Re: [PATCH v6 2/7] perf/x86/intel: Record branch type

On 5/9/2017 8:39 PM, Jiri Olsa wrote:

On Tue, May 09, 2017 at 07:57:11PM +0800, Jin, Yao wrote:

SNIP

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+
+	type >>= 2; /* skip X86_BR_USER and X86_BR_KERNEL */
+	mask = ~(~0 << 1);

is that a fancy way to get 1 into the mask? what do I miss?

you did not comment on this one

Sorry, I misunderstood that this comment and the next comment had the same
meaning.

In the previous version, I used the switch/case to convert from X86_BR to
PERF_BR. I got a comment from community that it'd better use a lookup table
for conversion.

Since each bit in type represents a X86_BR type so I use a mask (0x1) to
filter the bit. Yes, it looks I can also directly set 0x1 to mask.

I write the code "mask = ~(~0 << 1)" according to my coding habits. If you
think I should change the code to "mask = 0x1", that's OK  :)

im ok with that.. was just wondering for the reason
I guess compiler will make it single constant assignment anyway

I think so.  The compiler should be clever enough for this optimization.

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+
+	for (i = 0; i < X86_BR_TYPE_MAP_MAX; i++) {
+		if (type & mask)
+			return branch_map[i];

I wonder some bit search would be faster in here, but maybe not big deal

jirka

I just think the branch_map[] doesn't contain many entries (16 entries
here), so maybe checking 1 bit one time should be acceptable. I just want to
keep the code simple.

But if the number of entries is more (e.g. 64), maybe it'd better check 2 or
4 bits one time.

ook

jirka

Sorry, what's the meaning of ook? Does it mean "OK"?

just means ok ;-)

thanks,
jirka

Thanks so much!

Jin Yao