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cmd/objdump: amd64 popcnt disassembled as rep popcnt #19986

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rasky opened this issue Apr 15, 2017 · 3 comments
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cmd/objdump: amd64 popcnt disassembled as rep popcnt #19986

rasky opened this issue Apr 15, 2017 · 3 comments
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@rasky
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rasky commented Apr 15, 2017 

	return uint(bits.OnesCount16(val))
  0x4286bda		0fb635f27c3800		MOVZX 0x387cf2(IP), SI
  0x4286be1		4084f6			TESTL SI, SI
  0x4286be4		0f84f8000000		JE 0x4286ce2
  0x4286bea		0fb7f1			MOVZX CX, SI
  0x4286bed		f30fb8f6		REP POPCNT SI, SI
@randall77
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I think this is intentional. Those opcodes really are encoded with a REP prefix.
A lot of the SSE instructions are in the same boat (e.g. SQRTSS).
We even have tests that make sure the "REP" is there.

@rsc, any reason why we do this?

@rsc
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rsc commented Apr 20, 2017

If the REP is implied by the opcode, then it shouldn't be printed. Tests requiring it are likely just buggy tests.

@randall77
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Dup of #17410. It's been fixed in the x repo, it just needs to be pulled into master. I'm going to leave this issue open until that happens.

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@rsc @rasky @randall77 @gopherbot