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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/52134] New: Does not fold (x * 4) & -4
Date: Mon, 06 Feb 2012 10:51:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52134

             Bug #: 52134
           Summary: Does not fold (x * 4) & -4
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: rguenth@gcc.gnu.org
                CC: ebotcazou@gcc.gnu.org


We appearantly do not fold

((sizetype) MAX_EXPR <R1b, 0> + 2) * 4

BIT_AND_EXPR

-4

as it happens for TYPE_SIZE_UNIT of a struct for gnat.dg/opt9.adb.

This causes an issue with trying to compute the extent/padding of
a trailing bitfield in such a struct when you do

size_diffop (TYPE_SIZE_UNIT (DECL_CONTEXT (field)),
             DECL_FIELD_OFFSET (repr));

where repr is the first field of a bitfield group and field is the last
field of a bitfield group.

The above expression computes non-constant

((ssizetype) (((sizetype) MAX_EXPR <R1b, 0> + 2) * 4) & -4) - (ssizetype)
((sizetype) MAX_EXPR <R1b, 0> * 4)

Not sure if such an expression cannot be reliably required to be constant
in ada though.

Certainly this looks like a missed folding (not sure how often this
odd BIT_AND_EXPR for sizes happen in Ada).