Proposal: Register-based Go calling convention

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2021-08-17 04:00:05

Author: Austin Clements, with input from Cherry Zhang, Michael Knyszek, Martin Möhrmann, Michael Pratt, David Chase, Keith Randall, Dan Scales, and Ian Lance Taylor.

We propose switching the Go ABI from its current stack-based calling convention to a register-based calling convention. Preliminary experiments indicate this will achieve at least a 5–10% throughput improvement across a range of applications. This will remain backwards compatible with existing assembly code that assumes Go’s current stack-based calling convention through Go’s multiple ABI mechanism.

Since its initial release, Go has used a stack-based calling convention based on the Plan 9 ABI, in which arguments and result values are passed via memory on the stack. This has significant simplicity benefits: the rules of the calling convention are simple and build on existing struct layout rules; all platforms can use essentially the same conventions, leading to shared, portable compiler and runtime code; and call frames have an obvious first-class representation, which simplifies the implementation of the go and defer statements and reflection calls. Furthermore, the current Go ABI has no callee-save registers, meaning that no register contents live across a function call (any live state in a function must be flushed to the stack before a call). This simplifies stack tracing for garbage collection and stack growth and stack unwinding during panic recovery.

Unfortunately, Go’s stack-based calling convention leaves a lot of performance on the table. While modern high-performance CPUs heavily optimize stack access, accessing arguments in registers is still roughly 40% faster than accessing arguments on the stack. Furthermore, a stack-based calling convention, especially one with no callee-save registers, induces additional memory traffic, which has secondary effects on overall performance.

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