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On RISC-V the JAL and JALR instructions provide Return Address Stack(RAS) prediction hints based on the registers used (as per section 2.5.1 of the RISC-V ISA manual). When a JALR instruction uses X1 or X5 as the source register, it hints that a pop should occur. When making a function call, avoid the use of X5 as a source register since this results in the RAS performing a pop-then-push instead of a push, breaking call/return pairing and significantly degrading front-end branch prediction performance. Based on test result of golang.org/x/benchmarks/json on SpacemiT K1, fix version has a performance improvement of about 7% Fixes #76654 Change-Id: I867c8d7cfb54f5decbe176f3ab3bb3d78af1cf64 Reviewed-on: https://go-review.googlesource.com/c/go/+/726760 Reviewed-by: Dmitri Shuralyov <dmitshur@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com> LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Joel Sing <joel@sing.id.au> Run-TryBot: Joel Sing <joel@sing.id.au>
The Go Programming Language
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