[Bug Report] Wrong sign bit when 0 is multiplied by a negative number

[zhangkanqi]

Environment:

RSD version: bd7c5c1

Bug Description:

There is an instruction fmul.s ft9, fs4, ft8, where fs4=0x00000000 and ft8=0x998a3664.

After the fmul.s instruction, the result of ft9 in RSD is 0x00000000. However, the result in spike is 0x80000000.

It seems that RSD mistakes the sign bit of the final result?

[lpha-z]

Thank you for finding the bug.

@ RSD team: Fix the following 32'h00000000 to 32'h80000000. The identity element of floating-point addition is -0.0, not +0.0.

            if(fpuCode[i] == FC_MUL) begin
                fmaAddend[i] = 32'h00000000;
            end

https://github.com/rsd-devel/rsd/blob/master/Processor/Src/Pipeline/FPBackEnd/FPExecutionStage.sv#L180

[zhangkanqi]

Thanks for your confirmation!

[lpha-z]

We reopen this report because this bug has not been fixed yet.
When we finish fixing, we will close this report.

[zhangkanqi]

Hi! I was wondering if this bug has been resolved eventually?

Besides, I described this bug to Claude Code, and it identified the root cause of the vulnerability as:

rsd/Processor/Src/FloatingPointUnit/FP32PipelinedFMA.sv

Line 197 in 7b65f6b

wire[31:0] zero = { is_subtract ? 1'b0 : addend_sign, 8'h00, 23'h0 };

Claude Code suggested that this line of code should be modified to:

wire[31:0] zero         = { result_sign, 8'h00, 23'h0 };

So, I would also like to know if Claude Code's reasoning about the root cause of this bug is correct.

[lpha-z]

This bug will be fixed when PR #98 is merged, but I do not know why it has not been merged yet, and when it will be.

I think that the modification of Claude Code is wrong.
fma( +0.0f, -0.0f, +0.0f ) should be +0.0f, but it will return -0.0f.

[zhangkanqi]

Yes, Claude's fix suggestion is wrong.

And I would like to know what is the root cause of this bug?

[lpha-z]

The root cause of this bug would be assuming that ab and fma(a,b,0.0) are equal whereas ab and fma(a,b,-0.0) are actually equal when nearest rounding mode is used.