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From: "peteraward+gcc at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/51506] New: Function cloning misses constant struct
Date: Sun, 11 Dec 2011 23:58:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D51506

             Bug #: 51506
           Summary: Function cloning misses constant struct
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: peteraward+gcc@gmail.com


The actual problem I=E2=80=99m dealing with is with avr-gcc, so the goal is=
 to achieve
a small code size. I=E2=80=99m trying to write my code like this:
lcd_init(lcd_t l, ...)
where the first parameter is passed a *constant* struct which contains the
memory addresses of each of the pins for the LCD. Thus, I want the compiler=
 to
note that all calls have the same first argument, clone the function, and
propagate the constant.

However, it doesn=E2=80=99t seem to be working in practice.
In trying to build this test case, I found the compiler would just inline a=
ll
the functions, which defeats the point (in the actual code, the cost of
inlining is too high). So, I=E2=80=99ve added the noinline attribute, which=
 I don=E2=80=99t
think should stop this optimisation, but apologies if it does.

Anyhow, here=E2=80=99s the testcase.
(using gcc version 4.6.2 (Debian 4.6.2-5), on 64-bit Linux)

$ cat test.c
typedef struct {
    int a;
    int b;
} dint;

__attribute__((noinline))
static int compute_int(int x, int var) {
    int y =3D 0;
    for (int i =3D 0; i < x; i++)
        y +=3D i * x;
    return y + var;
}

__attribute__((noinline))
static int compute_dint(dint x, int var) {
    int z =3D x.a + x.b;
    int y =3D 0;
    for (int i =3D 0; i < z; i++)
        y +=3D i * z;
    return y + var;
}

int main() {
    int rv;
    rv +=3D compute_dint((dint) {6, 1}, 1);
    rv +=3D compute_dint((dint) {6, 1}, 2);
    rv +=3D compute_dint((dint) {6, 1}, 3);
    rv +=3D compute_int(5, 1);
    rv +=3D compute_int(5, 2);
    rv +=3D compute_int(5, 3);
    return rv;
}
$ gcc -fdump-ipa-all -fipa-cp -fipa-cp-clone -Os -std=3Dc99 test.c

Expected result:
both compute_int and compute_dint should be optimised to versions where "x"=
 is
constant.

Actual reslut:
only compute_int is optimised.