[Bug target/60863] New: Incorrect codegen in ix86_expand_clear for -Os

[Bug target/60863] New: Incorrect codegen in ix86_expand_clear for -Os

[hjl.tools at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60863

            Bug ID: 60863
           Summary: Incorrect codegen in ix86_expand_clear for -Os
           Product: gcc
           Version: 4.10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hjl.tools at gmail dot com
                CC: ubizjak at gmail dot com

ix86_expand_clear has

  /* This predicate should match that for movsi_xor and movdi_xor_rex64.  */
  if (!TARGET_USE_MOV0 || optimize_insn_for_speed_p ())
    {
      rtx clob = gen_rtx_CLOBBER (VOIDmode, gen_rtx_REG (CCmode, FLAGS_REG));
      tmp = gen_rtx_PARALLEL (VOIDmode, gen_rtvec (2, tmp, clob));
    }

But "xor reg,reg" has shorter encoding than "mov $0, reg".  If
"xor reg,reg" is generated for -O, shouldn't it also be generated
for -Os?

[Bug target/60863] Incorrect codegen in ix86_expand_clear for -Os

[ubizjak at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60863

--- Comment #1 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to H.J. Lu from comment #0)
> ix86_expand_clear has
> 
>   /* This predicate should match that for movsi_xor and movdi_xor_rex64.  */
>   if (!TARGET_USE_MOV0 || optimize_insn_for_speed_p ())
>     {
>       rtx clob = gen_rtx_CLOBBER (VOIDmode, gen_rtx_REG (CCmode, FLAGS_REG));
>       tmp = gen_rtx_PARALLEL (VOIDmode, gen_rtvec (2, tmp, clob));
>     }
> 
> But "xor reg,reg" has shorter encoding than "mov $0, reg".  If
> "xor reg,reg" is generated for -O, shouldn't it also be generated
> for -Os?

This is a typo, the condition should be optimize_insn_for_size_p. Just look
peepholes in i386.md after "Attempt to always use XOR for zeroing registers."
comment.

The patch that fixes the condition is preapproved - please also remove outdated
comment about predicates.
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Subject: [Bug c/60256] No -Wuninitialized warning for strcpy
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id`256

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
We call c_fully_fold on strcpy (s, s);, and because this CALL_EXPR is
tcc_vl_exp, we call fold () on it.  fold () then via fold_call_expr -> ...
calls fold_builtin_strcpy and that hits
  /* If SRC and DEST are the same (and not volatile), return DEST.  */
  if (operand_equal_p (src, dest, 0))
    return fold_convert_loc (loc, TREE_TYPE (TREE_TYPE (fndecl)), dest);

But on second thought, delayed folding won't help, as uninit passes are run
quite late and at that point strcpy(s, s); will be already gone.

[Bug target/60863] Incorrect codegen in ix86_expand_clear for -Os

[hjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60863

--- Comment #2 from hjl at gcc dot gnu.org <hjl at gcc dot gnu.org> ---
Author: hjl
Date: Thu Apr 17 15:19:34 2014
New Revision: 209488

URL: http://gcc.gnu.org/viewcvs?rev=209488&root=gcc&view=rev
Log:
Generate "xor reg, reg" if optimizing for size

    PR target/60863
    * config/i386/i386.c (ix86_expand_clear): Remove outdated
    comment.  Check optimize_insn_for_size_p instead of
    optimize_insn_for_speed_p.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/config/i386/i386.c

[Bug target/60863] Incorrect codegen in ix86_expand_clear for -Os

[hjl.tools at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60863

H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |4.10.0

--- Comment #3 from H.J. Lu <hjl.tools at gmail dot com> ---
Fixed.