Re: [PATCH]middle-end Use subregs to expand COMPLEX_EXPR to set the lowpart.

[Jeff Law <jeffreyalaw@gmail.com>]

On 6/9/2022 1:52 AM, Tamar Christina via Gcc-patches wrote:
> Hi All,
>
> When lowering COMPLEX_EXPR we currently emit two VEC_EXTRACTs.  One for the
> lowpart and one for the highpart.
>
> The problem with this is that in RTL the lvalue of the RTX is the only thing
> tying the two instructions together.
>
> This means that e.g. combine is unable to try to combine the two instructions
> for setting the lowpart and highpart.
>
> For ISAs that have bit extract instructions we can eliminate one of the extracts
> if, and only if we're setting the entire complex number.
>
> This change changes the expand code when we're setting the entire complex number
> to generate a subreg for the lowpart instead of a vec_extract.
>
> This allows us to optimize sequences such as:
Just a note.  I regularly see subregs significantly interfere with 
optimization, particularly register allocation.  So be aware that 
subregs can often get in the way of generating good code.  When changing 
something to use subregs I like to run real benchmarks rather than 
working with code snippets.


>
> _Complex int f(int a, int b) {
>      _Complex int t = a + b * 1i;
>      return t;
> }
>
> from:
>
> f:
> 	bfi     x2, x0, 0, 32
> 	bfi     x2, x1, 32, 32
> 	mov     x0, x2
> 	ret
>
> into:
>
> f:
> 	bfi	x0, x1, 32, 32
> 	ret
>
> I have also confirmed the codegen for x86_64 did not change.
>
> Bootstrapped Regtested on aarch64-none-linux-gnu, x86_64-pc-linux-gnu
> and no issues.
>
> Ok for master?
>
> Thanks,
> Tamar
>
> gcc/ChangeLog:
>
> 	* emit-rtl.cc (validate_subreg): Accept subregs of complex modes.
> 	* expr.cc (emit_move_complex_parts): Emit subreg of lowpart if possible.
>
> gcc/testsuite/ChangeLog:
>
> 	* g++.target/aarch64/complex-init.C: New test.
OK.

On a related topic, any thoughts on keeping complex objects as complex 
types/modes through gimple and into at least parts of the RTL pipeline?

The way complex arithmetic instructions work on our chip is going to be 
extremely tough to utilize in GCC -- we really need to the complex 
types/arithmetic up through RTL generation at the least. Ideally we'd 
even expose complex modes all the way to final.    Is that something 
y'all could benefit from as well?  Have y'all poked at this problem at all?

jeff