Re: [PATCH] [i386] Fix _mm256_zeroupper to notify LRA that vzeroupper will kill sse registers. [PR target/82735]

[Uros Bizjak <ubizjak@gmail.com>]

On Thu, May 27, 2021 at 7:03 AM Hongtao Liu <crazylht@gmail.com> wrote:
>
> Hi:
>   This is an updated patch which implements vzeroupper as call_insn
> which has a special vzeroupper ABI, also in this patch i reverted
> r11-7684, r10-6451, r10-3677 which seems to fix the same issue but in
> a different way.
>   Bootstrapped and regtested on x86_64-linux-gnux{-m32,} and
> x86_64-linux-gnux{-m32 \-march=cascadelake,-march=cascadelake}.
>   Also test the patch on SPEC2017 and eembc, no performance impact as expected.
>   Ok for trunk?
>
> gcc/ChangeLog:
>
>         PR target/82735
>         * config/i386/i386-expand.c (ix86_expand_builtin): Remove
>         assignment of cfun->machine->has_explicit_vzeroupper.
>         * config/i386/i386-features.c
>         (ix86_add_reg_usage_to_vzerouppers): Delete.
>         (ix86_add_reg_usage_to_vzeroupper): Ditto.
>         (rest_of_handle_insert_vzeroupper): Remove
>         ix86_add_reg_usage_to_vzerouppers, add df_analyze at the end
>         of the function.
>         (gate): Remove cfun->machine->has_explicit_vzeroupper.
>         * config/i386/i386-protos.h (ix86_expand_avx_vzeroupper):
>         Declared.
>         * config/i386/i386.c (ix86_insn_callee_abi): New function.
>         (ix86_initialize_callee_abi): Ditto.
>         (ix86_expand_avx_vzeroupper): Ditto.
>         (ix86_hard_regno_call_part_clobbered): Adjust for vzeroupper
>         ABI.
>         (TARGET_INSN_CALLEE_ABI): Define as ix86_insn_callee_abi.
>         * config/i386/i386.h (enum i386_insn_callee_abi_index): New.
>         (struct GTY(()) machine_function): Delete
>         has_explicit_vzeroupper.
>         * config/i386/i386.md (enum unspec): New member
>         UNSPEC_CALLEE_ABI.
>         * config/i386/predicates.md (vzeroupper_pattern): Adjust.
>         * config/i386/sse.md (avx_vzeroupper): Call
>         ix86_expand_avx_vzeroupper.
>         (*avx_vzeroupper): Rename to ..
>         (avx_vzeroupper_callee_abi): .. this, and adjust pattern as
>         call_insn which has a special vzeroupper ABI.
>         (*avx_vzeroupper_1): Deleted.
>         * df-scan.c (df_get_call_refs): When call_insn is a fake call,
>         it won't use stack pointer reg.
>         * final.c (leaf_function_p): When call_insn is a fake call, it
>         won't affect caller as a leaf function.
>         * reg-stack.c (callee_clobbers_any_stack_reg): New.
>         (subst_stack_regs): When call_insn doesn't clobber any stack
>         reg, don't clear the arguments.
>         * rtl.c (shallow_copy_rtx): Don't clear flag used when orig is
>         a insn.
>         * shrink-wrap.c (requires_stack_frame_p): No need for stack
>         frame for a fake call.
>
> gcc/testsuite/ChangeLog:
>
>         PR target/82735
>         * gcc.target/i386/pr82735-1.c: New test.
>         * gcc.target/i386/pr82735-2.c: New test.
>         * gcc.target/i386/pr82735-3.c: New test.
>         * gcc.target/i386/pr82735-4.c: New test.
>         * gcc.target/i386/pr82735-5.c: New test.

Please split the patch to middle-end and target part. The middle-end
should be approved first.

 (define_expand "avx_vzeroupper"
-  [(parallel [(unspec_volatile [(const_int 0)] UNSPECV_VZEROUPPER)])]
-  "TARGET_AVX")
+  [(parallel [(call (mem:QI (unspec_volatile [(const_int 0)]
UNSPECV_VZEROUPPER))
+            (const_int 0))
+         (unspec [(const_int 1)] UNSPEC_CALLEE_ABI)])]

The call insn doesn't look like a valid RTX. Why not just:

+  [(parallel [(call (mem:QI (const_int 0)
+            (const_int 0))

for a fake call? Also, UNSPEC_VZEROUPPER can be removed this way since
the const_int 1 of UNSPEC_CALLEE_ABI is now used to detect vzeroupper.

Also, you don't need the avx_vzeroupper pattern to just call
ix86_expand_avx_vzeroupper. Just call the function directly from the
call site:

    case AVX_U128:
      if (mode == AVX_U128_CLEAN)
    emit_insn (gen_avx_vzeroupper ());
      break;

+         (unspec [(const_int 1)] UNSPEC_CALLEE_ABI)])]

Can this const_int 1 be somehow more descriptive? Perhaps use
define_constant to define I386_VZEROUPPER ABI and use it in .md as
well as .c files.

Uros.