Re: [PATCH, ARM] Fix PR60609 (Error: value of 256 too large for field of 1 bytes)

[Jeff Law <law at redhat dot com>]

On 04/03/14 11:02, Ramana Radhakrishnan wrote:
> On Thu, Apr 3, 2014 at 2:27 PM, Charles Baylis
> <charles.baylis@linaro.org> wrote:
> > Hi
> >
> > This bug causes the compiler to create a Thumb-2 TBB instruction with
> > a jump table containing an out of range value in a .byte field:
> >
> > whatever.s:148: Error: value of 256 too large for field of 1 bytes at 100
> >
> > This occurs because the jump table is followed with a ".align 1" due
> > to ASM_OUTPUT_CASE_END, but the 'shorten' phase does not account for
> > the space taken by this align directive.
>
> My first reaction is to wonder why this is this not a bug in the
> "shorten" phase.
I don't think that code ever expected an alignment directive to be 
emitted by ASM_OUTPUT_CASE_END :(


> > This patch addresses the issue by removing ASM_OUTPUT_CASE_END from
> > arm.h, and ensuring that the alignment after an ADDR_DIFF_VEC is
> > instead inserted by aligning the label following the barrier which
> > follows it. This is achieved by defining LABEL_ALIGN_AFTER_BARRIER
> > appropriately.
>
> On first glance this feels like a blunt hammer, what's the code size
> bloat with putting out such an alignment after each barrier that the
> compiler emits rather than tracking this in ASM_OUTPUT_CASE_END.
I'd tend to agree that emitting an alignment after each barrier would be 
a blunt hammer in this case.

ISTM we really want a new target hook to define the alignment after a 
the jump table, independent of the other alignment directives.  Then 
we'd have to teach shorten_branches about that.

Perhaps the blunt hammer for 4.9 and the new alignemnt-after-jump-table 
for the next stage1?

jeff