[U-Boot] [PATCH v2] arm: Switch to -mno-unaligned-access when supported by the compiler

Tom Rini
2014-02-10T13:21:39Z

On Mon, Feb 10, 2014 at 10:24:47AM +0100, Albert ARIBAUD wrote:
> Hi Tom,
>
> On Tue, 4 Feb 2014 12:05:33 -0500, Tom Rini <trini@ti.com> wrote:
>
> > When we tell the compiler to optimize for ARMv7 it assumes a default of
> > unaligned accesses being supported at the hardware level and can make
> > use of this to perform what it deems as an optimization in any case,
> > including allowing for data to become unaligned. We explicitly disallow
> > this hardware feature so we must tell the compiler.
> >
> > Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
> > Cc: Mans Rullgard <mans@mansr.com>
> > Signed-off-by: Tom Rini <trini@ti.com>
>
> NAK -- the discrepancy between the compiler being told to allow native
> unaligned accesses while at the same time telling the hardware to trap
> them is conscious and voluntary. It was chosen to help detect unaligned
> accesses which are rarely necessary and can be explicitly performed by
> software on a case by case basis.
>
> If and when a specific file requires unaligned access which cannot be
> made by some other mean than enabling -mno-unaligned-access, then this
> file should have it added, not the whole of U-Boot.

Right, I recall the discussion, and we chose wrong. We aren't being
clever and making problems that would appear on armv5 and lower (or
non-ARM never allows unaligned access platforms) problems to appear on
more common armv7 platforms. We're telling the compiler it's OK to do
one thing when it's not and then getting annoying problems such as the
EFI partition one where the compiler looks at everything, says we can do
$this and then fails at runtime because we lied to it. The whole
splashguard set of options is another place where I believe we've shot
ourselves in the foot, quite likely.

--
Tom
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