Re: [PATCH][RTL-ifcvt] PR rtl-optimization/67749: Do not emit separate SET insn in IF-ELSE case

[Jeff Law <law@redhat.com>]

On 10/27/2015 08:49 AM, Kyrill Tkachov wrote:
> Hi all,
>
> This patch fixes the gcc.dg/ifcvt-2.c test for x86_64 where we were
> failing to if-convert. This was because in my patch at
> https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=228194 which
> tried to emit a SET to move the source of insn_a or insn_b (that came
> from the test block) into a temporary. A SET however, is not always
> enough. For example, on x86_64 in order for the resulting insn to be
> recognised it frequently needs to be in PARALLEL with a (clobber
> (reg:CC FLAGS_REG)). This leads to the insn not being recognised.
I don't think it affects the approach you've chosen, but I'm mentioning 
it for future reference.

gcse.c has some helper code (that probably ought to move into a more 
generic file) that will test for this situation.  Search for the 
instances of recog.  It essentially does something like

emit_insn (gen_rtx_SET (...))

And tries to recognize the result to determine if it's valid.


>
> So this patch removes that SET and instead generates a couple of
> temporary pseudos that gets passed on a bit later to the code that
> loads the operands into registers when they're not general_operand.
> This way we just modify the existing (recognisable) sets, allowing us
> to if-convert the testcase.
That sounds much more reasonable, assuming that the original 
destinations were just used once and those uses are guaranteed to be 
going away.



>
> Bootstrapped and tested on x86_64, arm, aarch64.
>
> Ok for trunk?

What happens in the case were noce_emit_bb returns a failure?  We've 
modified the original insns to use the new pseudos.  Won't this result 
in the original pseudo's uses using undefined values?

jeff