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 <ro...@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
