Label alignments in shorten_branches

[Joern Rennecke]

Quoting Bernd Schmidt <bernds_cb1@t-online.de>:

> Revision 18357, in 1998, introduced a mechanism to keep track of label
> alignments during shorten branches.  It tries to determine the known
> alignment at every point in the function, and compute the amount of
> padding needed at every label.
>
> The problem is, to do this, we need to know the exact length of every
> instruction.  The Blackfin has a few patterns where the length
> attribute gives a maximum rather than an exact value, but even if the
> machine description is 100% accurate, there is in general no way of
> knowing the exact length of an asm statement.

There is no need to know the exact length, only conservative estimates are
needed.

> I have a testcase where the compiler produces an assembly file that has
> an out-of-range jump, because it thought the label at the start of a
> jump table (which has an alignment of 4) needs no padding, when in fact
> it needs 2 bytes of padding.
>
> Can anyone think of a way to make this code work, or should I just post
> a patch to mostly revert 18357?

This code should already work if it is used properly.
the align_fuzz function is provided to give a conservative estimate of
possible changes in the distance of two code locations due to
some instructions actually being shorter than their estimates, which
can cause alignment padding increases.