[RFC PATCH, i386]: Rewrite nearbyint expanders

[Uros Bizjak <ubizjak at gmail dot com>]

Hello!

Attached patch rewrites nearbyint expanders. The problem with the
current approach is, that setting mask flag of precision interrupt on
x87 only prevents interrupt generation, but the insn still sets
interrupt flag. fclex clears the flag, but it also clears other flags
(e.g. invalid flag, set by sNaN).

This issue can be "solved" by disabling x87 insn for
flag_trapping_math. The flag is considered by other instructions (e.g.
ceil, floor) as "we don't care if insn throws an exception"), so based
on this reasoning, we can as well convert nearbyint to rint (which
doesn't require complex control word set-up).

There is no issue with traps with SSE 4.1 round instruction. This
instruction can suppress generation of precision interrupt by setting
ROUND_NO_EXC bit in its immediate operand.

2018-09-24  Uros Bizjak  <ubizjak@gmail.com>

    * config/i386/i386.md (frndintxf2_mask_pm): Remove.
    (frndintxf2_mask_pm_i387): Ditto.
    (nearbyintxf2): Rewrite expander pattern to match rintxf2.
    Enable for !flag_trapping_math.
    (nearbyint<mode>2): Enable x87 modes for !flag_trapping_math.
    Enable SSE modes for TARGET_SSE4_1 and expand them with round insn.
    Change operand 1 predicate to nonimmediate_operand.
    (attr "i387_cw"): Remove mask_pm.
    * config/i386/i386.h (enum ix86_stack_slot): Remove SLOT_CW_MASK_PM.
    (enum ix86_entity): Remove I387_MASK_PM.
    * config/i386/i386.c (ix86_i387_mode_needed): Do not
    handle I387_MASK_PM.
    (ix86_mode_needed): Ditto.
    (ix86_mode_after): Ditto.
    (ix86_mode_entry): Ditto.
    (ix86_mode_exit): Ditto.
    (emit_i387_cw_initialization): Do not handle I387_CW_MASK_PM.

Patch was bootstrapped and regression tested on x86_64-linux-gnu.

I'll wait a bit for possible comments on the approach.

Uros.

Attachment: p.diff.txt
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