[llvm-dev] Getting consistent block frequency information

[Joshua Cranmer via llvm-dev]

Hi,

I've been trying to get frequency information for basic blocks and edges 
between basic blocks using MachineBlockFrequencyInfo and 
MachineBranchProbabilityInfo. Unfortunately, the results returned don't 
seem to give high consistency.

If I have code like the following:

define i32 @foo(i32 %i, i32* %addr) {
entry:
   %tobool = icmp ne i32 %i, 0
   br i1 %tobool, label %if.end, label %if.then

if.then:
   %add = add i32 %i, 1
   store i32 %add, i32* %addr
   br label %return

if.end:
   %mul = mul i32 %i, 2
   store i32 %mul, i32* %addr
   br label %return

return:
   %ret = load i32, i32* %addr
   ret i32 %ret
}

The branch probability information suggests that the edge from 
entry->if.then and the edge from entry->if.end are executed in a 5:3 
ratio. The resulting block frequency information, when queried in 
integer form, reports that entry and return has an integer count of 21, 
if.then as 13, and if.end as 8. I get different results if I do the 
obvious thing of getting the branch probability and multiplying it by 
the block frequency information to derive edge frequency; for example, 
in this case, multiplying 21 by 3/8 returns 7 instead of 8 for the edge 
from entry to if.end. Choosing a particular order to generate the block 
orders (to match up with the derivation of block mass estimates) doesn't 
appear to yield the correct results.

Is there any way to get more accurate results?

-- 
Joshua Cranmer
Thunderbird module owner
DXR coauthor

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