Maybe Dmitry can shed some light on this. He actually suggested that 
optimization.

- Alex

On 29.09.21 10:33, Peter Zijlstra wrote:
> On Mon, Sep 27, 2021 at 07:33:40PM +0200, Alexander Lochmann wrote:
>> The existing trace mode stores PCs in execution order. This could lead
>> to a buffer overflow if sufficient amonut of kernel code is executed.
>> Thus, a user might not see all executed PCs. KCOV_MODE_UNIQUE favors
>> completeness over execution order. While ignoring the execution order,
>> it marks a PC as exectued by setting a bit representing that PC. Each
>> bit in the shared buffer represents every fourth byte of the text
>> segment.  Since a call instruction on every supported architecture is
>> at least four bytes, it is safe to just store every fourth byte of the
>> text segment.
> 
> I'm still trying to wake up, but why are call instruction more important
> than other instructions? Specifically, I'd think any branch instruction
> matters for coverage.
> 
> More specifically, x86 can do a tail call with just 2 bytes.
> 

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