Re: [PATCH] PR rtl-optimization/7061: Complex number arguments on x86_64-like ABIs.

[Jeff Law <jeffreyalaw@gmail.com>]

On 5/30/2022 4:06 AM, Roger Sayle wrote:
> This patch addresses the issue in comment #6 of PR rtl-optimization/7061
> (a four digit PR number) from 2006 where on x86_64 complex number arguments
> are unconditionally spilled to the stack.
>
> For the test cases below:
> float re(float _Complex a) { return __real__ a; }
> float im(float _Complex a) { return __imag__ a; }
>
> GCC with -O2 currently generates:
>
> re:     movq    %xmm0, -8(%rsp)
>          movss   -8(%rsp), %xmm0
>          ret
> im:     movq    %xmm0, -8(%rsp)
>          movss   -4(%rsp), %xmm0
>          ret
>
> with this patch we now generate:
>
> re:     ret
> im:     movq    %xmm0, %rax
>          shrq    $32, %rax
>          movd    %eax, %xmm0
>          ret
>
> [Technically, this shift can be performed on %xmm0 in a single
> instruction, but the backend needs to be taught to do that, the
> important bit is that the SCmode argument isn't written to the
> stack].
>
> The patch itself is to emit_group_store where just before RTL
> expansion commits to writing to the stack, we check if the store
> group consists of a single scalar integer register that holds
> a complex mode value; on x86_64 SCmode arguments are passed in
> DImode registers.  If this is the case, we can use a SUBREG to
> "view_convert" the integer to the equivalent complex mode.
>
> An interesting corner case that showed up during testing is that
> x86_64 also passes HCmode arguments in DImode registers(!), i.e.
> using modes of different sizes.  This is easily handled/supported
> by first converting to an integer mode of the correct size, and
> then generating a complex mode SUBREG of this.  This is similar
> in concept to the patch I proposed here:
> https://gcc.gnu.org/pipermail/gcc-patches/2022-February/590139.html
> which was almost (but not quite) approved here:
> https://gcc.gnu.org/pipermail/gcc-patches/2022-March/591139.html
Yea, sorry.  Too much to do at the new job.  Trying to work my way 
through queued up stuff now...


>
> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check, both with and without --target_board=unix{-m32},
> with no new failures.  Ok for mainline?
>
>
> 2020-05-30  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>          PR rtl-optimization/7061
>          * expr.cc (emit_group_stote): For groups that consist of a single
>          scalar integer register that hold a complex mode value, use
>          gen_lowpart to generate a SUBREG to "view_convert" to the complex
>          mode.  For modes of different sizes, first convert to an integer
>          mode of the appropriate size.
>
> gcc/testsuite/ChangeLog
>          PR rtl-optimization/7061
>          * gcc.target/i386/pr7061-1.c: New test case.
>          * gcc.target/i386/pr7061-2.c: New test case.
OK
jeff