From: dengjianbo <dengjianbo@loongson.cn>
To: Xi Ruoyao <xry111@xry111.site>, Andreas Schwab <schwab@linux-m68k.org>
Cc: adhemerval.zanella@linaro.org, libc-alpha@sourceware.org,
caiyinyu@loongson.cn, i.swmail@xen0n.name,
joseph_myers@mentor.com, carlos@redhat.com,
xuchenghua@loongson.cn
Subject: Re: [PATCH] LoongArch: Optimize math barriers
Date: Fri, 8 May 2026 17:47:24 +0800 [thread overview]
Message-ID: <75617e1e-54db-c3cb-1601-7dab304020e7@loongson.cn> (raw)
In-Reply-To: <460de7efbecb99e8ee40d21d5ed3c451da89d5a8.camel@xry111.site>
On 5/8/26 5:29 PM, Xi Ruoyao wrote:
> On Thu, 2026-05-07 at 17:30 +0800, dengjianbo wrote:
>> Yes, unnecessary moves can occur if the value is already in a general
>> reg. For example, when invoking math_opt_barrier with fixed-point
>> parameters. However, in glibc, these two macros are invoked with
>> floating-point parameters, and I didn’t find the unnecessary moves
>> being generated. For other architectures, aarch64 and s390 also do
>> not include the 'r' constraint.
>>
>> For gcc part, to my understanding, when 'fr' is present, GCC selects the
>> union of FP_REGS and GR_REGS as the destination register class, which is
>> initialized to GR_REGS. Then compute the cost of choosing different
>> register classes, choosing GR_REGS incurs no extra cost, while choosing
>> FP_REGS adds an extra move cost. I am not sure if it's a strategy here.
>
> Where does this issue manifest (i.e. in which glibc source file)? I'll
> try to reduce a test case from that. To me this seems a compiler bug
> too.
>>
>
One example is the xflow function, compiler generates the unnecessary
moves for this function, the source file is
sysdeps/ieee754/dbl-64/math_err.c．
next prev parent reply other threads:[~2026-05-08 9:47 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-07 2:17 Deng Jianbo
2026-05-07 6:46 ` Andreas Schwab
2026-05-07 9:30 ` dengjianbo
2026-05-08 9:29 ` Xi Ruoyao
2026-05-08 9:47 ` dengjianbo [this message]
2026-05-25 1:09 ` dengjianbo
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