Add unit tests for Branch module

[pavelkryukov]

Branch module is the simplest module to test. Its inputs are completed instructions, and its outputs are very few.

The idea is to wrap Branch module inside a single environment with ports

class BranchTesting {
    Branch branch;
    WritePort<Instr>* wp_exe_datapath;
    ReadPort<Instr>* rp_wb_datapath;
    ReadPort<BPInteface>* rp_bp_inteface;
};

Then, you may generate a test case in a following manner:

TEST_CASE(" Branch move")
{
    MIPS32Instr beq( "beq");
    wp_exe_datapath->wirte( beq, 0);
    branch.clock( 0);
    CHECK( rp_bp_bypass->is_ready( 1) ); // Data ready next cycle
}

[graudtv]

Hi. I've written some facilities for testing, what is the best way to share this code with you? It is quite big to paste it here, but not finished yet to make a pull request.

[pavelkryukov]

Thanks. I see it in a fork repo and can comment.

[pavelkryukov]

We finalizing rating on 07.12.2020.
In order to get points for this, please complete this before this Sunday.

[graudtv]

Ok. Could you please have a look? If it's ok, I will add some more tests

[pavelkryukov]

If everything looks, please submit pull request to see all changes at once.

[graudtv]

I am trying to write tests for predicted and not predicted instructions, and I a bit don't understand how Branch::is_misprediction() works.
Let we have an instruction, which is predicted as taken and is actually taken. This is not a misprediction. Condition on line 51 returns false (it is in BP). Condition on line 57 also returns false, because actual information is the same as in bp_data. The last expression returns true, because target != new_PC. So functions says it is misprediction.

mipt-mips/simulator/modules/branch/branch.h

Lines 49 to 61 in a39e74d

	bool is_misprediction( const Instr& instr, const BPInterface& bp_data) const
	{
	if ( !bp_data.is_hit)
	if ( ( instr.is_common_branch() && instr.is_taken())
	|| ( instr.is_likely_branch() && !instr.is_taken())
	|| instr.is_indirect_jump())
	return true;
	if ( bp_data.is_hit && ( bp_data.is_taken != instr.is_taken()))
	return true;
	return ( bp_data.is_taken || instr.is_indirect_jump()) && ( bp_data.target != instr.get_new_PC());
	}

[pavelkryukov]

which is predicted as taken and is actually taken. This is not a misprediction.
because target != new_PC. So functions says it is misprediction

Correct: you predicted direction, but not the target (i.e. taken, but somewhere else).
That may happen in case of aliasing in branch prediction table.

[pavelkryukov]

It is a great achievement for our project, as your infrastructure has high quality and may be easily extended for more modules.
Thank you, and keep that level in future work!