Subject: Re: Possible bug in arm32 strongarm optimisations.
To: Reinoud Zandijk <email@example.com>
From: Chris Gilbert <firstname.lastname@example.org>
Date: 10/25/2000 09:25:06
On Wed, 25 Oct 2000, Reinoud Zandijk wrote:
> Hi Chris,
> On Wed, 25 Oct 2000, Chris Gilbert wrote:
> > I've been trying to build an arm32 kernel with the -mcpu=strongarm
> > -mtune=strongarm.
> Oh ok
> > The problem I've had is that the podulebus doesn't get devices attaching
> > properly. Anyway, after doing some rummaging around in the .s files from
> > a kernel with and without the above options it seems that it's doing the
> > following optimsation:
> > turning:
> > ldr r3, [r5, #6]
> > mov r0, r3, lsr #16
> > into
> > ldrh r0, [r5, #4]
> > This occurs at about line 289/290 of sys/arch/arm32/podulebus/podulebus.c
> > I believe it should actually produce:
> > ldrh r0, [r5, #8]
> Well i chalenge that :)
> Lets say the memory is represented as follows from r5 :
> 7 6 5 4 | 3 2 1 0
> ldr r3, [r5, #6]
> r3 <-  // (in)famous rotating
> mov r0, r3, lsr # 16
> r0 <- [--54] // with the `-' zero
What is this rotating thing? I don't remember coming across it before. Is
it documented anywhere?
> The optimalisation implemented is :
> ldrh r0, [r5, #4]
> r0 <- [--54] // the same
doh, helps if I'd actually looked at the structure, the value should be at +4
into the struct...
perhaps a commented line in the RISCPC config file, saying use these flags
for a SA on a RiscPC, as RPC doesn't support halfword load and stores...