On 07/29/2015 08:55 AM, Michael Ellerman wrote:
> On Tue, 2015-30-06 at 08:20:30 UTC, Anshuman Khandual wrote:
>> > Branch record attributes 'mispred' and 'predicted' are single bit
>> > fields as defined in the perf ABI. Hence the data type of the field
>> > 'pred' used during BHRB processing should be changed from integer
>> > to bool. This patch also changes the name of the variable from 'pred'
>> > to 'mispred' making the logical inversion process more meaningful
>> > and readable.
> This whole function is a mess.
>
> There's no good reason why we're doing the assignment to pred/mispred in two
> places to begin with, so if that was eliminated we wouldn't need a local for
> mispred to begin with.
Not sure whether I got this right. We are assigning mispred once with
the value (val & BHRB_PREDICTION) and then assigning mispred and it's
inversion to two different fields of the branch entry as required.
>
> Then there's the type juggling, all of which probably works but is fishy and
> horrible.
With this patch and one more (2nd patch of the BHRB SW filter series)
patch, we are trying to make it better.
>
> You take a u64, bitwise and it with a mask, assign that to a boolean, then
> take
So that any residual positive value after the "AND" operation will
become logical TRUE for the boolean. We dont use any shifting here
as BHRB_PREDICTION checks for the right most (least significant) bit
in the sequence.
> the boolean, *bitwise* negate that and assign the result to a single bit
> bitfield.
This is getting fixed with a subsequent patch (2nd patch of the BHRB
SW filter series) in a new function called insert_branch.
+static inline void insert_branch(struct cpu_hw_events *cpuhw,
+ int index, u64 from, u64 to, bool mispred)
+{
+ cpuhw->bhrb_entries[index].from = from;
+ cpuhw->bhrb_entries[index].to = to;
+ cpuhw->bhrb_entries[index].mispred = mispred;
+ cpuhw->bhrb_entries[index].predicted = !mispred;
+}
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