[Bug libfortran/93727] Fortran 2018: EX edit descriptor

[jvdelisle at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93727

--- Comment #33 from Jerry DeLisle <jvdelisle at gcc dot gnu.org> ---
Created attachment 64540
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=64540&action=edit
Proposed patch to handle rounding of hexadecimal formats

>From a31c9ee1433e2f5813985ca9ba2d660ffcdfe995 Mon Sep 17 00:00:00 2001
From: Jerry DeLisle <jvdelisle@gcc.gnu.org>
Date: Sat, 23 May 2026 20:28:43 -0700
Subject: [PATCH] Fortran: [PR93727] Add EX format rounding for truncated hex
 mantissa
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Implement proper rounding of the hex mantissa in write_ex when the
user specifies a d smaller than full precision.  All Fortran ROUND=
modes are supported: ROUND_NEAREST (ties-to-even), ROUND_COMPATIBLE
(ties away from zero), ROUND_UP, ROUND_DOWN, and ROUND_ZERO.
ROUND_PROCDEFINED and ROUND_UNSPECIFIED default to ROUND_NEAREST on
IEEE 754 systems, consistent with the decimal format behaviour.

Carry propagation handles the case where incrementing a string of
trailing F hex digits reaches the integer digit; if that overflows
(F → 16) the output is normalized by setting the integer digit to 8
and incrementing the binary exponent by one.

        PR fortran/93727

libgfortran/ChangeLog:

        * io/write.c (write_ex): Replace simple truncation with
        rounding-aware logic respecting dtp round_status.  Add carry
        propagation and integer-digit normalization.

gcc/testsuite/ChangeLog:

        * gfortran.dg/EXformat_3.F90: New test covering rounding for
        KIND=4, 8, 10, and 16: clear round-up, ties-to-even (truncate
        and round-up cases), carry propagation, and normalization.

Assisted by: Claude Sonnet 4.6