FP Denorm ACT

[jjscheel]

Technical Group

Architecture Test SIG

ratification-pkg

all FP extensions

Technical Liaison

Allen Baum

Task Category

Arch Tests

Task Sub Category

• gcc
• binutils
• gdb
• intrinsics
• Java
• KVM
• ld
• llvm
• Linux kernel
• QEMU
• Spike

Ratification Target

Legacy

Statement of Work (SOW)

Component names:
D,F,Q, Zfh, Zfinx, Zdinx, Zhinx

Requirements:
Add directed test cases for FP ops that generate a normalized result, but also set the underflow flag

Origin
For an obscure IEEE FP reason, it is possible to generate a normalized FP result while still setting the underflow
flag. This is because the underflow is set after rounding – but for some corner cases, two roundings can occur.
This happens when a denormalized number , after rounding, becomes the largest denorm.
When converted to a denorm format, a second rounding (if the result is odd) occurs, which can round up and
overflow for some rounding modes, and it becomes normalized.

Test Details of Required Tests

For each FP op (FADD, FSUB, FMUL, FDIV, FSQRT, FMUL[N][ADD/SUB},
                             FCVT.S.D, FCVT.D.Q, FCVT.H.x
                             (any FPCVT from a longer FP format to a shorter FP format)

Choose operands that result in raw values for each FP format (before any rounding) of
   EXP = -bias, (is 127 for single, 2047 for double, etc)
   SGN= 0 and1
   MANT = all 1s & all1-1 (including hidden bit) for each FP format
   All 4 combinations of guard and sticky bits

Deliverables:
Since the real requirement is generation of an internal rounded result of all 1s, which is not architecturally visible, we can’t write the coverage except to verify the output is either the smallest normalized number or the largest denorm.

• Assembly language tests that meet the TestFormatSpec,
• Coverage models using riscv-ctg YAML formatted schema Acceptance criteria

Acceptance Criteria:

• Tests pass using the riscof framework

Projected timeframe: 3 months

SOW Signoffs: (delete those not needed)

• Task group liaison sign-off
• Development partner sign-off
• ACT SIG sign-off (if ACT work)

Waiver

• Freeze
• Ratification

Pull Request Details

TBD

[jjscheel]

@ptprasanna, please review this SOW and ask any questions here. The text came from @allenjbaum. He will be your primary resource on this item.

We should discuss more in our next meeting.

[jjscheel]

Updated typos in the description section.

[jjscheel]

Comments from @allenjbaum via email:

I'm wondering if we should include a requirement to show that the intermediate result is in the required range (e.g. with a manual calculation). That's pretty easy for most of these on RV32 (with a hex calculator) except divide and sqrt perhaps,
but more difficult with longer word lengths than hex calculators normally support.

Actually, the tricky bit (sic) is to see that guard/round bits have all 4 combinations

[ptprasanna]

@jjscheel and @allenjbaum, Had a detailed discussion with @anuani21 on this, looks like we do have the tests available and running on the given combination. It's just that the underflow flag which we failed to set on fcsr to acknowledge the result is a denorm, is bit of a gap in the development, which is been caught now. @anuani21 is working on it to fix this up.

[jjscheel]

Thanks, @ptprasanna! Please keep working with @allenjbaum and the sig-arch-test group.

[allenjbaum]

Note that the result that is stored is NOT a denorm ! The result at some intermediate stage could be considered to be a denorm, and then rounded up to be normalized, but the denorm flag is still set.

…

On Thu, Nov 2, 2023 at 9:51 PM Prasanna T ***@***.***> wrote: @jjscheel <https://github.com/jjscheel> and @allenjbaum <https://github.com/allenjbaum>, Had a detailed discussion with @anuani21 <https://github.com/anuani21> on this, looks like we do have the tests available and running on the given combination. It's just that the underflow flag which we failed to set on fcsr to acknowledge the result is a denorm, is bit of a gap in the development, which is been caught now. @anuani21 <https://github.com/anuani21> is working on it to fix this up. — Reply to this email directly, view it on GitHub <#36 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AHPXVJQZXDCG6SJ3DDSLKDDYCRZ5VAVCNFSM6AAAAAA5J4FNMWVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMYTOOJRHA4TCMZQGQ> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[jjscheel]

@ptprasanna, any chance on an update here?