[Qemu-devel] Fwd: Re: [PATCH] tcg-ppc64: Support the ELFv2 ABI

[Richard Henderson]

Bah.  Copied Uli's typo to qemu address.


r~


-------- Original Message --------
Subject: Re: [PATCH] tcg-ppc64: Support the ELFv2 ABI
Date: Tue, 22 Apr 2014 10:53:18 -0700
From: Richard Henderson <address@hidden>
To: Ulrich Weigand <address@hidden>, address@hidden
CC: address@hidden, address@hidden

On 04/22/2014 09:20 AM, Ulrich Weigand wrote:
> +#elif _CALL_ELF == 2
> +    /* In the ELFv2 ABI, we do not need to set up the TOC pointer in r2,
> +       but instead we have to set up r12 to contain the destination address
> +       when performing an indirect call.  */
> +    TCGReg reg = arg;
> +    if (const_arg) {
> +        /* FIXME: we could use bl if we knew that the destination uses
> +           the same TOC, and what its local entry point offset is.
> +           For now, always perform an indirect call.  */
> +        tcg_out_movi(s, TCG_TYPE_PTR, TCG_REG_R12, arg);
> +        reg = TCG_REG_R12;
> +    } else {
> +        tcg_out_mov(s, TCG_TYPE_PTR, TCG_REG_R12, arg);
> +    }
> +    tcg_out32(s, MTSPR | RS(reg) | CTR);
> +    tcg_out32(s, BCCTR | BO_ALWAYS | LK);

Looks ok.

Any chance we can get assurances on the same TOC from the compiler+linker?

E.g. for Alpha gcc has -msmall-data, so the compiler can assume a single gp for
local functions, optimizing more calls itself.  Then the linker has
--warn-multiple-gp to check this assumption at link time.


> -#ifndef __APPLE__
> +#if !defined(__APPLE__) && _CALL_ELF != 2
>      /* First emit adhoc function descriptor */
>      tcg_out64(s, (uint64_t)s->code_ptr + 24); /* entry point */
>      s->code_ptr += 16;          /* skip TOC and environment pointer */

The proper test here is #ifdef _CALL_AIX.

Please check out

  git://github.com/rth7680/qemu.git tcg-ppc-merge

which merges the ppc32 and ppc64 backends.  I also started to
support the ELFv2 ABI.  I believe that you technically need

 #ifdef _CALL_AIX
 # define LINK_AREA_SIZE                (6 * SZR)
 # define LR_OFFSET                     (1 * SZR)
 # define TCG_TARGET_CALL_STACK_OFFSET  (LINK_AREA_SIZE + 8 * SZR)
+#elif defined(_CALL_ELF) && _CALL_ELF == 2
+# define LINK_AREA_SIZE                (4 * SZR)
+# define LR_OFFSET                     (1 * SZR)
+# define TCG_TARGET_CALL_STACK_OFFSET  LINK_AREA_SIZE

although we don't have helpers that will actually exercise 9 arguments.
But I had missed this R12 feature of the call conventions, so thanks for that.