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Re: [PATCH] Rewrite pic.md to improve medany and pic code size.


On Wed, Aug 29, 2018 at 9:22 AM, Palmer Dabbelt <palmer@sifive.com> wrote:
> Thanks Jim -- I'm afraid at least part of this was my mess, as I had to go
> add in the ZERO_EXTEND_LOAD hackery to work around some bug in this file
> that I couldn't figure out how to fix in a better way.

ZERO_EXTEND_LOAD is exactly the same as SUBX, so is redundant.  That
is the main reason why I dropped it.

> IIRC the issue I found was in booting a Linux kernel that was built with
> "-mcmodel=medany".  If I'm reading our current kernel port correctly that's
> the default for 64-bit systems, but I'm not sure when we flipped over so I
> think it's worth checking -- I know at the time I fixed the bug it wasn't
> the default :).  You should be able to do a "make V=1" inside Linux to just
> see the compiler command lines.

By default, 32-bit kernels are built medlow, and 64-bit kernels are
built medany.  That is in the riscv Kconfig file.  Before committing
the patch, I did apply it to freedom-u-sdk, build a 64-bit kernel, and
boot both on qemu and spike, so I think we are OK here.

I did find a few optimization problems via the gcc testsuite while
trying to improve the file.  The most interesting one, and maybe the
one you hit earlier, is with non-extended loads, which because of
LOAD_EXTEND_OP we actually have to use zero-extending loads for
char/short loads, but sign-extending loads for words, and I had to add
a new mode attribute for that.  That one took a little time to debug
because it didn't fail with trivial testcases, and the miscompilation
was non-obvious.

Jim


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