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From: Jeffrey Walton <noloader@gmail.com>
Date: Tue, 21 May 2019 02:52:00 -0000
Message-ID: <CAH8yC8=ULU8tz-XrQvy2gnJ2B5tgSj-s-aMpd5m8E2wmTR4sRw@mail.gmail.com>
Subject: getauxv and ARMv7 platform detection
To: libc-help@sourceware.org
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Hi Everyone,

I just noticed the following code was failing on ARMv7 platforms:

// For older machines, like GCC 4.3 on ARMv6
#ifndef HWCAP_ARMv7
# define HWCAP_ARMv7 (1 << 29)
#endif

inline bool CPU_QueryARMv7()
{
    if (getauxval(AT_HWCAP) & HWCAP_ARMv7) != 0)
        return true;
    return false;
}

Looking through Torvalds GitHub [1] I don't see a reliable define for
HWCAP_ARMv7 in hwcap.h.

I know I can fallback to NEON, but that misses earlier devices that
are ARMv7 but lacks NEON.

Searching is turning up a lot of spurious noise (for Aarch64/ARMv8),
and one hit for Mozilla working around the same problem [2].

My question is, how do we use getauxval to detect ARMv7 platforms?

Thanks in advance.

[1] https://github.com/torvalds/linux/blob/master/arch/arm/include/uapi/asm/hwcap.h
[2] https://hg.mozilla.org/releases/mozilla-aurora/rev/522951ff7046fd2bdf3916c973fbfde27be0af31