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cmd/compile: deadcode for LoweredMuluhilo on riscv64
This is a follow up of CL 425101 on RISCV64. According to RISCV Volume 1, Unprivileged Spec v. 20191213 Chapter 7.1: If both the high and low bits of the same product are required, then the recommended code sequence is: MULH[[S]U] rdh, rs1, rs2; MUL rdl, rs1, rs2 (source register specifiers must be in same order and rdh cannot be the same as rs1 or rs2). Microarchitectures can then fuse these into a single multiply operation instead of performing two separate multiplies. So we should not split Muluhilo to separate instructions. Updates #54607 Change-Id: If47461f3aaaf00e27cd583a9990e144fb8bcdb17 Reviewed-on: https://go-review.googlesource.com/c/go/+/425203 Auto-Submit: Keith Randall <khr@golang.org> TryBot-Result: Gopher Robot <gobot@golang.org> Run-TryBot: Wayne Zuo <wdvxdr@golangcn.org> Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com>
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