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Date: Sun, 15 Dec 2019 13:06:29 -0500
From: Jeffrey Walton <>
Subject: Re: max_align_t mess on i386

On Sat, Dec 14, 2019 at 10:19 AM Rich Felker <> wrote:
> In reserching how much memory could be saved, and how practical it
> would be, for the new malloc to align only to 8-byte boundaries
> instead of 16-byte on archs where alignof(max_align_t) is 8 (pretty
> much all 32-bit archs), I discovered that GCC quietly changed its
> idead of i386 max_align_t to 16-byte alignment in GCC 7, to better
> accommodate the new _Float128 access via SSE. Presumably (I haven't
> checked) the change is reflected with changes in the psABI document to
> make it "official".

Be careful with policy changes like this. The malloc (3) man page says:

    The malloc() and calloc() functions return a pointer to the
    allocated memory that is suitably aligned for any kind of variable.

I expect to be able to use a pointer returned by malloc (and friends)
in MMX, SSE and AVX functions.


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