. This preliminary action ensures registers are reserved before they are actually needed, maintaining reliability without requiring continuous reservation. The release instructions are also inserted preliminarily to prepare for upcoming phases where registers can be freed.
Solution Approach 2:
The patent transforms register allocation from static conservative parameters to dynamic parameters that change based on program phase. The compiler analyzes the program to determine when register needs increase or decrease, adjusting allocation parameters accordingly. This prevents the waste of continuously reserving registers at maximum levels when actual needs vary over time.
Data Source
AI summary
Systems, apparatuses, and methods for efficiently sharing registers among threads are disclosed. A system includes at least a processor, control logic, and a register file with a plurality of registers. The processor assigns a base set of registers to each thread of a plurality of threads executing on the processor. When a given thread needs more than the base set of registers to execute a given phase of program code, the given thread executes an acquire instruction to acquire exclusive access to an extended set of registers from a shared resource pool. When the given thread no longer needs additional registers, the given thread executes a release instruction to release the extended set of registers back into the shared register pool for other threads to use. In one implementation, the compiler inserts acquire and release instructions into the program code based on a register liveness analysis performed during compilation.