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Is th.flurw from xtheadfmemidx really supported on RV32? #49

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topperc opened this issue Apr 24, 2024 · 1 comment · May be fixed by #50
Open

Is th.flurw from xtheadfmemidx really supported on RV32? #49

topperc opened this issue Apr 24, 2024 · 1 comment · May be fixed by #50

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@topperc
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topperc commented Apr 24, 2024

The xtheadfmemidx doc says "All instructions are available for RV32 and RV64". But th.flurw seems identical to th.flrw on RV32.

I found this patch that removed some of the "u" instructions from RV32 for xtheadmemidx. 5d38f4f
cmuellner added a commit to cmuellner/thead-extension-spec that referenced this issue Apr 25, 2024
@cmuellner
xtheadfmemidx: Fix instruction depenencies
f9fe30c 
The XTheadFMemIdx instruction that perform a zero-extension of the index
register (those with a `u` in the name) are identical to the
sign-extension counterparts on RV32. Therefore, it does not make sense
to offer both for RV32.

Further, the table lists the 8-byte loads/stores as compatible with the
`F` extensions. However, 8-byte loads won't fit into single-precision
floating-point register. And 8-byte stores can't be performed from a
single-precision floating-point register. Instead of defining truncation
and extension for these cases, let's make the instructions only
available for those cases where they make sense.

Fixes XUANTIE-RV#49.

Signed-off-by: Christoph Müllner <[email protected]>
@cmuellner
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We should drop th.flurw from RV32, just like we did with th.lur* and th.sur* in the referenced change for the XTheadMemidx extension. The whole table in the XTheadFMemIdx seems to be incorrect and deserves a rework.

Afaik, there is no implementation of XTheadFMemIdx in any of T-Head's RV32 cores. The E902 and the E906 only implement XTheadCmo and XTheadSync. So we can safely edit the spec (and all implementations that follow) without worrying too much.

@cmuellner cmuellner linked a pull request Apr 25, 2024 that will close this issue
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xtheadfmemidx: Fix instruction depenencies cmuellner/thead-extension-spec
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@cmuellner @topperc