Re: [04/32] [x86] Robustify vzeroupper handling across calls

[Uros Bizjak <ubizjak@gmail.com>]

On Wed, Sep 25, 2019 at 5:48 PM Richard Sandiford
<richard.sandiford@arm.com> wrote:

> > The comment suggests that this code is only needed for Win64 and that
> > not testing for Win64 is just a simplification.  But in practice it was
> > needed for correctness on GNU/Linux and other targets too, since without
> > it the RA would be able to keep 256-bit and 512-bit values in SSE
> > registers across calls that are known not to clobber them.
> >
> > This patch conservatively treats calls as AVX_U128_ANY if the RA can see
> > that some SSE registers are not touched by a call.  There are then no
> > regressions if the ix86_hard_regno_call_part_clobbered check is disabled
> > for GNU/Linux (not something we should do, was just for testing).

If RA can sse that some SSE regs are not touched by the call, then we
are sure that the called function is part of the current TU. In this
case, the called function will be compiled using VEX instructions,
where there is no AVX-SSE transition penalty. So, skipping VZEROUPPER
is beneficial here.

Uros.

> > If in fact we want -fipa-ra to pretend that all functions clobber
> > SSE registers above 128 bits, it'd certainly be possible to arrange
> > that.  But IMO that would be an optimisation decision, whereas what
> > the patch is fixing is a correctness decision.  So I think we should
> > have this check even so.
>
> 2019-09-25  Richard Sandiford  <richard.sandiford@arm.com>
>
> gcc/
>         * config/i386/i386.c: Include function-abi.h.
>         (ix86_avx_u128_mode_needed): Treat function calls as AVX_U128_ANY
>         if they preserve some 256-bit or 512-bit SSE registers.
>
> Index: gcc/config/i386/i386.c
> ===================================================================
> --- gcc/config/i386/i386.c      2019-09-25 16:47:48.000000000 +0100
> +++ gcc/config/i386/i386.c      2019-09-25 16:47:49.089962608 +0100
> @@ -95,6 +95,7 @@ #define IN_TARGET_CODE 1
>  #include "i386-builtins.h"
>  #include "i386-expand.h"
>  #include "i386-features.h"
> +#include "function-abi.h"
>
>  /* This file should be included last.  */
>  #include "target-def.h"
> @@ -13511,6 +13512,15 @@ ix86_avx_u128_mode_needed (rtx_insn *ins
>             }
>         }
>
> +      /* If the function is known to preserve some SSE registers,
> +        RA and previous passes can legitimately rely on that for
> +        modes wider than 256 bits.  It's only safe to issue a
> +        vzeroupper if all SSE registers are clobbered.  */
> +      const function_abi &abi = insn_callee_abi (insn);
> +      if (!hard_reg_set_subset_p (reg_class_contents[ALL_SSE_REGS],
> +                                 abi.mode_clobbers (V4DImode)))
> +       return AVX_U128_ANY;
> +
>        return AVX_U128_CLEAN;
>      }
>