[PATCH][optabs.c] Fix PR 61713: ICE when expanding single-threaded version of atomic_test_and_set

[Kyrill Tkachov <kyrylo dot tkachov at arm dot com>]

Hi all,

This fixes the PR mentioned in the subject. When expanding 
atomic_test_and_set we try the corresponding sync optabs and if none 
exist, like for pre-SMP ARM architectures (-march=armv6 and earlier) the 
code in optabs.c reverts to a single-threaded implementation that just 
does a load and store.

However, if the result of the operation is to be ignored the code in 
builtins.c follows the convention that the target RTX is set to 
const0_rtx to signify that the result is ignored and the expand 
functions in optabs.c are supposed to check for that and act appropriately.

The code in the single-threaded codepath in expand_atomic_test_and_set 
in optabs.c didn't perform this check and instead tried to emit a move 
to a const0_rtx which ICEs further down the line.

This patch fixes that by checking if the result is ignored, and if it is 
omits the load.
I wouldn't dare to remove the load in normal atomic handling code due to 
all the memory ordering subtleties, but this code path occurs only when 
we have a trivially single-threaded bare-metal target and the compiler 
assumes no races anyway and no dodgy context switches and tries to 
implement this with a ldrb+strb, so I think removing the ldrb is valid.

Bootstrapped on arm, aarch64 and x86 and tested as well.

Ok for trunk?
This appears on 4.9 as well, I'll test it on that branch as well

2014-07-16  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    PR target/61713
    * gcc/optabs.c (expand_atomic_test_and_set): Do not try to emit
    move to subtarget in serial version if result is ignored.