linux-kernel - Re: [PATCH v8 2/7] perf/x86/intel: Record branch type

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Date:   Thu, 13 Jul 2017 22:53:04 +0800
From:   "Jin, Yao" <yao.jin@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     acme@...nel.org, jolsa@...nel.org, mingo@...hat.com,
        alexander.shishkin@...ux.intel.com, mpe@...erman.id.au,
        Linux-kernel@...r.kernel.org, ak@...ux.intel.com,
        kan.liang@...el.com, yao.jin@...el.com
Subject: Re: [PATCH v8 2/7] perf/x86/intel: Record branch type



On 7/13/2017 10:31 PM, Peter Zijlstra wrote:
> On Thu, Jul 13, 2017 at 08:04:14PM +0800, Jin Yao wrote:
>> +#define X86_BR_TYPE_MAP_MAX	16
>> +
>> +static int
>> +common_branch_type(int type)
>> +{
>> +	int i, mask;
>> +	const int branch_map[X86_BR_TYPE_MAP_MAX] = {
>> +		PERF_BR_CALL,		/* X86_BR_CALL */
>> +		PERF_BR_RET,		/* X86_BR_RET */
>> +		PERF_BR_SYSCALL,	/* X86_BR_SYSCALL */
>> +		PERF_BR_SYSRET,		/* X86_BR_SYSRET */
>> +		PERF_BR_UNKNOWN,	/* X86_BR_INT */
>> +		PERF_BR_UNKNOWN,	/* X86_BR_IRET */
>> +		PERF_BR_COND,		/* X86_BR_JCC */
>> +		PERF_BR_UNCOND,		/* X86_BR_JMP */
>> +		PERF_BR_UNKNOWN,	/* X86_BR_IRQ */
>> +		PERF_BR_IND_CALL,	/* X86_BR_IND_CALL */
>> +		PERF_BR_UNKNOWN,	/* X86_BR_ABORT */
>> +		PERF_BR_UNKNOWN,	/* X86_BR_IN_TX */
>> +		PERF_BR_UNKNOWN,	/* X86_BR_NO_TX */
>> +		PERF_BR_CALL,		/* X86_BR_ZERO_CALL */
>> +		PERF_BR_UNKNOWN,	/* X86_BR_CALL_STACK */
>> +		PERF_BR_IND,		/* X86_BR_IND_JMP */
>> +	};
>> +
>> +	type >>= 2; /* skip X86_BR_USER and X86_BR_KERNEL */
>
>> +	mask = ~(~0 << 1);
> OCC worthy means of writing: 1
Yes, mask = 1. I just want to represent the bit 0 of mask is 1.
>> +
>> +	for (i = 0; i < X86_BR_TYPE_MAP_MAX; i++) {
>> +		if (type & mask)
>> +			return branch_map[i];
>> +
>> +		type >>= 1;
>> +	}
> That is some of the more confused code I've seen in a while :/
>
> 	if (type)
> 		return branch_map[__ffs(type)];
>
> is what you meant to write, no?

Not write, I just want to return the element of branch_map[].

The input type is one of X86_BR and the X86_BR is:

X86_BR_CALL        = 1 << 2, /* call */
X86_BR_RET        = 1 << 3, /* return */
X86_BR_SYSCALL        = 1 << 4, /* syscall */
X86_BR_SYSRET        = 1 << 5, /* syscall return */
X86_BR_INT        = 1 << 6, /* sw interrupt */
X86_BR_IRET        = 1 << 7, /* return from interrupt */
X86_BR_JCC        = 1 << 8, /* conditional */
X86_BR_JMP        = 1 << 9, /* jump */
X86_BR_IRQ        = 1 << 10,/* hw interrupt or trap or fault */
X86_BR_IND_CALL        = 1 << 11,/* indirect calls */
X86_BR_ABORT        = 1 << 12,/* transaction abort */
X86_BR_IN_TX        = 1 << 13,/* in transaction */
X86_BR_NO_TX        = 1 << 14,/* not in transaction */
X86_BR_ZERO_CALL    = 1 << 15,/* zero length call */
X86_BR_CALL_STACK    = 1 << 16,/* call stack */
X86_BR_IND_JMP        = 1 << 17,/* indirect jump */

Lookup the table to get the common branch type.

Sorry, what is the __ffs()?

Thanks
Jin Yao

>> +
>> +	return PERF_BR_UNKNOWN;
>> +}