golang/go

cmd/objdump: new GOAMD64=v3 instructions don't disassemble properly

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#48,584 opened on Sep 23, 2021

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NeedsFixcompiler/runtimehelp wanted

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Description

Build this program with GOAMD64=v3:

package main

//go:noinline
func f(x, y uint64) uint64 {
	return x &^ y
}
func main() {
	f(0, 0)
}

Disassembling with the native objdump -d, we get

 1053b20:       c4 e2 e0 f2 c0  andnq   %rax, %rbx, %rax

With go tool objdump we get

  :0                    0x1053b20               c4e2e0f2c0c3cc          ROLL $0xcc, BL

@martisch

Side note, the popcount instruction disassembles as POPCNT, it should really disassemble as POPCNTQ or POPCNTL, as appropriate.

Contributor guide

Research direction
Inspect the disassembly code in cmd/objdump and trace how it handles the ANDN instruction with GOAMD64=v3 variants.
Tech stack
go
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
Go
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
75