Re: [PATCH 4/4] arm: Enable ARM mode for armv6 strlen

[Ramana Radhakrishnan <ramana.gcc@googlemail.com>]

On Fri, Apr 13, 2018 at 10:56 AM, Phil Blundell <pb@pbcl.net> wrote:
> On Thu, 2018-04-12 at 17:49 -0300, Adhemerval Zanella wrote:
>> Even though it configure the toolchain to not emit thumb, if user
>> tries to optimize for armv7 (by tinkering with CC or CFLAGS) it will
>> still emit thumb instructions because of the optimized assembly
>> implementations.
>> And the expectation imho is to if user explicit builds with -marm
>> the resulting library should not user thumb instructions.
>
> I think the precedent on other architectures is that glibc will use the
> instruction set appropriate to the target triple it was given.  For
> example, if you configure glibc for i686-linux-gnu then it will use
> CMOV instructions, and setting -march=i586 in CFLAGS won't prevent
> this.
>
> I continue to feel that the scenario mentioned in the bug report you
> linked to (configuring for armv7 but disabling Thumb in the kernel) is
> just silly and we should not be indicating to users that this is
> supported.  T32 is an integral part of ARMv7 (indeed, the M profile
> doesn't support A32 at all) and if you take it out then the resulting
> architecture is no longer ARMv7.  It seems undesirable to force all the
> ARMv7-optimised assembly routines to also provide an ARM-only version
> even if the resulting performance is the same or worse than the Thumb
> implementation.  This code is never going to get tested in practice
> (witness the fact that you found it's been broken for several years)
> and it's just a liability.

>
>> In fact, the whole idea of current code is indeed to prevent thumb
>> instructions in such cases
>
> That's true.  It's not entirely clear to me why Roland made that change
>  in the first place but I think it was roughly contemporaneous with the
> NaCl port and I'm sort of guessing it was something to do with that.
> Does anybody else know/remember?

That change coming in with the NaCl port makes sense to me. IIRC NaCl
wanted fixed length encodings, (thus )everything to be in Arm state
only : instructions that were "forbidden" / not allowed as part of the
whole NaCl strategy for providing sand boxes including switching to
Thumb. I also seem to remember that they had stuff that checked
instructions in the binary were in a particular set and anything
outside was not considered to be a safe binary.

Aha - https://developer.chrome.com/native-client/reference/sandbox_internals/arm-32-bit-sandbox#arm-32-bit-sandbox



regards
Ramana


> p.
>