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openhwgroup / cva6

The CORE-V CVA6 is an Application class 6-stage RISC-V CPU capable of booting Linux
https://docs.openhwgroup.org/projects/cva6-user-manual/
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[BUG] Unexpected Rounding Behavior During fsqrt.d Execution #2057

Open youzi27 opened 5 months ago

youzi27 commented 5 months ago

Is there an existing CVA6 bug for this?

Bug Description

In some rounding mode, precision errors occur when calculating fsqrt.d. And the results differ from those of Spike. Testing has shown that RDN, RUP, RMM and DYN all experience rounding errors.

How to reproduce and the logs are as follows: Example: Initialize fa7with 0x402e000000000000 and execute fsqrt.d ft10, fa7. Under RDN, DYN Mode:

Spike results are: ft10 = 0x400efbdeb14f4eda CVA6 results are: ft10 = 0x400efbdeb14f4ed9

Under RUP , RMM Mode: Spike results are: ft10 = 0x400efbdeb14f4ed9 CVA6 results are: ft10 = 0x400efbdeb14f4eda

youzi27 commented 5 months ago

The vulnerability of the rounding mode mentioned above also occurs in the fdiv instruction.

Example: When executing the instruction fdiv.d fs9, fs10, fa4 with the following register values:

fs10 = 0x41ddc16575c00000
fa4  = 0x41e0000c09000000

The results differ between CVA6 and Spike simulator as follows: Round Toward Zero (RTZ) Mode: CVA6 Result: 0x3fedc14f1407f47f Spike Result: 0x3fedc14f1407f47e (correct value as verified) Round Up (RUP), Round to Nearest, Tie to Max Magnitude (RMM), and Dynamic (DYN) Rounding Modes: CVA6 Result: 0x3fedc14f1407f47e Spike Result: 0x3fedc14f1407f47f

JeanRochCoulon commented 5 months ago

This Github issue can also be reported on cvfpu side, thanks

Nolan-Ashford commented 3 months ago

Hi. This issue is the same as https://github.com/openhwgroup/cva6/issues/2060, and it is a known issue. For more details, please refer to: https://github.com/openhwgroup/cvfpu#operations. It seems that you have submitted duplicate issues more than once. Please carefully review the previously mentioned related issues before submitting a new one.

youzi27 commented 3 months ago

Hi @Nolan-Ashford .

Thank you for your attention to this issue.

However, in your response and the links you provided, I did not see any content related to the DYN and RMM rounding modes. Please carefully read the specific rounding modes mentioned in the issue I submitted.

Therefore, this is different from the previous issue and is a new bug.

Thank you for the discussion, but I hope you will conduct a more thorough review.