[Bug rtl-optimization/55950] New: Invalid sqrt constant propagation with -frounding-mode

[Bug rtl-optimization/55950] New: Invalid sqrt constant propagation with -frounding-mode

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55950

             Bug #: 55950
           Summary: Invalid sqrt constant propagation with -frounding-mode
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: glisse@gcc.gnu.org


simplify_const_unary_operation (in simplify-rtx.c) does constant propagation
for SQRT protected only by:

       if (HONOR_SNANS (mode) && real_isnan (&d))
         return 0;

With -frounding-math, it should check whether sqrt was exact and only propagate
in that case (sqrt(1)=1, sqrt(.25)=.5, but sqrt(2) should wait for execution).

(note that this doesn't happen on a trivial sqrt(2.) example, but I've already
seen it happen)

[Bug rtl-optimization/55950] Invalid sqrt constant propagation with -frounding-mode

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55950

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-01-14
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> 2013-01-14 13:21:45 UTC ---
Confirmed.  real.c has all the bells and whistles to implement this I think.

[Bug rtl-optimization/55950] Invalid sqrt constant propagation with -frounding-mode

[jsm28 at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55950

--- Comment #2 from Joseph S. Myers <jsm28 at gcc dot gnu.org> ---
Author: jsm28
Date: Thu Nov 21 16:20:28 2013
New Revision: 205223

URL: http://gcc.gnu.org/viewcvs?rev=205223&root=gcc&view=rev
Log:
    PR rtl-optimization/55950
    * real.c (real_sqrt): Remove function.
    * real.h (real_sqrt): Remove prototype.
    * simplify-rtx.c (simplify_const_unary_operation): Do not fold
    SQRT using real_sqrt.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/real.c
    trunk/gcc/real.h
    trunk/gcc/simplify-rtx.c

[Bug rtl-optimization/55950] Invalid sqrt constant propagation with -frounding-mode

[jsm28 at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55950

Joseph S. Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |4.9.0

--- Comment #3 from Joseph S. Myers <jsm28 at gcc dot gnu.org> ---
Fixed for 4.9 by removing this optimization, in the expectation that any
important cases of sqrt constant folding are handled at the GIMPLE level which
has proper checks for this.