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Date: Tue, 20 Nov 2007 12:57:00 -0000
Subject: [Bug c/34161]  New: -Os produces 32-bit load from 16-bit variable
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From: "vegard at peltkore dot net" <gcc-bugzilla@gcc.gnu.org>
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When compiling the following program with -Os, the short member x of struct a
is read using a movl instruction instead of movzwl.

struct a {
        int dummy;
        short x;
};

struct b {
        unsigned short x;
};

extern void dummy(struct b *b);

void f(struct a *a, struct b *b)
{
        dummy(b);

        if (a) {
                asm("/* THIS FIELD IS ONLY 16 BITS */");
                b->x = a->x;
        }
}

With -O3:
        movzwl  4(%ebx), %eax
        movw    %ax, (%esi)

And -Os:
        movl    4(%ebx), %eax
        movw    %ax, (%esi)


-- 
           Summary: -Os produces 32-bit load from 16-bit variable
           Product: gcc
           Version: 4.1.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: vegard at peltkore dot net
 GCC build triplet: i386-redhat-linux
  GCC host triplet: i386-redhat-linux
GCC target triplet: i386-redhat-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34161