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Re: [RFC PATCH 0/3] tests/tcg/ppc64le: fix the build of TCG tests with C

From: Matheus K. Ferst
Subject: Re: [RFC PATCH 0/3] tests/tcg/ppc64le: fix the build of TCG tests with Clang
Date: Fri, 11 Feb 2022 17:22:20 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0

On 09/02/2022 09:31, Cédric Le Goater wrote:
Hello Matheus,

[ Adding Miroslav ]

On 2/8/22 21:31, matheus.ferst@eldorado.org.br wrote:
From: Matheus Ferst <matheus.ferst@eldorado.org.br>

Based-on: https://lists.gnu.org/archive/html/qemu-devel/2022-01/msg06506.html

As the configuration scripts used -mbig and -mlittle, building PPC tests
with Clang was silently skipped. With the patch to fix these options[1],
"make check-tcg" fails because of build and runtime errors. This patch
series tries to fix some of these problems.

The first patch fixes "tests/tcg/ppc64le/mtfsf.c" by removing the
GCC-only builtins used to emit mtfsf and mffs. We can emit these insns
with inline asm instead.

The second patch addresses differences in the output of float_madds.c.
The __builtin_fmaf used in this test emits fmadds with GCC and xsmaddasp
with LLVM. The first insn had rounding errors fixed in
d04ca895dc7f ("target/ppc: Add helpers for fmadds et al"), we apply
a similar fix to xsmaddasp.

Then we have the build errors of tests/tcg/ppc64le/bcdsub.c. According
to GCC docs[2], the '-mpower8-vector' flag provides some bcdsub
builtins, so it'd be reasonable to assume that the rest of the toolchain
knows about the insn if the compiler accepts this flag. Clang supports
this flag since version 3.6[3], but the insn and builtins were only
added in LLVM 14[4]. I couldn't find a good solution. Should we write a
test to check for this insn at configuration time? Should we detect the
compiler at build time and emit the insns with ".long" and fixed

Even building with Clang 14, the test will fail in runtime because
LLVM doesn't like "__int128" in inline asm. No error or warning is
emitted, but the generated code only loads one doubleword of the VSR.
The third patch of this series avoids this issue by using a vector
type for VSR values.

Finally, it seems that the insns tested by
tests/tcg/ppc64le/byte_reverse.c are not yet supported by LLVM. Since
the configuration script uses '-mpower10' to check for POWER10 support
and Clang doesn't support this flag, "make check-tcg" doesn't fail. We
should probably change this check in the future, but since LLVM support
of POWER10 seems incomplete, I guess we can leave it for now.

gitlab didn't spot any issues with the 4 patches applied.

AFAICT, CI wouldn't run into this kind of problem because we don't have PPC runners, and the cross-compiler containers use GCC.

Should we merge all patches :

   Use long endian options for ppc64
   tests/tcg/ppc64le: Use vector types instead of __int128
   target/ppc: change xs[n]madd[am]sp to use float64r32_muladd
   tests/tcg/ppc64le: use inline asm instead of __builtin_mtfsf

and see how we can address the LLVM support for P10 later ?

The problems with bcdsub.c are not resolved for Clang < 14, but I guess it's ok to merge anyway.

Matheus K. Ferst
Instituto de Pesquisas ELDORADO <http://www.eldorado.org.br/>
Analista de Software
Aviso Legal - Disclaimer <https://www.eldorado.org.br/disclaimer.html>

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