• Re: Bug#1001314: mozjs78: FTBFS on armhf: '-mfloat-abi=hard': selected

    From Wookey@21:1/5 to peter green on Thu Dec 9 15:20:01 2021
    On 2021-12-08 20:11 +0000, peter green wrote:
    The result of this change is if you manually set "-march" then it
    overrides the built-in defaults for both the CPU and FPU rather than
    only overriding the CPU.

    OK. That explains some things I've been noticing recently.

    The default -march value on Debian armhf is "armv7-a+fp". You should

    Shouldn't it be "armv7-a+nosimd+fp", because neon is not assumed to be present on armhf?
    All neon code has to be gated on a HWCAP check, so the default should exclude it.

    I recently found a case where gcc11 helpfully put a neon optimisation
    into the init code of a complilation unit (zeroing variables), which
    of course means the program crashes on started on non-neon hardware.
    To be fair it only did that with -mfpu=neon set, but I'm not sure that it can't happen with a
    default march=armv7-a+fp

    OK. I just checked and in fact it doesn't do this optimisation if
    built with -march=armv7-a+fp so I guess there is an implicit assumption that everything not listed is disabled?
    Do we actually know for sure or shall I try and find out from some compiler engineers?

    Principal hats: Linaro, Debian, Wookware, ARM


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